=head1 NAME perlfaq4 - Data Manipulation =head1 VERSION version 5.20210520 =head1 DESCRIPTION This section of the FAQ answers questions related to manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues. =head1 Data: Numbers =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? For the long explanation, see David Goldberg's "What Every Computer Scientist Should Know About Floating-Point Arithmetic" (L). Internally, your computer represents floating-point numbers in binary. Digital (as in powers of two) computers cannot store all numbers exactly. Some real numbers lose precision in the process. This is a problem with how computers store numbers and affects all computer languages, not just Perl. L shows the gory details of number representations and conversions. To limit the number of decimal places in your numbers, you can use the C or C function. See L for more details. printf "%.2f", 10/3; my \$number = sprintf "%.2f", 10/3; =head2 Why is int() broken? Your C is most probably working just fine. It's the numbers that aren't quite what you think. First, see the answer to "Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?". For example, this print int(0.6/0.2-2), "\n"; will in most computers print 0, not 1, because even such simple numbers as 0.6 and 0.2 cannot be presented exactly by floating-point numbers. What you think in the above as 'three' is really more like 2.9999999999999995559. =head2 Why isn't my octal data interpreted correctly? (contributed by brian d foy) You're probably trying to convert a string to a number, which Perl only converts as a decimal number. When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest of the digits are in base 10: my \$string = '0644'; print \$string + 0; # prints 644 print \$string + 44; # prints 688, certainly not octal! This problem usually involves one of the Perl built-ins that has the same name a Unix command that uses octal numbers as arguments on the command line. In this example, C on the command line knows that its first argument is octal because that's what it does: %prompt> chmod 644 file If you want to use the same literal digits (644) in Perl, you have to tell Perl to treat them as octal numbers either by prefixing the digits with a C<0> or using C: chmod( 0644, \$filename ); # right, has leading zero chmod( oct(644), \$filename ); # also correct The problem comes in when you take your numbers from something that Perl thinks is a string, such as a command line argument in C<@ARGV>: chmod( \$ARGV[0], \$filename ); # wrong, even if "0644" chmod( oct(\$ARGV[0]), \$filename ); # correct, treat string as octal You can always check the value you're using by printing it in octal notation to ensure it matches what you think it should be. Print it in octal and decimal format: printf "0%o %d", \$number, \$number; =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? Remember that C merely truncates toward 0. For rounding to a certain number of digits, C or C is usually the easiest route. printf("%.3f", 3.1415926535); # prints 3.142 The L module (part of the standard Perl distribution) implements C, C, and a number of other mathematical and trigonometric functions. use POSIX; my \$ceil = ceil(3.5); # 4 my \$floor = floor(3.5); # 3 In 5.000 to 5.003 perls, trigonometry was done in the L module. With 5.004, the L module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the L module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system of rounding is being used by Perl, but instead to implement the rounding function you need yourself. To see why, notice how you'll still have an issue on half-way-point alternation: for (my \$i = -5; \$i <= 5; \$i += 0.5) { printf "%.0f ",\$i } -5 -4 -4 -4 -3 -2 -2 -2 -1 -0 0 0 1 2 2 2 3 4 4 4 5 Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on 32-bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed. =head2 How do I convert between numeric representations/bases/radixes? As always with Perl there is more than one way to do it. Below are a few examples of approaches to making common conversions between number representations. This is intended to be representational rather than exhaustive. Some of the examples later in L use the L module from CPAN. The reason you might choose L over the perl built-in functions is that it works with numbers of ANY size, that it is optimized for speed on some operations, and for at least some programmers the notation might be familiar. =over 4 =item How do I convert hexadecimal into decimal Using perl's built in conversion of C<0x> notation: my \$dec = 0xDEADBEEF; Using the C function: my \$dec = hex("DEADBEEF"); Using C: my \$dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); Using the CPAN module C: use Bit::Vector; my \$vec = Bit::Vector->new_Hex(32, "DEADBEEF"); my \$dec = \$vec->to_Dec(); =item How do I convert from decimal to hexadecimal Using C: my \$hex = sprintf("%X", 3735928559); # upper case A-F my \$hex = sprintf("%x", 3735928559); # lower case a-f Using C: my \$hex = unpack("H*", pack("N", 3735928559)); Using L: use Bit::Vector; my \$vec = Bit::Vector->new_Dec(32, -559038737); my \$hex = \$vec->to_Hex(); And L supports odd bit counts: use Bit::Vector; my \$vec = Bit::Vector->new_Dec(33, 3735928559); \$vec->Resize(32); # suppress leading 0 if unwanted my \$hex = \$vec->to_Hex(); =item How do I convert from octal to decimal Using Perl's built in conversion of numbers with leading zeros: my \$dec = 033653337357; # note the leading 0! Using the C function: my \$dec = oct("33653337357"); Using L: use Bit::Vector; my \$vec = Bit::Vector->new(32); \$vec->Chunk_List_Store(3, split(//, reverse "33653337357")); my \$dec = \$vec->to_Dec(); =item How do I convert from decimal to octal Using C: my \$oct = sprintf("%o", 3735928559); Using L: use Bit::Vector; my \$vec = Bit::Vector->new_Dec(32, -559038737); my \$oct = reverse join('', \$vec->Chunk_List_Read(3)); =item How do I convert from binary to decimal Perl 5.6 lets you write binary numbers directly with the C<0b> notation: my \$number = 0b10110110; Using C: my \$input = "10110110"; my \$decimal = oct( "0b\$input" ); Using C and C: my \$decimal = ord(pack('B8', '10110110')); Using C and C for larger strings: my \$int = unpack("N", pack("B32", substr("0" x 32 . "11110101011011011111011101111", -32))); my \$dec = sprintf("%d", \$int); # substr() is used to left-pad a 32-character string with zeros. Using L: my \$vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); my \$dec = \$vec->to_Dec(); =item How do I convert from decimal to binary Using C (perl 5.6+): my \$bin = sprintf("%b", 3735928559); Using C: my \$bin = unpack("B*", pack("N", 3735928559)); Using L: use Bit::Vector; my \$vec = Bit::Vector->new_Dec(32, -559038737); my \$bin = \$vec->to_Bin(); The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader. =back =head2 Why doesn't & work the way I want it to? The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a series of bits and work with that (the string C<"3"> is the bit pattern C<00110011>). The operators work with the binary form of a number (the number C<3> is treated as the bit pattern C<00000011>). So, saying C<11 & 3> performs the "and" operation on numbers (yielding C<3>). Saying C<"11" & "3"> performs the "and" operation on strings (yielding C<"1">). Most problems with C<&> and C<|> arise because the programmer thinks they have a number but really it's a string or vice versa. To avoid this, stringify the arguments explicitly (using C<""> or C) or convert them to numbers explicitly (using C<0+\$arg>). The rest arise because the programmer says: if ("\020\020" & "\101\101") { # ... } but a string consisting of two null bytes (the result of C<"\020\020" & "\101\101">) is not a false value in Perl. You need: if ( ("\020\020" & "\101\101") !~ /[^\000]/) { # ... } =head2 How do I multiply matrices? Use the L or L modules (available from CPAN) or the L extension (also available from CPAN). =head2 How do I perform an operation on a series of integers? To call a function on each element in an array, and collect the results, use: my @results = map { my_func(\$_) } @array; For example: my @triple = map { 3 * \$_ } @single; To call a function on each element of an array, but ignore the results: foreach my \$iterator (@array) { some_func(\$iterator); } To call a function on each integer in a (small) range, you B use: my @results = map { some_func(\$_) } (5 .. 25); but you should be aware that in this form, the C<..> operator creates a list of all integers in the range, which can take a lot of memory for large ranges. However, the problem does not occur when using C<..> within a C loop, because in that case the range operator is optimized to I over the range, without creating the entire list. So my @results = (); for my \$i (5 .. 500_005) { push(@results, some_func(\$i)); } or even push(@results, some_func(\$_)) for 5 .. 500_005; will not create an intermediate list of 500,000 integers. =head2 How can I output Roman numerals? Get the L module. =head2 Why aren't my random numbers random? If you're using a version of Perl before 5.004, you must call C once at the start of your program to seed the random number generator. BEGIN { srand() if \$] < 5.004 } 5.004 and later automatically call C at the beginning. Don't call C more than once--you make your numbers less random, rather than more. Computers are good at being predictable and bad at being random (despite appearances caused by bugs in your programs :-). The F article in the "Far More Than You Ever Wanted To Know" collection in L, courtesy of Tom Phoenix, talks more about this. John von Neumann said, "Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin." Perl relies on the underlying system for the implementation of C and C; on some systems, the generated numbers are not random enough (especially on Windows : see L). Several CPAN modules in the C option squashes duplicated and consecutive characters in the string so a character does not show up next to itself my \$str = 'Haarlem'; # in the Netherlands \$str =~ tr///cs; # Now Harlem, like in New York =head2 How do I expand function calls in a string? (contributed by brian d foy) This is documented in L, and although it's not the easiest thing to read, it does work. In each of these examples, we call the function inside the braces used to dereference a reference. If we have more than one return value, we can construct and dereference an anonymous array. In this case, we call the function in list context. print "The time values are @{ [localtime] }.\n"; If we want to call the function in scalar context, we have to do a bit more work. We can really have any code we like inside the braces, so we simply have to end with the scalar reference, although how you do that is up to you, and you can use code inside the braces. Note that the use of parens creates a list context, so we need C to force the scalar context on the function: print "The time is \${\(scalar localtime)}.\n" print "The time is \${ my \$x = localtime; \\$x }.\n"; If your function already returns a reference, you don't need to create the reference yourself. sub timestamp { my \$t = localtime; \\$t } print "The time is \${ timestamp() }.\n"; The C module can also do a lot of magic for you. You can specify a variable name, in this case C, to set up a tied hash that does the interpolation for you. It has several other methods to do this as well. use Interpolation E => 'eval'; print "The time values are \$E{localtime()}.\n"; In most cases, it is probably easier to simply use string concatenation, which also forces scalar context. print "The time is " . localtime() . ".\n"; =head2 How do I find matching/nesting anything? To find something between two single characters, a pattern like C will get the intervening bits in \$1. For multiple ones, then something more like C would be needed. For nested patterns and/or balanced expressions, see the so-called L<< (?PARNO)|perlre/C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> >> construct (available since perl 5.10). The CPAN module L can help to build such regular expressions (see in particular L and L). More complex cases will require to write a parser, probably using a parsing module from CPAN, like L, L, L, L, or L. =head2 How do I reverse a string? Use C in scalar context, as documented in L. my \$reversed = reverse \$string; =head2 How do I expand tabs in a string? You can do it yourself: 1 while \$string =~ s/\t+/' ' x (length(\$&) * 8 - length(\$`) % 8)/e; Or you can just use the L module (part of the standard Perl distribution). use Text::Tabs; my @expanded_lines = expand(@lines_with_tabs); =head2 How do I reformat a paragraph? Use L (part of the standard Perl distribution): use Text::Wrap; print wrap("\t", ' ', @paragraphs); The paragraphs you give to L should not contain embedded newlines. L doesn't justify the lines (flush-right). Or use the CPAN module L. Formatting files can be easily done by making a shell alias, like so: alias fmt="perl -i -MText::Autoformat -n0777 \ -e 'print autoformat \$_, {all=>1}' \$*" See the documentation for L to appreciate its many capabilities. =head2 How can I access or change N characters of a string? You can access the first characters of a string with substr(). To get the first character, for example, start at position 0 and grab the string of length 1. my \$string = "Just another Perl Hacker"; my \$first_char = substr( \$string, 0, 1 ); # 'J' To change part of a string, you can use the optional fourth argument which is the replacement string. substr( \$string, 13, 4, "Perl 5.8.0" ); You can also use substr() as an lvalue. substr( \$string, 13, 4 ) = "Perl 5.8.0"; =head2 How do I change the Nth occurrence of something? You have to keep track of N yourself. For example, let's say you want to change the fifth occurrence of C<"whoever"> or C<"whomever"> into C<"whosoever"> or C<"whomsoever">, case insensitively. These all assume that \$_ contains the string to be altered. \$count = 0; s{((whom?)ever)}{ ++\$count == 5 # is it the 5th? ? "\${2}soever" # yes, swap : \$1 # renege and leave it there }ige; In the more general case, you can use the C modifier in a C loop, keeping count of matches. \$WANT = 3; \$count = 0; \$_ = "One fish two fish red fish blue fish"; while (/(\w+)\s+fish\b/gi) { if (++\$count == \$WANT) { print "The third fish is a \$1 one.\n"; } } That prints out: C<"The third fish is a red one."> You can also use a repetition count and repeated pattern like this: /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; =head2 How can I count the number of occurrences of a substring within a string? There are a number of ways, with varying efficiency. If you want a count of a certain single character (X) within a string, you can use the C function like so: my \$string = "ThisXlineXhasXsomeXx'sXinXit"; my \$count = (\$string =~ tr/X//); print "There are \$count X characters in the string"; This is fine if you are just looking for a single character. However, if you are trying to count multiple character substrings within a larger string, C won't work. What you can do is wrap a while() loop around a global pattern match. For example, let's count negative integers: my \$string = "-9 55 48 -2 23 -76 4 14 -44"; my \$count = 0; while (\$string =~ /-\d+/g) { \$count++ } print "There are \$count negative numbers in the string"; Another version uses a global match in list context, then assigns the result to a scalar, producing a count of the number of matches. my \$count = () = \$string =~ /-\d+/g; =head2 How do I capitalize all the words on one line? X X X X (contributed by brian d foy) Damian Conway's L handles all of the thinking for you. use Text::Autoformat; my \$x = "Dr. Strangelove or: How I Learned to Stop ". "Worrying and Love the Bomb"; print \$x, "\n"; for my \$style (qw( sentence title highlight )) { print autoformat(\$x, { case => \$style }), "\n"; } How do you want to capitalize those words? FRED AND BARNEY'S LODGE # all uppercase Fred And Barney's Lodge # title case Fred and Barney's Lodge # highlight case It's not as easy a problem as it looks. How many words do you think are in there? Wait for it... wait for it.... If you answered 5 you're right. Perl words are groups of C<\w+>, but that's not what you want to capitalize. How is Perl supposed to know not to capitalize that C after the apostrophe? You could try a regular expression: \$string =~ s/ ( (^\w) #at the beginning of the line | # or (\s\w) #preceded by whitespace ) /\U\$1/xg; \$string =~ s/([\w']+)/\u\L\$1/g; Now, what if you don't want to capitalize that "and"? Just use L and get on with the next problem. :) =head2 How can I split a [character]-delimited string except when inside [character]? Several modules can handle this sort of parsing--L, L, L, and L, among others. Take the example case of trying to split a string that is comma-separated into its different fields. You can't use C because you shouldn't split if the comma is inside quotes. For example, take a data line like this: SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of I, to handle these for us. He suggests (assuming your string is contained in C<\$text>): my @new = (); push(@new, \$+) while \$text =~ m{ "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | ([^,]+),? | , }gx; push(@new, undef) if substr(\$text,-1,1) eq ','; If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, C<"like \"this\"">. Alternatively, the L module (part of the standard Perl distribution) lets you say: use Text::ParseWords; @new = quotewords(",", 0, \$text); For parsing or generating CSV, though, using L rather than implementing it yourself is highly recommended; you'll save yourself odd bugs popping up later by just using code which has already been tried and tested in production for years. =head2 How do I strip blank space from the beginning/end of a string? (contributed by brian d foy) A substitution can do this for you. For a single line, you want to replace all the leading or trailing whitespace with nothing. You can do that with a pair of substitutions: s/^\s+//; s/\s+\$//; You can also write that as a single substitution, although it turns out the combined statement is slower than the separate ones. That might not matter to you, though: s/^\s+|\s+\$//g; In this regular expression, the alternation matches either at the beginning or the end of the string since the anchors have a lower precedence than the alternation. With the C flag, the substitution makes all possible matches, so it gets both. Remember, the trailing newline matches the C<\s+>, and the C<\$> anchor can match to the absolute end of the string, so the newline disappears too. Just add the newline to the output, which has the added benefit of preserving "blank" (consisting entirely of whitespace) lines which the C<^\s+> would remove all by itself: while( <> ) { s/^\s+|\s+\$//g; print "\$_\n"; } For a multi-line string, you can apply the regular expression to each logical line in the string by adding the C flag (for "multi-line"). With the C flag, the C<\$> matches I an embedded newline, so it doesn't remove it. This pattern still removes the newline at the end of the string: \$string =~ s/^\s+|\s+\$//gm; Remember that lines consisting entirely of whitespace will disappear, since the first part of the alternation can match the entire string and replace it with nothing. If you need to keep embedded blank lines, you have to do a little more work. Instead of matching any whitespace (since that includes a newline), just match the other whitespace: \$string =~ s/^[\t\f ]+|[\t\f ]+\$//mg; =head2 How do I pad a string with blanks or pad a number with zeroes? In the following examples, C<\$pad_len> is the length to which you wish to pad the string, C<\$text> or C<\$num> contains the string to be padded, and C<\$pad_char> contains the padding character. You can use a single character string constant instead of the C<\$pad_char> variable if you know what it is in advance. And in the same way you can use an integer in place of C<\$pad_len> if you know the pad length in advance. The simplest method uses the C function. It can pad on the left or right with blanks and on the left with zeroes and it will not truncate the result. The C function can only pad strings on the right with blanks and it will truncate the result to a maximum length of C<\$pad_len>. # Left padding a string with blanks (no truncation): my \$padded = sprintf("%\${pad_len}s", \$text); my \$padded = sprintf("%*s", \$pad_len, \$text); # same thing # Right padding a string with blanks (no truncation): my \$padded = sprintf("%-\${pad_len}s", \$text); my \$padded = sprintf("%-*s", \$pad_len, \$text); # same thing # Left padding a number with 0 (no truncation): my \$padded = sprintf("%0\${pad_len}d", \$num); my \$padded = sprintf("%0*d", \$pad_len, \$num); # same thing # Right padding a string with blanks using pack (will truncate): my \$padded = pack("A\$pad_len",\$text); If you need to pad with a character other than blank or zero you can use one of the following methods. They all generate a pad string with the C operator and combine that with C<\$text>. These methods do not truncate C<\$text>. Left and right padding with any character, creating a new string: my \$padded = \$pad_char x ( \$pad_len - length( \$text ) ) . \$text; my \$padded = \$text . \$pad_char x ( \$pad_len - length( \$text ) ); Left and right padding with any character, modifying C<\$text> directly: substr( \$text, 0, 0 ) = \$pad_char x ( \$pad_len - length( \$text ) ); \$text .= \$pad_char x ( \$pad_len - length( \$text ) ); =head2 How do I extract selected columns from a string? (contributed by brian d foy) If you know the columns that contain the data, you can use C to extract a single column. my \$column = substr( \$line, \$start_column, \$length ); You can use C if the columns are separated by whitespace or some other delimiter, as long as whitespace or the delimiter cannot appear as part of the data. my \$line = ' fred barney betty '; my @columns = split /\s+/, \$line; # ( '', 'fred', 'barney', 'betty' ); my \$line = 'fred||barney||betty'; my @columns = split /\|/, \$line; # ( 'fred', '', 'barney', '', 'betty' ); If you want to work with comma-separated values, don't do this since that format is a bit more complicated. Use one of the modules that handle that format, such as L, L, or L. If you want to break apart an entire line of fixed columns, you can use C with the A (ASCII) format. By using a number after the format specifier, you can denote the column width. See the C and C entries in L for more details. my @fields = unpack( \$line, "A8 A8 A8 A16 A4" ); Note that spaces in the format argument to C do not denote literal spaces. If you have space separated data, you may want C instead. =head2 How do I find the soundex value of a string? (contributed by brian d foy) You can use the C module. If you want to do fuzzy or close matching, you might also try the L, and L, and L modules. =head2 How can I expand variables in text strings? (contributed by brian d foy) If you can avoid it, don't, or if you can use a templating system, such as L or L
Toolkit, do that instead. You might even be able to get the job done with C or C: my \$string = sprintf 'Say hello to %s and %s', \$foo, \$bar; However, for the one-off simple case where I don't want to pull out a full templating system, I'll use a string that has two Perl scalar variables in it. In this example, I want to expand C<\$foo> and C<\$bar> to their variable's values: my \$foo = 'Fred'; my \$bar = 'Barney'; \$string = 'Say hello to \$foo and \$bar'; One way I can do this involves the substitution operator and a double C flag. The first C evaluates C<\$1> on the replacement side and turns it into C<\$foo>. The second /e starts with C<\$foo> and replaces it with its value. C<\$foo>, then, turns into 'Fred', and that's finally what's left in the string: \$string =~ s/(\\$\w+)/\$1/eeg; # 'Say hello to Fred and Barney' The C will also silently ignore violations of strict, replacing undefined variable names with the empty string. Since I'm using the C flag (twice even!), I have all of the same security problems I have with C in its string form. If there's something odd in C<\$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then I could get myself in trouble. To get around the security problem, I could also pull the values from a hash instead of evaluating variable names. Using a single C, I can check the hash to ensure the value exists, and if it doesn't, I can replace the missing value with a marker, in this case C to signal that I missed something: my \$string = 'This has \$foo and \$bar'; my %Replacements = ( foo => 'Fred', ); # \$string =~ s/\\$(\w+)/\$Replacements{\$1}/g; \$string =~ s/\\$(\w+)/ exists \$Replacements{\$1} ? \$Replacements{\$1} : '???' /eg; print \$string; =head2 Does Perl have anything like Ruby's #{} or Python's f string? Unlike the others, Perl allows you to embed a variable naked in a double quoted string, e.g. C<"variable \$variable">. When there isn't whitespace or other non-word characters following the variable name, you can add braces (e.g. C<"foo \${foo}bar">) to ensure correct parsing. An array can also be embedded directly in a string, and will be expanded by default with spaces between the elements. The default L can be changed by assigning a different string to the special variable C<\$">, such as C. Perl also supports references within a string providing the equivalent of the features in the other two languages. C<\${\ ... }> embedded within a string will work for most simple statements such as an object->method call. More complex code can be wrapped in a do block C<\${\ do{...} }>. When you want a list to be expanded per C<\$">, use C<@{[ ... ]}>. use Time::Piece; use Time::Seconds; my \$scalar = 'STRING'; my @array = ( 'zorro', 'a', 1, 'B', 3 ); # Print the current date and time and then Tommorrow my \$t = Time::Piece->new; say "Now is: \${\ \$t->cdate() }"; say "Tomorrow: \${\ do{ my \$T=Time::Piece->new + ONE_DAY ; \$T->fullday }}"; # some variables in strings say "This is some scalar I have \$scalar, this is an array @array."; say "You can also write it like this \${scalar} @{array}."; # Change the \$LIST_SEPARATOR local \$" = ':'; say "Set \\$\" to delimit with ':' and sort the Array @{[ sort @array ]}"; You may also want to look at the module L, and templating tools such as L and L. See also: L and L in this FAQ. =head2 What's wrong with always quoting "\$vars"? The problem is that those double-quotes force stringification--coercing numbers and references into strings--even when you don't want them to be strings. Think of it this way: double-quote expansion is used to produce new strings. If you already have a string, why do you need more? If you get used to writing odd things like these: print "\$var"; # BAD my \$new = "\$old"; # BAD somefunc("\$var"); # BAD You'll be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct: print \$var; my \$new = \$old; somefunc(\$var); Otherwise, besides slowing you down, you're going to break code when the thing in the scalar is actually neither a string nor a number, but a reference: func(\@array); sub func { my \$aref = shift; my \$oref = "\$aref"; # WRONG } You can also get into subtle problems on those few operations in Perl that actually do care about the difference between a string and a number, such as the magical C<++> autoincrement operator or the syscall() function. Stringification also destroys arrays. my @lines = `command`; print "@lines"; # WRONG - extra blanks print @lines; # right =head2 Why don't my EEHERE documents work? Here documents are found in L. Check for these three things: =over 4 =item There must be no space after the EE part. =item There (probably) should be a semicolon at the end of the opening token =item You can't (easily) have any space in front of the tag. =item There needs to be at least a line separator after the end token. =back If you want to indent the text in the here document, you can do this: # all in one (my \$VAR = <op_ppaddr)() ); @@@ TAINT_NOT; @@@ return 0; @@@ } MAIN_INTERPRETER_LOOP Or with a fixed amount of leading whitespace, with remaining indentation correctly preserved: my \$poem = fix< for details. =head1 Data: Arrays =head2 What is the difference between a list and an array? (contributed by brian d foy) A list is a fixed collection of scalars. An array is a variable that holds a variable collection of scalars. An array can supply its collection for list operations, so list operations also work on arrays: # slices ( 'dog', 'cat', 'bird' )[2,3]; @animals[2,3]; # iteration foreach ( qw( dog cat bird ) ) { ... } foreach ( @animals ) { ... } my @three = grep { length == 3 } qw( dog cat bird ); my @three = grep { length == 3 } @animals; # supply an argument list wash_animals( qw( dog cat bird ) ); wash_animals( @animals ); Array operations, which change the scalars, rearrange them, or add or subtract some scalars, only work on arrays. These can't work on a list, which is fixed. Array operations include C, C, C, C, and C. An array can also change its length: \$#animals = 1; # truncate to two elements \$#animals = 10000; # pre-extend to 10,001 elements You can change an array element, but you can't change a list element: \$animals[0] = 'Rottweiler'; qw( dog cat bird )[0] = 'Rottweiler'; # syntax error! foreach ( @animals ) { s/^d/fr/; # works fine } foreach ( qw( dog cat bird ) ) { s/^d/fr/; # Error! Modification of read only value! } However, if the list element is itself a variable, it appears that you can change a list element. However, the list element is the variable, not the data. You're not changing the list element, but something the list element refers to. The list element itself doesn't change: it's still the same variable. You also have to be careful about context. You can assign an array to a scalar to get the number of elements in the array. This only works for arrays, though: my \$count = @animals; # only works with arrays If you try to do the same thing with what you think is a list, you get a quite different result. Although it looks like you have a list on the righthand side, Perl actually sees a bunch of scalars separated by a comma: my \$scalar = ( 'dog', 'cat', 'bird' ); # \$scalar gets bird Since you're assigning to a scalar, the righthand side is in scalar context. The comma operator (yes, it's an operator!) in scalar context evaluates its lefthand side, throws away the result, and evaluates it's righthand side and returns the result. In effect, that list-lookalike assigns to C<\$scalar> it's rightmost value. Many people mess this up because they choose a list-lookalike whose last element is also the count they expect: my \$scalar = ( 1, 2, 3 ); # \$scalar gets 3, accidentally =head2 What is the difference between \$array[1] and @array[1]? (contributed by brian d foy) The difference is the sigil, that special character in front of the array name. The C<\$> sigil means "exactly one item", while the C<@> sigil means "zero or more items". The C<\$> gets you a single scalar, while the C<@> gets you a list. The confusion arises because people incorrectly assume that the sigil denotes the variable type. The C<\$array[1]> is a single-element access to the array. It's going to return the item in index 1 (or undef if there is no item there). If you intend to get exactly one element from the array, this is the form you should use. The C<@array[1]> is an array slice, although it has only one index. You can pull out multiple elements simultaneously by specifying additional indices as a list, like C<@array[1,4,3,0]>. Using a slice on the lefthand side of the assignment supplies list context to the righthand side. This can lead to unexpected results. For instance, if you want to read a single line from a filehandle, assigning to a scalar value is fine: \$array[1] = ; However, in list context, the line input operator returns all of the lines as a list. The first line goes into C<@array[1]> and the rest of the lines mysteriously disappear: @array[1] = ; # most likely not what you want Either the C pragma or the B<-w> flag will warn you when you use an array slice with a single index. =head2 How can I remove duplicate elements from a list or array? (contributed by brian d foy) Use a hash. When you think the words "unique" or "duplicated", think "hash keys". If you don't care about the order of the elements, you could just create the hash then extract the keys. It's not important how you create that hash: just that you use C to get the unique elements. my %hash = map { \$_, 1 } @array; # or a hash slice: @hash{ @array } = (); # or a foreach: \$hash{\$_} = 1 foreach ( @array ); my @unique = keys %hash; If you want to use a module, try the C function from L. In list context it returns the unique elements, preserving their order in the list. In scalar context, it returns the number of unique elements. use List::MoreUtils qw(uniq); my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7 my \$unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7 You can also go through each element and skip the ones you've seen before. Use a hash to keep track. The first time the loop sees an element, that element has no key in C<%Seen>. The C statement creates the key and immediately uses its value, which is C, so the loop continues to the C and increments the value for that key. The next time the loop sees that same element, its key exists in the hash I the value for that key is true (since it's not 0 or C), so the next skips that iteration and the loop goes to the next element. my @unique = (); my %seen = (); foreach my \$elem ( @array ) { next if \$seen{ \$elem }++; push @unique, \$elem; } You can write this more briefly using a grep, which does the same thing. my %seen = (); my @unique = grep { ! \$seen{ \$_ }++ } @array; =head2 How can I tell whether a certain element is contained in a list or array? (portions of this answer contributed by Anno Siegel and brian d foy) Hearing the word "in" is an Idication that you probably should have used a hash, not a list or array, to store your data. Hashes are designed to answer this question quickly and efficiently. Arrays aren't. That being said, there are several ways to approach this. If you are going to make this query many times over arbitrary string values, the fastest way is probably to invert the original array and maintain a hash whose keys are the first array's values: my @blues = qw/azure cerulean teal turquoise lapis-lazuli/; my %is_blue = (); for (@blues) { \$is_blue{\$_} = 1 } Now you can check whether C<\$is_blue{\$some_color}>. It might have been a good idea to keep the blues all in a hash in the first place. If the values are all small integers, you could use a simple indexed array. This kind of an array will take up less space: my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); my @is_tiny_prime = (); for (@primes) { \$is_tiny_prime[\$_] = 1 } # or simply @istiny_prime[@primes] = (1) x @primes; Now you check whether \$is_tiny_prime[\$some_number]. If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead: my @articles = ( 1..10, 150..2000, 2017 ); undef \$read; for (@articles) { vec(\$read,\$_,1) = 1 } Now check whether C is true for some C<\$n>. These methods guarantee fast individual tests but require a re-organization of the original list or array. They only pay off if you have to test multiple values against the same array. If you are testing only once, the standard module L exports the function C for this purpose. It works by stopping once it finds the element. It's written in C for speed, and its Perl equivalent looks like this subroutine: sub any (&@) { my \$code = shift; foreach (@_) { return 1 if \$code->(); } return 0; } If speed is of little concern, the common idiom uses grep in scalar context (which returns the number of items that passed its condition) to traverse the entire list. This does have the benefit of telling you how many matches it found, though. my \$is_there = grep \$_ eq \$whatever, @array; If you want to actually extract the matching elements, simply use grep in list context. my @matches = grep \$_ eq \$whatever, @array; =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? Use a hash. Here's code to do both and more. It assumes that each element is unique in a given array: my (@union, @intersection, @difference); my %count = (); foreach my \$element (@array1, @array2) { \$count{\$element}++ } foreach my \$element (keys %count) { push @union, \$element; push @{ \$count{\$element} > 1 ? \@intersection : \@difference }, \$element; } Note that this is the I, that is, all elements in either A or in B but not in both. Think of it as an xor operation. =head2 How do I test whether two arrays or hashes are equal? The following code works for single-level arrays. It uses a stringwise comparison, and does not distinguish defined versus undefined empty strings. Modify if you have other needs. \$are_equal = compare_arrays(\@frogs, \@toads); sub compare_arrays { my (\$first, \$second) = @_; no warnings; # silence spurious -w undef complaints return 0 unless @\$first == @\$second; for (my \$i = 0; \$i < @\$first; \$i++) { return 0 if \$first->[\$i] ne \$second->[\$i]; } return 1; } For multilevel structures, you may wish to use an approach more like this one. It uses the CPAN module L: use FreezeThaw qw(cmpStr); my @a = my @b = ( "this", "that", [ "more", "stuff" ] ); printf "a and b contain %s arrays\n", cmpStr(\@a, \@b) == 0 ? "the same" : "different"; This approach also works for comparing hashes. Here we'll demonstrate two different answers: use FreezeThaw qw(cmpStr cmpStrHard); my %a = my %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); \$a{EXTRA} = \%b; \$b{EXTRA} = \%a; printf "a and b contain %s hashes\n", cmpStr(\%a, \%b) == 0 ? "the same" : "different"; printf "a and b contain %s hashes\n", cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; The first reports that both those the hashes contain the same data, while the second reports that they do not. Which you prefer is left as an exercise to the reader. =head2 How do I find the first array element for which a condition is true? To find the first array element which satisfies a condition, you can use the C function in the L module, which comes with Perl 5.8. This example finds the first element that contains "Perl". use List::Util qw(first); my \$element = first { /Perl/ } @array; If you cannot use L, you can make your own loop to do the same thing. Once you find the element, you stop the loop with last. my \$found; foreach ( @array ) { if( /Perl/ ) { \$found = \$_; last } } If you want the array index, use the C function from C: use List::MoreUtils qw(firstidx); my \$index = firstidx { /Perl/ } @array; Or write it yourself, iterating through the indices and checking the array element at each index until you find one that satisfies the condition: my( \$found, \$index ) = ( undef, -1 ); for( \$i = 0; \$i < @array; \$i++ ) { if( \$array[\$i] =~ /Perl/ ) { \$found = \$array[\$i]; \$index = \$i; last; } } =head2 How do I handle linked lists? (contributed by brian d foy) Perl's arrays do not have a fixed size, so you don't need linked lists if you just want to add or remove items. You can use array operations such as C, C, C, C, or C to do that. Sometimes, however, linked lists can be useful in situations where you want to "shard" an array so you have many small arrays instead of a single big array. You can keep arrays longer than Perl's largest array index, lock smaller arrays separately in threaded programs, reallocate less memory, or quickly insert elements in the middle of the chain. Steve Lembark goes through the details in his YAPC::NA 2009 talk "Perly Linked Lists" ( L ), although you can just use his L module. =head2 How do I handle circular lists? X X X X X X (contributed by brian d foy) If you want to cycle through an array endlessly, you can increment the index modulo the number of elements in the array: my @array = qw( a b c ); my \$i = 0; while( 1 ) { print \$array[ \$i++ % @array ], "\n"; last if \$i > 20; } You can also use L to use a scalar that always has the next element of the circular array: use Tie::Cycle; tie my \$cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ]; print \$cycle; # FFFFFF print \$cycle; # 000000 print \$cycle; # FFFF00 The L creates an iterator object for circular arrays: use Array::Iterator::Circular; my \$color_iterator = Array::Iterator::Circular->new( qw(red green blue orange) ); foreach ( 1 .. 20 ) { print \$color_iterator->next, "\n"; } =head2 How do I shuffle an array randomly? If you either have Perl 5.8.0 or later installed, or if you have Scalar-List-Utils 1.03 or later installed, you can say: use List::Util 'shuffle'; @shuffled = shuffle(@list); If not, you can use a Fisher-Yates shuffle. sub fisher_yates_shuffle { my \$deck = shift; # \$deck is a reference to an array return unless @\$deck; # must not be empty! my \$i = @\$deck; while (--\$i) { my \$j = int rand (\$i+1); @\$deck[\$i,\$j] = @\$deck[\$j,\$i]; } } # shuffle my mpeg collection # my @mpeg =