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Understanding Perl Lists & Arrays: Concepts and Examples

Learn about Perl lists and arrays for efficient programming. Discover how to access array elements, special array indices, list literals, and more through this comprehensive guide.

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Understanding Perl Lists & Arrays: Concepts and Examples

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  1. Programming Language ConceptsPerl Lists & Arrays Adapted by Carl Reynolds from materials by Sean P. Strout PLC - Perl Lists & Arrays (v2.00)

  2. Values 0 35 1 12.4 2 ’hello’ Indices 1.72e30 3 4 “bye\n” Lists and Arrays • Plurals are represented by lists and arrays • A list is an ordered collection of scalars (i.e. the data) • An array is a variable that contains a list • A list can hold any mixture of scalar values PLC - Perl Lists & Arrays (v2.00)

  3. Accessing Array Elements • Array elements begin at zero and increase by one $fred[0] = “yabba”; $fred[1] = “dabba”; $fred[2] = “doo”; • Arrays use a different namespace than scalars, so variables can have the same names: $fred = 65; print “scalar fred = $fred\n”; print “array fred[0] = $fred[0]”; PLC - Perl Lists & Arrays (v2.00)

  4. Accessing Array Elements • The subscript is truncated to an integer $fred[.7] ; # $fred[0], or “yabba” • Accessing an element beyond the end of an array yields an undef value (same behavior as scalars) $blank = $fred[ 142_897 ]; # $blank = undef $darl = $mcbride; # unused $mcbride undef PLC - Perl Lists & Arrays (v2.00)

  5. Special Array Indices • Arrays are automatically extended as needed, so it’s impossible to overrun other data • C vs. Perl example void main() { $x = 10; int x = 10; $array[0] = 0; int array[5]; for ($i=0;$i<=5;$i++) { int i; $array[$i] = $i; } for (i=0; i<=5; i++) { print “x=$x\n”; array[i] = i; } printf(“x=%d\n”, x); } • Intervening elements are automatically created as undef values. PLC - Perl Lists & Arrays (v2.00)

  6. Special Array Indices $rocks[0] = ’bedrock’; $rocks[1] = ’slate’; $rocks[2] = ’lava’; $rocks[3] = ’crushed rock’; $rocks[99] = ’schist’; # 95 undef elements • The last element index is $#array_name $rocks = $#rocks; # 99 $num_rocks = $rocks + 1 # ok but not ideal $#rocks = 2 # forget all rocks after lava $#rocks = 99 # add 97 new undef elements $rocks[ $#rocks ] = ’hard rock’ # add this rock at end • Negative indices wrap around $rocks[ -1 ] = ’hard rock’ # easier way $dead_rock = $rocks[ -100 ]; # gets ’bedrock’ $rocks[ -200 ] = ’crystal’ # fatal error PLC - Perl Lists & Arrays (v2.00)

  7. List Literals • A list is represented in your program as a list of comma-separated values enclosed in parentheses. (1, 2, 3) # list of three values, 1, 2 and 3 (1, 2, 3,) # trailing comma is ignored (“fred”, 4.5) # two values, “fred” and 4.5 () # empty list – zero elements • The range operator creates a series of scalars (1..100) # list of 100 ints (5..1) # empty list – only counts “uphill” (0, 2..6, 10) # 0, 2, 3, 4, 5, 6, 10 ($a..$b) # range determined by scalar vals (0..$#rocks) # the indices of the $rocks[] array ($a, 17) # two values, $a and 17 ($b+$c, $d+$e) # two values (“fred”, “barney”, “wilma”, “dino”) # 4 strings PLC - Perl Lists & Arrays (v2.00)

  8. Quoted Words • Use qw to generate a list of words without typing quote marks qw / fred barney betty wilma dino / • The whitespace is discarded and the elements are treated as single quoted strings • Can’t use \n or $scalar_name • How can you make a quoted word list out of these pathnames? Aren’t the forward slashes a problem? /usr/local/pub /home/adjunct/sps You can pick a delimiter for qw other than /: qw{/usr/local/pub /home/adjunct/sps} PLC - Perl Lists & Arrays (v2.00)

  9. List Assignment • List values can be assigned to scalar variables ($fred, $barney, $dino) = (“flintstone”, “rubble”, undef); $fred = (“a”, “b”, “c”); # bad things! (last in, sticks) ($fred, $barney) = ($barney, $fred); # swapping ($betty[1], $betty[0]) = ($betty[0], $betty[1]); ($fred, $barney) = qw< flintstone rubble slate granite >; ($wilma, $barney) = qw[flintstone]; • Use the at-sign, @ , to refer to the entire array @rocks = qw/ bedrock slate lava /; @tiny = (); # empty list @giant = 1..1e5; # list of 100,000 elements @stuff = (@giant, undef, @giant) # 200,001 elements $dino = “granite”; @quarry = (@rocks, “crushed rock”, @tiny, $dino); # 5 elements PLC - Perl Lists & Arrays (v2.00)

  10. List Assignment • Arrays cannot contain other arrays, so any assignment from one array to another is a copy. @copy = @quarry # copy a list from one array to another • Use pop to remove the last element from an array @array = 5..9; # 5 elements $fred = pop(@array); # 4 elements, $fred = 9 $barney = pop @array; # 3 elements, $barney = 8 pop @array; # ignore return value • A pop of an empty array returns undef PLC - Perl Lists & Arrays (v2.00)

  11. List Assignment • Use push to add elements to the end of an array. @array = 5..9; # 5 elements $fred = pop(@array); # 4 elements, $fred = 9 $barney = pop @array; # 3 elements, $barney = 8 pop @array; # ignore return value push(@array, 0); # (5, 6, 0) push(@array, 8); # (5, 6, 0, 8) push(@array, 1..10); # add 10 more elements @others = qw/ 9 0 2 1 0 /; push @array, @others; # 5 more elements to @array @array: 5 6 0 8 1 2 3 4 5 6 7 8 9 10 9 0 2 1 0 PLC - Perl Lists & Arrays (v2.00)

  12. List Assignment • Use unshift and shift to work on the start of the array. @array = qw/ dino fred barney /; $a = shift(@array); # $a=“dino”, @array has 2 elements $b = shift(@array); # $b = “fred”, @array has 1 element shift @array; # @array is now empty $c = shift @array; # $c gets undef, @array is still empty unshift(@array, 5); # @array has (5) unshift @array, 4; # @array has (4,5) @others = 1..3; unshift @array, @others; # array has (1,2,3,4,5) • Returns undef for an empty array variable PLC - Perl Lists & Arrays (v2.00)

  13. Interpolating Arrays into Strings • Arrays can be interpolated into double quoted strings • Elements of the array are automatically separated by the value of the special $” variable (default is space) $” = “, “; @rocks = qw{ flintstone slate rubble }; print “quartz @rocks limestone\n”; # output is: quartz flintstone, slate, rubble limestone • Be careful when building strings with the @ symbol $email=“nospam@cs.rit.edu” # error! PLC - Perl Lists & Arrays (v2.00)

  14. Interpolating Arrays into Strings • Is the index variable, $y, interpolated? • No, the index variable is interpreted as an ordinary expression @fred = qw\ hello dolly \; $y = “2*4”; print “This is $fred[$y-1]’s place”; • What happens here? @fred = qw/ eating rocks is wrong /; $fred = “right”; # want to say “this is right[3]”: print “this is $fred[3]\n”; #this is wrong print “this is ${fred}[3]\n”; #this is right[3] print “this is $fred\[3]\n”; #this is right[3] PLC - Perl Lists & Arrays (v2.00)

  15. Foreach Control Structure • Process an entire array in a list using foreach foreach $rock (qw/ bedrock slate lava /) { print “One rock is $rock.\n”; } • The control variable is the list element (not a copy) @rocks = qw/ bedrock slate lava /; $rock = “crush”; foreach $rock (@rocks) { $rock = “\t$rock”; $rock .= “\n”; } print “The rocks are:\n”, @rocks; # tab+each+newline print “\$rock=$rock\n”; # $rock=“crush” PLC - Perl Lists & Arrays (v2.00)

  16. Default Variable $_ • The default scalar variable is $_ foreach (1..10) { # uses $_ by default print “I can count to $_\n”; } • Used as default in many cases $_ = “Yabba dabba doo\n”; print; # prints $_ by default PLC - Perl Lists & Arrays (v2.00)

  17. Reverse and Sort • Use reverse to invert the order of a list @fred = 6..10; @barney = reverse(@fred); # 10,9,8,7,6 @wilma = reverse 6..10; # same @fred = reverse @fred; # store result back in @fred • Use sort to order a list of values by their ASCII character values: @rocks = qw / b s r g /; @barney = sort(@rocks); # b, g, r, s @back = reverse sort @rocks; # s, r, g, b @rocks = sort @rocks; # stores result back @numbers = sort 97..102; # 100,101,102,97,98,99 PLC - Perl Lists & Arrays (v2.00)

  18. Scalar and List Contexts • A given expression means different things depending on the context it is found in 5 + something; # something must be a scalar sort something; # something must be a list @people = qw/ fred barney betty /;# list context @sorted = sort @people; # list context $number = 5 + @people; # scalar context @list = @people; # list of 3 people $n = @people; # the number 3 PLC - Perl Lists & Arrays (v2.00)

  19. Scalar and List Contexts • Here are some more contexts: $fred = something; # scalar context @pebbles = something; # list context ($wilma, $betty) = something; # list context ($dino) = something; # list context PLC - Perl Lists & Arrays (v2.00)

  20. More Scalar and List Contexts #!perl @forwards = qw/ yabba dabba doo/; @backwards = reverse @forwards; print "Quoted forwards list: @forwards\n"; # yabba dabba doo print "Unquoted forwards list: " . @forwards . "\n";; # 3 print "\n"; print "Quoted backwards list: @backwards\n"; # doo dabba yabba print "Unquoted backwards list: " . @backwards . "\n";; # 3 print "\n"; @backwardsList = reverse @backwards; print "Quoted reversed backwardsList: " . "@backwardsList" . "\n"; # yabba dabba doo print "Unquoted reversed backwardsList: " . @backwardsList . "\n"; # 3 print "\n"; $backwardsScalar = reverse @backwards; print "Quoted reversed backwards scalar: " . "$backwardsScalar" . "\n"; # abbayabbadood print "Unquoted reversed backwardsScalar: " . $backwardsScalar . "\n"; # abbayabbadood PLC - Perl Lists & Arrays (v2.00)

  21. Scalar and List Contexts • What are these contexts? $fred[something] = something; push @fred, something; foreach $something1 (something2) { … } 123 + something; if (something) { … } print something; PLC - Perl Lists & Arrays (v2.00)

  22. Scalar and List Contexts • To force a scalar context when Perl is expecting a list, use the fake function scalar @rocks = qw/ talc quartz jade obsidian /; print “How many rocks do you have?\n”; print “I have “, @rocks, “ rocks!\n”; # incorrect print “I have “, scalar @rocks, “ rocks!\n”; # correct PLC - Perl Lists & Arrays (v2.00)

  23. Revision History • Revision History • v1.00, 10/10/2003 5:25 PM, sps Initial revision. -- v2.0, 1/9/2005, chr PLC - Perl Lists & Arrays (v2.00)

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