chen li wrote:
Hi all,Although I read the usage for grep and map using perldoc -f grep or map I don't think I really get the differences between them. I have a small data set: my @data=( [1,2,3], [4,5,6], [7,8,9] ); I want to reverse the elements in each row only but not rows themselves I would expect the following output: my @reverse=( [3,2,1], [6,5,4], [9,8,7] ); Here is the script I write. What I find is that the map function will give me the expected result but not the grep function. Any comments? Thanks, Li #!C:/Perl/bin/perl.exe use warnings; use strict; use Data::Dump qw(dump); my @data=( [1,2,3], [4,5,6], [7,8,9] ); my @reverse_map=map{[reverse @[EMAIL PROTECTED]; print "result of using map function\n",dump @reverse_map; my @reverse_grep=grep{[reverse @[EMAIL PROTECTED]; print "\n","result of using grep function\n",dump @reverse_grep; exit; ######output from screen####### result of using map function ([3, 2, 1], [6, 5, 4], [9, 8, 7]) result of using grep function ([1, 2, 3], [4, 5, 6], [7, 8, 9])
map and grep are very different things. What you want here is map, and you've used it exactly correctly, and you have a working program as evidence of this. Both map and grep take a rule and a list to work with, and apply that rule to each element of the list, but there the similarity ends. Think of map as a kind of list-to-list transform - a 'mapping' in fact. The result of map is the contents of the original list transformed by applying the rule to each element in turn. To double every element of a list, for instance, we could write map $_ * 2, (1, 2, 3) and map would return the list (2, 4, 6). grep, on the other hand, is best thought of as a list filter. Its result is all of the elements of the original list (unaltered) for which the rule evaluates to a true value. As an example, the expression below takes the numbers 1 to 10 and returns only the odd numbers in that list by using the remainder operator '%' to test each each element in turn. grep $_ % 2 == 1, (1 .. 10) Here, grep returns the list (1, 3, 5, 7, 9) In your own example, your call to grep used a rule which returned an anonymous array reference every time. Since any reference is a true value, the call simply passed through all of the elements of the original array. I hope this helps you to understand better. As food for thought consider this: I have said that map and grep are very different, but it is possible to implement the functionality of grep fairly simply using map; can you think how? As a clue, remember that map's rule - the first parameter - is evaluated in list context. Cheers, Rob -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] <http://learn.perl.org/> <http://learn.perl.org/first-response>
