--- Rob Dixon <[EMAIL PROTECTED]> wrote:
> >
> >
> > #!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
Thanks you the detailed explanation and good examples.
They really help me out.
Li
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