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Re^2: Replacing values in an array

by tonto (Pilgrim)
on Jan 26, 2013 at 23:54 UTC ( #1015550=note: print w/ replies, xml ) Need Help??


in reply to Re: Replacing values in an array
in thread Replacing values in an array

Brilliant!
Now I have three elegant solutions and a reason to study "map"!
Thank you!
-tonto


Comment on Re^2: Replacing values in an array
Re^3: Replacing values in an array
by muba (Priest) on Jan 27, 2013 at 05:37 UTC

    map and grep can be an intimidating functions, but quite useful once you understand them.

    Let's consider grep first, since this one is a little simpler to grasp — or at least, that was my experience. grep takes two arguments, the second being a list of elements to work with, the first being the work that you want to have done on that list. You can specify that as a BLOCK, a code reference, or just the name of a function. grep then loops over the list, aliasing $_ to each element in turn, and calls the given piece of code. Then it returns every element for which the code returned a true value.

    my @numbers = (0, 0.5, 1, 1.5, 2, 2.5); # Calling grep with a BLOCK: my @integers = grep {int($_) == $_} @numbers; print join(", ", @integers), " are integers.\n"; # Calling grep with a function name: my @basket = ("apple", undef, undef, undef, "banana", "cherry", undef, + "date"); print "The number of elements in \@basket is ", scalar(@basket), "\n"; my @basket_1 = grep defined, @basket; print "The number of *defined* elements in \@basket is ", scalar(@bask +et_1), "\n";

    See what happens? grep returns the elements of the list you gave it, for which the piece of code returns true. The non-grep equivalents would be:

    my @numbers = (0, 0.5, 1, 1.5, 2, 2.5); # Calling grep with a BLOCK: my @integers; for (@numbers) { push @integers, $_ if int($_) == $_; } print join(", ", @integers), " are integers.\n"; # Calling grep with a function name: my @basket = ("apple", undef, undef, undef, "banana", "cherry", undef, + "date"); print "The number of elements in \@basket is ", scalar(@basket), "\n"; my @basket_1; for (@basket) { push @basket_1, $_ if defined; } print "The number of *defined* elements in \@basket is ", scalar(@bask +et_1), "\n";

    Now, map is pretty similar, except that it allows you to change the elements:

    my @numbers = 1..10; my @times_ten = map { $_ * 10 } @numbers; print join(", ", @times_ten), "\n";

    Of course, these are just the basics — the range of things you can do with them is astonishing. I hope this helps you along.

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