Re: How do I un-map this code?

by dragonchild (Archbishop)
 on Mar 21, 2008 at 18:19 UTC ( #675494=note: print w/ replies, xml ) Need Help??

in reply to How do I un-map this code?

I have also tried to avoid the use of map because I really can never understand what it is doing

I translate this statement as "I'm scared, so I refuse to address my fear." If you really don't want to address your fear, stop now. I plan on getting you to understand map so that you never have to worry again.

map, grep, and sort all operate the exact same way. They take a list of stuff and return a list of stuff.

• sort takes the list and sorts it, returning back every item that was in the original list, but possibly in a different place.
• grep takes the list and applies a function to each item. If the function returns true, that item is in the result list. If the function returns false, that item is skipped. The items that make it are in the same order as they were originally.
• map takes the list and applies a function to each item. It takes the return value from the function and puts it into the result list. The items are returned in the same order.

So, an example:

```my @l = ( 5, 4, 3, 2, 1 );

my @sorted  = sort { \$a <=> \$b } @l; # Sorted is ( 1, 2, 3, 4, 5 )
my @grepped = grep { \$_ > 2 } @l;    # Grepped is ( 5, 4, 3 )
my @mapped  = map  { \$_ * 2 } @l;    @ Mapped is ( 10, 8, 6, 4, 2 )

Now, the biggest problem people usually have with map and grep (though, funnily enough, not sort) is that you have to read them from right to left. This is in direct opposition to how you read everything else (which is from left to right). Perl 6 will provide a mechanism where you can have map, grep, and other list operators that can be read from left to right. It will look something like:

```my @l = ( 5, 4, 3, 2, 1 );

@l ==> grep { \$_ > 2 }
==> sort { \$a <=> \$b }
==> map  { \$_ * 2 }
==> @final;

# Final contains ( 6, 8, 10 )

Oh, and fearing unless is just plain old laziness - the bad kind. unless(...) is exactly identical to if(!(...)). Nothing less, nothing more. Feel free to revise as desired.

My criteria for good software:
1. Does it work?
2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

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