xenoterracide: Teaching Perl - Week 2 ( part2 )

Apr 25, 2010

Teaching Perl - Week 2 ( part2 )

This part2 was prompted by Chromatic's post on state. I'd never heard of state before, and it's documentation is poor. Let's take a look at how state could affect our game.

use strict; use warnings; use feature qw( say state switch ); say 'welcome'; # generate the winning number between 1 and 10 # see perlfaq4 for algorithm details my $winning_num = 1 + int( rand( (10-1)+1 ) ); until ( $winning_num == state $guess ) { say 'Guess a number between 1 and 10: '; $guess = readline(*STDIN); # check to see if we have a winner, or the guess is to high, or low. given( $guess ) { when ( $_ > $winning_num ) { say 'Too high'; } when ( $_ < $winning_num ) { say 'Too low'; } when ( $winning_num ) { say 'You Win!'; } } } say 'Game over!';

As you can see this allows us to make our $guess variable lexically scoped, meaning it's contained within the loop. This however generates some warnings: Use of uninitialized value $guess in numeric eq (==) at ./game.pl line 14. and if you enter non-numeric input Argument "l\n" isn't numeric in numeric eq (==) at ./game.pl line 14, line 1.. Both are completely fixable, but lets fix the uninitialized first.

use strict; use warnings; use feature qw( say state switch ); say 'welcome'; # generate the winning number between 1 and 10 # see perlfaq4 for algorithm details my $winning_num = 1 + int( rand( (10-1)+1 ) ); until ( $winning_num == ( state $guess = 0 ) ) { say 'Guess a number between 1 and 10: '; $guess = readline(*STDIN); # check to see if we have a winner, or the guess is to high, or low. given( $guess ) { when ( $_ > $winning_num ) { say 'Too high'; } when ( $_ < $winning_num ) { say 'Too low'; } when ( $winning_num ) { say 'You Win!'; } } } say 'Game over!';

So all we've had to change is adding parenthesis around state and assign it to a value. If we had used a my declaration it would reassign to 0 every time and we'd never match the $winning_num. But since state will only ever initialize once we don't have that problem. We still have our problem if we enter a character though, let's sanitize that input.

use strict; use warnings; use feature qw( say state switch ); say 'welcome'; # generate the winning number between 1 and 10 # see perlfaq4 for algorithm details my $winning_num = 1 + int( rand( (10-1)+1 ) ); until ( $winning_num == ( state $guess = 0 ) ) { say 'Guess a number between 1 and 10: '; $guess = readline(*STDIN); # check to see if we have a winner, or the guess is to high, or low. given( $guess ) { # sanitize input # this must be first otherwise it won't work right when ( $_ !~ /[[:digit:]]/ ) { say 'not a number'; # set $guess to 0 so we don't have a numerical comparison warning $guess = 0; } when ( $_ > $winning_num ) { say 'Too high'; } when ( $_ < $winning_num ) { say 'Too low'; } when ( $winning_num ) { say 'You Win!'; } } } say 'Game over!';

So in order to fix the 'not a numerical comparison' issue we check $guess to see if it's a digit with a regular expression. If it isn't a digit we set it to 0 which is an invalid guess anyways, but fixes the warning. This actually fixes two bugs. Before if you entered a letter it would tell you that it was too low (which I don't actually understand, because aren't characters > numbers? if someone on Iron Man could explain it in a comment.)

2 comments:

Ruslan U. ZakirovApril 26, 2010 11:16 PM
Don't you think it's more natural to use loop with post-loop exit condition? Code will look much nicer and more natural.
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xenoterracideApril 27, 2010 1:41 AM
@Ruslan it looks nicer... (playing with that now) but I can't seem to get $guess initialized within the loop. which is the only point of this newer version. I think if you use a do ... while loop $guess must be initialized outside the loop.

I think maybe it might look a bit nicer if I broke the conditions up onto multiple lines too.
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Apr 25, 2010

Teaching Perl - Week 2 ( part2 )

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