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Loops and variable usage

by Anonymous Monk
on Mar 23, 2012 at 18:09 UTC ( #961275=perlquestion: print w/ replies, xml ) Need Help??
Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi all,

After working with R and other languages I have to do some work in Perl and it's giving me a headache.. I pass 3 parameters to my subroutine. Depending on the variable the script can change their value and the final section prints a custom XML script. But no matter what, the script will not work because of the local/global variables. How can I use the variable within a loop only when the 'if' condition is met and if not then use the pre-defined one?</p)

sub ddd { a = $_[0]; b = $_[1]; c = $_[2]; if ($a == '1') { $a = 'p'; } elsif ($a = '2') { $a = 'pp'; } if ($b == '3') { $b = 'w'} print v " < $a > < $b > < $c >"; }

Probably not the most elegant code but since there is so many variables I cannot make too many loops and start adding more variables as the code will grow too much. Any other way to do it?

Thanks for all your help.

Tom

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Re: Loops and variable usage
by Eliya (Vicar) on Mar 23, 2012 at 18:34 UTC

    I don't see any loop, but I do see a couple of things which most likely don't do what you intended.  Maybe retry with this:

    sub ddd { my $a = $_[0]; my $b = $_[1]; my $c = $_[2]; # could also be written as: # my ($a, $b, $c) = @_; if ($a == 1) { $a = 'p'; } elsif ($a == 2) { $a = 'pp'; } if ($b == 3) { $b = 'w'} print " < $a > < $b > < $c >"; }
      Sorry, the loop is on upper level I tried to make it simpler. I have tried different combinations with my and none worked. for example $a is always 'p' even if the $_[0] was '2'. the loop is feeding the subroutine the proper values.
Re: Loops and variable usage
by apprentice (Scribe) on Mar 23, 2012 at 18:44 UTC
    First, you have bad syntax in your elsif conditional, a single = which assigns $a the value of 2. It should be ==.

    I reformatted your code to be more readable, fixed the ==, added 'my' to the subroutine vars, removed the 'v' filehandle so I could just print to the screen, seems to work for me based on your logic. Here is the code (always 'use strict;'!).

    #!/usr/bin/perl -w use strict; $| = 1; my $a = 1; my $b = 1; my $c = 1; print "Passing in \$a: $a, \$b: $b, \$c: $c: expect p 1 1\n"; ddd($a,$b,$c); $a = 2; $b = 2; $c = 2; print "Passing in \$a: $a, \$b: $b, \$c: $c: expect pp 2 2\n"; ddd($a,$b,$c); $a = 3; $b = 3; $c = 3; print "Passing in \$a: $a, \$b: $b, \$c: $c: expect 3 w 3\n"; ddd($a,$b,$c); sub ddd { my $a = $_[0]; my $b = $_[1]; my $c = $_[2]; if ($a == '1') { $a = 'p'; } elsif ($a == '2') { $a = 'pp'; } if ($b == '3') { $b = 'w' } print " < $a > < $b > < $c > \n"; }
    and here is the output:
    #> ./test-subroutine.pl Passing in $a: 1, $b: 1, $c: 1: expect p 1 1 < p > < 1 > < 1 > Passing in $a: 2, $b: 2, $c: 2: expect pp 2 2 < pp > < 2 > < 2 > Passing in $a: 3, $b: 3, $c: 3: expect 3 w 3 < 3 > < w > < 3 > #>


    "Peace, love, and Perl...well, okay, mostly just Perl!" --me

    Apprentice
      This script was just an example not proper code, but thanks very much for all help. I just figured out that I used '==' to compare strings...

        Actually, you use 'eq' for strings and '==' for numbers.

        ...roboticus

        When your only tool is a hammer, all problems look like your thumb.

Re: Loops and variable usage
by JavaFan (Canon) on Mar 23, 2012 at 19:18 UTC
    It seems like your code is equivalent to:
    sub ddd { my $a = $_[0] == 1 ? 'p' : $_[0] == 2 ? 'pp' : $_[0]; my $b = $_[1] == 3 ? 'w' : $_[1]; my $c = $_[2]; print v " < $a > < $b > < $c >"; }
    Having said that, I don't which problem you are trying to solve. I don't see any loops.
      I find nested ternaries are much clearer using the code layout suggested by the book "Perl Best Practices"
      sub ddd { # code # value my $a = $_[0] == 1 ? 'p' : $_[0] == 2 ? 'pp' : $_[0] ; my $b = $_[1] == 3 ? 'w' : $_[1] ; my $c = $_[2]; print v " < $a > < $b > < $c >"; }
Re: Loops and variable usage
by tobyink (Abbot) on Mar 23, 2012 at 22:34 UTC

    Do not use variables named $a and $b.

    Perl has predefined variables named $a and $b which are used for sort. It is possible to use them for other purposes (List::Util does), but only via a lot of careful thought (or a lot of luck).

    perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

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