Loops and variable usage

[Anonymous Monk]

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

[Eliya]

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 >";
}

[Anonymous Monk]

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.

[apprentice]

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 > 
#>

[Anonymous Monk]

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...

[roboticus]

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

...roboticus

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

[tobyink]

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).

[JavaFan]

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.

[BillKSmith]

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 >";
}