JBzh has asked for the wisdom of the Perl Monks concerning the following question:

Good afternoon. I'm learning perl, and confused by a simple code which prints :

Arch bits :
OS Arch x86_64 : i686
OS Arch bits 1 : 64
OS Arch bits: 64
Now using for/if
OS Arch x86_64 : i686
OS Arch bits 1 : 64

[download]

instead of :

Arch bits :
OS Arch x86_64 : i686
OS Arch bits 3 : 32
OS Arch bits: 32
Now using for/if
OS Arch x86_64 : i686
OS Arch bits 3 : 32

[download]

Code is as follow :

#!/usr/bin/perl

print "Arch bits : $os_arch_bits\n";
#$os_arch = `uname -m`;
#$os_arch =~ s/\n//g;
#$os_arch =~ s/\r//g;
$os_arch = 'i686';
if    ($os_arch == 'x86_64') {
        print "OS Arch x86_64 : $os_arch\n";
        $os_arch_bits = '64';
        print "OS Arch bits 1 : $os_arch_bits\n";
}
elsif ($os_arch == 'i386') {
        print "OS Arch i386 : $os_arch\n";
        $os_arch_bits = '32';
        print "OS Arch bits 2 : $os_arch_bits\n";
}
elsif ($os_arch == 'i686') {
        print "OS Arch 64 : $os_arch\n";
        $os_arch_bits = '32';
        print "OS Arch bits 3 : $os_arch_bits\n";
}
else {
        print "OS Arch 64 : $os_arch\n";
        $os_arch_bits = 'XX';
        print "OS Arch bits 4 : $os_arch_bits\n";
}

print "OS Arch bits: $os_arch_bits\n";

print "Now using for/if\n";
for ($os_arch) {
        if ("x86_64") {
                print "OS Arch x86_64 : $os_arch\n";
                $os_arch_bits = '64';
                print "OS Arch bits 1 : $os_arch_bits\n";
        }
        elsif ("i386") {
                print "OS Arch i386 : $os_arch\n";
                $os_arch_bits = '32';
                print "OS Arch bits 2 : $os_arch_bits\n";
        }
        elsif ("i686") {
                print "OS Arch 64 : $os_arch\n";
                $os_arch_bits = '32';
                print "OS Arch bits 3 : $os_arch_bits\n";
        }
        else {
                print "OS Arch 64 : $os_arch\n";
                $os_arch_bits = 'XX';
                print "OS Arch bits 4 : $os_arch_bits\n";
        }
}

[download]
Help will be appreciated :)

Replies are listed 'Best First'.
Re: simple "if.. elsif.. else.." statements, trying "for.. if.." too
by toolic (Bishop) on May 18, 2012 at 12:24 UTC
    Additionally, this: 
    for ($os_arch) {
        if ("x86_64") {
                print "OS Arch x86_64 : $os_arch\n";
                $os_arch_bits = '64';
                print "OS Arch bits 1 : $os_arch_bits\n";
        }

    [download]

    is the same as: 

    for ($os_arch) {
        if (1) {  # <------------------------------------------
                print "OS Arch x86_64 : $os_arch\n";
                $os_arch_bits = '64';
                print "OS Arch bits 1 : $os_arch_bits\n";
        }

    [download]

    Therefore, the 1st "if" will always be true. None of the "else" clauses will be evaluated. You don't need the for loop: 

    if ($os_arch eq "x86_64") {
        print "OS Arch x86_64 : $os_arch\n";
        $os_arch_bits = '64';
        print "OS Arch bits 1 : $os_arch_bits\n";
}
elsif ($os_arch eq "i386") {
        print "OS Arch i386 : $os_arch\n";
        $os_arch_bits = '32';
        print "OS Arch bits 2 : $os_arch_bits\n";
}
elsif ($os_arch eq "i686") {
        print "OS Arch 64 : $os_arch\n";
        $os_arch_bits = '32';
        print "OS Arch bits 3 : $os_arch_bits\n";
}
else {
        print "OS Arch 64 : $os_arch\n";
        $os_arch_bits = 'XX';
        print "OS Arch bits 4 : $os_arch_bits\n";
}

    [download]

    See also Switch statements

    use strict and warnings

[reply]
[d/l]
[select]
      I usually use switch when coding in other langages. I can't find the website were I read this strange for loop. Thank you, I'll search in the switch doc.
[reply]

        What you probably saw was: 

        for ($os_arch) {
        if (/x86_64/) {

        [download]

        Which is short for: 

        for my $os_type ($os_arch) {
        if ($os_type =~ m/x86_64/) {

        [download]
[reply]
[d/l]
[select]

        I imagine you wanted this:

        for ($os_arch) {
   when ("x86_64") {
      ...
   }
   ...
}

        [download]

        Or:

        given ($os_arch) {
   when ("x86_64") {
      ...
   }
   ...
}

        [download]
        perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
[reply]
[d/l]
[select]
Re: simple "if.. elsif.. else.." statements, trying "for.. if.." too
by Anonymous Monk on May 18, 2012 at 12:13 UTC
    Wrong operator, see perlop. Had you enabled warnings, then Perl would alert you to your mistake. 
        $os_arch eq 'x86_64'

    [download]
[reply]
[d/l]
      it works, thank you :)
[reply]
Re: simple "if.. elsif.. else.." statements, trying "for.. if.." too
by LanX (Saint) on May 18, 2012 at 23:35 UTC
[reply]

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