print command runs after call to subroutine

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

Im still learning so this is a very low-level question. I just would like to understand what's going on here please. here is my code:

#! /usr/bin/perl
 
use strict;
use warnings;
use 5.14.0;
 
sub display
{       
    foreach(@_)
    {       
        print "$_\n"
        }

}
 
my @numberSet = 1..10;
 
print "\n show all the elements!\n". &display(@numberSet) ;
[download]

as you can you see, I want the output to print "show all the elements" and then display all the elements of the array using the subroutine. But the subroutine runs first and then the print happens. Why is this ?

1
2
3
4
5
6
7
8
9
10

 show all the elements!
[download]

Comment on print command runs after call to subroutine Select or Download Code

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Re: print command runs after call to subroutine by Corion (Patriarch) on Oct 19, 2016 at 12:32 UTC
Because the evaluation order is different from the order in your source code. To construct the argument passed to your final `print` line, Perl needs to run the `display` subroutine to get at its return value. The `display` subroutine prints out some stuff and returns nothing. This result is then appended to the final `print` statement. If you want a different order of execution, consider using one `print` statement and a simple call to `display`, or even better, consider making `display` return a string instead of printing it. `sub display { my( $result ) = ''; foreach( @_ ) { $result = $result . "$_\n"; }: return $result }` [download] Also, your style of calling `display` is somewhat fraught with dangers. I recommend using the more common style of simply using the subroutine name with parentheses instead: `display( 1..10 );` [download] Using `&display()` brings you close to the danger of using `&display;`, which has the subtle side-effect of using the (parent callers) `@_` as arguments instead of passing no arguments.	[reply] [d/l] [select]
Re^2: print command runs after call to subroutine by Fuhrer_of_Flapjacks (Initiate) on Oct 19, 2016 at 12:54 UTC
The alternative you suggest for the display subroutine makes sense. So I guess it's good practice to always have a sub return a value and have that display in the "main" portion of the code rather than print anything within the sub itself (unless of course there is a specific need for that to happen)? As for your other recommendations, to address those, I was just trying to write a sub that can accept any number of parameters rather than a fixed amount. I would imagine that in a more concrete scenario, a fixed number more likely be used. Thank you for the help.	[reply]
Re^3: print command runs after call to subroutine by Corion (Patriarch) on Oct 19, 2016 at 13:12 UTC
Personally, I've found that in most situations I don't want subroutines to print out stuff, because having them print stuff makes it harder to suppress that output later, when rewriting the script. Perl does not enforce the number of parameters anyway, so you can always write the following: `sub display { ... }; display(1,2); display(1,2,3,4);` [download]	[reply] [d/l]