What should If I want to check whether my union is list or string in perl

Question

I need to write an union function using Perl, and what I did is here, questions is followed by code.

sub union
{
    my @array1  = qw/a b c d/;
    my @array2  = qw/a f g h/;
    my %myunion = ();
    @myunion{ @array1, @array2 } = (1) x ( @array1 + @array2 );
    @myunion = keys %myunion;
    return @myunion;

    #if(){
    #    return @myunion;
    #}
    #else{
    #    return join ',', @myunion;
    #}
}
my @uString = union( [ 1, 2, 3 ], [ 2, 3, 4 ] );
my @uList   = union( [ 1, 2, 3 ], [ 2, 3, 4 ] );
print "$uString\n" 
print "@uList\n";

So without my command part, my out put is a b c d f g h in random order, but I want to make them as 1 2 3 4 in random order as what I write as input in my @uList. Also I need to check whether the caller requires a list, if it is it will return 1 2 3 4in random order, otherwise, it will return a comman-seperate string of the union which should be 1,2,3,4 in random order. So I want to know what should I do in the condition part of the if else statement.

Are you looking for [wantarray](https://perldoc.perl.org/functions/wantarray.html) function? — PerlDuck, Dec 09 '17 at 18:53
If you want the output to reflect the contents of the parameters, then you must use the values passed instead of the hard-coded arrays `@array1` and `@array2` — Borodin, Dec 09 '17 at 18:53
`$uString` and `@uString` are different variables. Also, your sub doesn't read the parameters. — choroba, Dec 09 '17 at 18:59
The line `my @uString = union(…)` imposes _list context_ because `@uString` is an array! — PerlDuck, Dec 09 '17 at 19:09
@nicken Apparently you are looking for `wantarray`. In this case you _must not_ ignore `@uString` vs. `$uString` because that's exactly what triggers `wantarray`. Please [edit] your question. — PerlDuck, Dec 10 '17 at 17:30

score 2 · Answer 1 · edited Dec 09 '17 at 20:47

2

Use the @_ variable to retrieve subroutine parameters.

To create the hash keys, you can let the values undefined.

#! /usr/bin/perl
use warnings;
use strict;

sub union {
    my ($aref1, $aref2) = @_;
    my %union;
    @union{ @$aref1, @$aref2 } = ();
    return keys %union
}

my @union = union([1, 2, 3], [2, 3, 4]);
print "@union\n";

or

my $union = join ',', union([1, 2, 3], [2, 3, 4]);
print "$union\n";

edited Dec 09 '17 at 20:47

ikegami

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answered Dec 09 '17 at 19:01

choroba

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so what if I call $uString and @uList, it will show different answer, as I mentioned before, if I call uString it will show 1,2,3,4 if I call uList it will show 1 2 3 4, one with comma, one not – nicken Dec 09 '17 at 20:39
@nicken, I made a change to the answer that shows how you can get `1,2,3,4` when that's what you want. – ikegami Dec 09 '17 at 20:47
@ikegami I know that way, but what if I call my $uString = union([1,2,3],[2,3,4]); then it will reaturn 1,2,3,4 ,so I think I need if else statement to chck whther it call is string or list. – nicken Dec 09 '17 at 20:58
@nicken: When I do `$uString = union(...)`, I'm getting the number of elements back, not `1,2,3,4`. That's what [keys](http://p3rl.org/keys) returns in scalar context. – choroba Dec 09 '17 at 21:16
@choroba, The OP asking how to write `union` so its result is based on context. – ikegami Dec 09 '17 at 21:44
@nicken, The bit you are missing is `wantarray`, but it's a bad idea to use it here. Finding the union of sets and formatting numbers for output are two unrelated tasks. Consult the answer to see how you can get `1,2,3,4` when that's what you want.1 – ikegami Dec 09 '17 at 21:46

What should If I want to check whether my union is list or string in perl

1 Answers1