Strange "Useless use of constant" message: should not appear at all!

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

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Re: Strange "Useless use of constant" message: should not appear at all! by japhy (Canon) on May 10, 2004 at 21:45 UTC
You can't do array comparisons like that. You're getting the warning because `@x == (1,2)` is evaluating `(1,2)` in scalar context. The comma operator, in scalar context, evaluates its left-hand operand and discards its value, and returns its right-hand operand. So the 1 is useless. Perhaps you want: `if ("@movement" eq "1 0") { ... } elsif ("@movement" eq "0 1") { ... } # etc.` [download] _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area) `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l] [select]
Re: Strange "Useless use of constant" message: should not appear at all! by blokhead (Monsignor) on May 10, 2004 at 21:49 UTC
`@movements == (0, 1);` [download] You can't compare arrays/lists for equality directly. The == and `eq` operators work on scalars, so `@movement` and `(0, 1)` are both being evaluated in scalar context. Of course, `(0, 1)` in scalar context simply throws away the 0, which is the source of the error. If you want to compare these arrays for equality, since they are so simple, you can stringify them: `local $" = ","; if ("@movement" eq "0,1") { ... } elsif ("@movement" eq "1,0") { ... } ...` [download] Or a more general comment, even use a lookup table: `local $" = ","; my %dir = ( "0,1" => "north", "1,0" => "east", "0,-1" => "south", "-1,0" => "west" ); $from = $dir{"@movement"};` [download] blokhead	[reply] [d/l] [select]
Re: Strange "Useless use of constant" message: should not appear at all! by Juerd (Abbot) on May 10, 2004 at 22:45 UTC
Others have pointed out what is wrong with your code. Some have suggested interpolating the array, which I think is a good strategy here. However, I haven't seen ?: in this thread yet. Which is strange, as all this if/elsif/... thing seems to do is assign to $from. `my $from = "@movement" eq '0 1' ? 'north' : "@movement" eq '1 0' ? 'east' : "@movement" eq '0 -1' ? 'south' : "@movement" eq '-1 0' ? 'west' : die "Unknown direction: (@movement)";` [download] I probably wouldn't use a lookup hash here. Redundancy's not so bad with only 4 items. Juerd # { site => 'juerd.nl', plp_site => 'plp.juerd.nl', do_not_use => 'spamtrap' }	[reply] [d/l]
Re: Strange "Useless use of constant" message: should not appear at all! by Joost (Canon) on May 10, 2004 at 21:52 UTC
Please note that this: `if (@movement == (0, 1)) {$from = "north"}` [download] is equivalent to `if (@movement == ($any_other_value, 1)) {$from = "north"}` [download] ~~Because it only tests the equivalence between the last element of the list and the array.~~ update: no it doesn't; see above. You probably want something like `my $from = ""; if ($movement[0] == 0 && $movement[1] == 1) {$from = "north"} elsif ($movement[0] == 1 && $movement[1] == 0) {$from = "east"} elsif ($movement[0] == 0 && $movement[1] == -1) {$from = "south"} elsif ($movement[0] == -1 && $movement[1] == 0) {$from = "west"}` [download] This also solves your warning by the way. Hope this helps Joost "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: Strange "Useless use of constant" message: should not appear at all! by cLive ;-) (Prior) on May 10, 2004 at 21:50 UTC
Perhaps you should run this: `my @movement = (0, 1); my $from = ""; if (@movement == (0, 1)) {$from = "north"} elsif (@movement == (1, 0)) {$from = "east"} elsif (@movement == (0, -1)) {$from = "south"} elsif (@movement == (-1, 0)) {$from = "west"} print "$from\n";` [download] Then read up on == working on numerical values :) cLive ;-)	[reply] [d/l]
Re: Strange "Useless use of constant" message: should not appear at all! by Sandy (Curate) on May 10, 2004 at 21:53 UTC
Your code is not a functioning programming. Within the `if` statement, `@movement` is evaluated in scalar context, and consequently gives a number that is the size of the array. `#!/usr/bin/perl -w my @movement = (0,1); my $from = ""; if (@movement == 2) {$from = "north"} elsif (@movement == (1, 0)) {$from = "east"} elsif (@movement == (0, -1)) {$from = "south"} # elsif (@movement == (-1, 0)) {$from = "west"} print "\$from = <$from>\n";` [download] gives "north" as the answer. Similarly, the list, (not array) of `(0,1)` returns the last thing in the list. The 'useless use of number in void context' happens when numbers in the list (not the last one) are thrown away. For some reason this doesn't happen if the number thrown away is a 'zero'. Example 1 `my @movement = (0,1); my $from = ""; if (5 == (3,4,5)) {$from = "north"} elsif (@movement == (1, 0)) {$from = "east"} elsif (@movement == (0, -1)) {$from = "south"} # elsif (@movement == (-1, 0)) {$from = "west"} print "\$from = <$from>\n";` [download] gives `Useless use of a constant in void context at ./t.pl line 4. Useless use of a constant in void context at ./t.pl line 4. $from = <north>` [download] Example 2 `#!/usr/bin/perl -w my @movement = (0,1); my $from = ""; if (5 == (0,0,5)) {$from = "north"} elsif (@movement == (1, 0)) {$from = "east"} elsif (@movement == (0, -1)) {$from = "south"} # elsif (@movement == (-1, 0)) {$from = "west"} print "\$from = <$from>\n";` [download] `$from = <north>` [download] I'm not quite sure why the `(@movement == (1,0))` doesn't give a warning, but the `(@movement == (-1,0))` does. Example 3 `#!/usr/bin/perl -w my @movement = (0,1); my $from = ""; if (@movement == (1, 0)) {$from = "east"} if (@movement == (-1, 0)) {$from = "west"} print "\$from = <$from>\n";` [download] Gives `Useless use of a constant in void context at ./t.pl line 5. $from = <>` [download] Sandy UPDATE: Looking at a previous post, I thought that I misunderstood how @movement would be read within the if statement. Does it compare the last element of @movement with the list within the if block. This little test says no, unless I am missing somehting? `#!/usr/bin/perl -w my @movement = (0,1); my $from = ""; if (@movement == (0,1)) {$from = "east"} print "\$from = <$from>\n";` [download] Gives `$from = <>` [download]	[reply] [d/l] [select]
Re: Strange "Useless use of constant" message: should not appear at all! by tinita (Parson) on May 11, 2004 at 10:15 UTC
others have addressed your problem, however, the reason for the warning only showing up at the last of the elsif's can be shortened to this: `perl -wle'-2; -1; 0; 1; 2;' Useless use of a constant in void context at -e line 1. Useless use of a constant in void context at -e line 2. Useless use of a constant in void context at -e line 5.` [download] 1 or 0 in void context aren't treated as useless. that's why you can write `1;` at the end of a module, and, uhm, i can't think of an example for the zero at the moment...	[reply] [d/l] [select]
Re: Strange "Useless use of constant" message: should not appear at all! by neniro (Priest) on May 10, 2004 at 21:40 UTC
~~A small change and it's okay:~~	[reply]
Re: Re: Strange "Useless use of constant" message: should not appear at all! by muba (Priest) on May 10, 2004 at 21:45 UTC
Hmm, I first ++ voted your message after testing it and your small change, which you have now deleted, seems to work. Strange, but thank you! Update: Well, it did at first sight.	[reply]
Re: Re: Re: Strange "Useless use of constant" message: should not appear at all! by neniro (Priest) on May 10, 2004 at 22:05 UTC
Sorry, but it seems that you can't compare arrays directly to lists (thats why I removed my original post). Meantime there are a lot of good suggestions below. Using a lookup-hash is a fine idea.	[reply]