Perl ternary errantly enters "else" clause?

Question

I have the following code:

# List of tests
my $tests = [("system_test_builtins_sin", "system_test_builtins_cos", "system_test_builtins_tan")];

# Provide overrides for certain variables that may be needed because of special cases
# For example, cos must be executed 100 times and sin only 5 times.
my %testOverrides = (
    system_test_builtins_sin => {
        reps => 5,
    },
    system_test_builtins_cos => {
        reps => 100,
    },
);

my %testDefaults = (
    system_test_reps => 10,
);

# Execute a system tests
foreach my $testName (@$tests)
{
    print "Executing $testName\n";
    my $reps;

    if (exists $testOverrides{$testName}{reps})
        { $reps = $testOverrides{$testName}{reps}; }
    else
        { $reps = $testDefaults{system_test_reps}; }
    print "After long if: $reps\n";
    exists $testOverrides{$testName}{reps} ? $reps = $testOverrides{$testName}{reps} : $reps = $testDefaults{system_test_reps};
    print "After first ternary: $reps\n";
    exists $testOverrides{$testName}{reps} ? $reps = $testOverrides{$testName}{reps} : print "Override not found.\n";
    print "After second ternary: $reps\n";
}

This gives the following output:

Executing system_test_builtins_sin
After long if: 5
After first ternary: 10
After second ternary: 5
Executing system_test_builtins_cos
After long if: 100
After first ternary: 10
After second ternary: 100
Executing system_test_builtins_tan
After long if: 10
After first ternary: 10
Override not found.
After second ternary: 10

This output is most unexpected! I don't understand why the first ternary seems to always be executing the "if false" clause. It is always assigning a value of 10. I also tried changing the "false" clause to $reps = 6, and I saw that it always got the value of 6. Why does the ternary's logic depend on the content of the third (if false) clause?

Answer 1

Here's a simpler script that illustrates the problem:

#!/usr/bin/perl

use strict;
use warnings;

my $x;

1 ? $x = 1 : $x = 0;
print "Without parentheses, \$x = $x\n";

1 ? ($x = 1) : ($x = 0);
print "With parentheses, \$x = $x\n";

It produces this output:

Without parentheses, $x = 0
With parentheses, $x = 1

I'm not sure that the relationship between assignment and ?: can be complete explained by operator precedence. (For example, I believe C and C++ can behave differently in some cases.)

Run perldoc perlop and search for "Conditional Operator", or look here; it covers this exact issue (more concisely than I did here).

In any case, I think that using an if/else statement would be clearer than using the ?: operator. Or, since both the "true" and "false" branches assign to the same variable, a better use of ?: would be to change this:

exists $testOverrides{$testName}{reps}
    ? $reps = $testOverrides{$testName}{reps}
    : $reps = $testDefaults{system_test_reps};

to this:

$reps = ( exists $testOverrides{$testName}{reps}
          ? testOverrides{$testName}{reps}
          : $testDefaults{system_test_reps} );

But again, the fact that I had to wrap the line to avoid scrolling is a good indication that an if/else would be clearer.

You might also consider using the // operator, unless you're stuck with an ancient version of Perl that doesn't support it. (It was introduced by Perl 5.10.) It's also known as the "defined-or" operator. This:

$x // $y

is equivalent to

defined($x) ? $x : $y

So you could write:

$reps = $testOverrides{$testName}{reps} // $testDefaults{system_test_reps};

This doesn't have exactly the same semantics, since it tests the expression using defined rather than exists; it will behave differently if $testOverrides{$testName}{reps} exists but has the value undef.