Question

In Perl, I can do:

my @unit_indices = sort { 
    $units{$b}[0] <=> $units{$a}[0] 
        or
    $a cmp $b
} keys %units;

which sorts by one field (array element) descending and the other (hash key) ascending, but causes perlcritic to complain:

Forbid $b before $a in sort blocks at line X, column Y. See page 152 of PBP. (Severity: 1)

Perl Best Practices recommends using reverse instead.

But the operation would be much more comprehensible if you wrote:
@sorted_results = reverse sort @unsorted_results;

However, I haven’t found a way to have subsorts that run in opposite directions.

Obviously, I can tell perlcritic to ignore this, but I’d like to know how to accomplish what I need and make perlcritic happy.

Answer 1

I agree with tchrist’s comment, but taking your question at face value:

1) use ## no critic

my @unit_indices = sort {   ## no critic (ReverseSortBlock)
    $units{$b}[0] <=> $units{$a}[0] 
        or
    $a cmp $b
} keys %units;

2) Use a .perlcriticrc file

[-BuiltinFunctions::ProhibitReverseSortBlock]

3) Change the sense of your comparison

my @unit_indices = sort { 
    -($units{$a}[0] <=> $units{$b}[0])
        or
    $a cmp $b
} keys %units;

my @unit_indices = sort { 
    -$units{$a}[0] <=> -$units{$b}[0])
        or
    $a cmp $b
} keys %units;