Question

I am somewhat new to Perl, and this bug has puzzled me for a couple of days. I’ve had difficulty finding anything on Google for a problem this specific. I will try to present the “clues” as clearly as possible. I am using Perl v5.16.1. The relevant lines in my code are these:

my %result = ();

...

$result{'TABLENAME'} = $tableName;

...

for my $i (1..$numberOfColumns) {
    $result{$columnNames[$i-1]} = $columnValues[$i-1];
}

In my test, $numberOfColumns is 7. The problem is that the for-loop does not create the key-value pairs as I expect. I will explain. I’ve tried debugging using Perl’s debugger with perl -d. The debugger output below shows things going fine until the last line.

  DB<2> c 219
testcode::testsub(modules/testcode.pm:219):
219:        $result{'TABLENAME'} = $tableName;

  DB<3> c 239
testcode::testsub(modules/testcode.pm:239):
239:        for my $i (1..$numberOfColumns) {

  DB<4> p %result
TABLENAMEmyowntableitis

  DB<5> p $result{TABLENAME}
myowntableitis

  DB<6> s
testcode::testsub(modules/testcode.pm:240):
240:            $result{$columnNames[$i-1]} = $columnValues[$i-1];

  DB<6> p $i
1

  DB<7> p $columnNames[0]
id

  DB<8> p $columnValues[0]
1

  DB<9> s
testcode::testsub(modules/testcode.pm:240):
240:            $result{$columnNames[$i-1]} = $columnValues[$i-1];

  DB<9> p $i
2

  DB<10> p %result
TABLENAMEmyowntableitisid
1

  DB<11> p $result{TABLENAME}
    myowntableitis

  DB<12> p $result{id}

  DB<13>

I expected the last p $result{id} to return 1 instead of nothing. Does anyone have an idea what could be happening here?

Answer 1

If we look at DB<10>, we see that the 1 is printed on the next line:

DB<10> p %result
TABLENAMEmyowntableitisid
1

This means that $result{"id\n"} eq "1", and you are using an undefined key.

A style note: foreaching over two arrays to build a hash is possible, of course. But then again, Perl has slices, which makes life much easier (unless you are processing incredible large amounts of data)

@result{@columnNames} = @columnValues;

(assuming @columnNames <= @columnValues)