Condition in ternary operator doesn't cause any change

Question

Here is my code sample:

for $i(1..100){
   if ($i%15==0){$s="Divisible by 15"}
   elsif($i%5==0){$s="Divisible by 5"}
   else {$i%3==0 ? $s="Divisible by 3" : $s=$i};
   print $s."\n"}

This displays partial correct results, if a number if divisible by 3, it displays the "number" and not "Divisible by 3".

Example of output:

1
2
3
4
Divisible by 5
6
7
8
9
Divisible by 5

PS: I have to write this code in the minimum no. of characters possible. (The reason why the code is so sluggish)

Duplicate: http://stackoverflow.com/questions/14650715/using-ternary-operator-while-printing-in-perl — squiguy, Feb 01 '13 at 17:13
@squiguy: that started out as a different question; this question was added later, but I've rolled it back now — ysth, Feb 01 '13 at 20:45

score 3 · Answer 1 · answered Feb 01 '13 at 17:11

The fourth line is parsed as

(((($i % 3) == 0) ? ($s = 'Divisible by 3') : $s) = $i)

meaning that even when $i is divisible by 3, the assignment looks like

($s = 'Divisible by 3') = $i

Some fixes:

$i%3==0 ? ($s="Divisible by 3") : ($s=$i)
$s = ($i%3==0 ? "Divisible by 3" : $i)

Pro-tip: B::Deparse is very helpful for figuring this stuff out. I ran the command

perl -MO=Deparse,-p -e '$i%3==0 ? $s="Divisible by 3" : $s=$i'

to see exactly what the problem was.

score 0 · Accepted Answer · answered Feb 01 '13 at 17:10

The ternary ?: has higher precedence than assignment. Use parens to sort that out:

for my $i (1 .. 100) {
   my $s = "Divisible by ";
     $i%15==0 ? ($s .= 15)
   : $i%5==0  ? ($s .= 5)
   : $i%3==0  ? ($s .= 3)
   :            ($s  =$i);
   print "$s\n";
}

Output:

1
2
Divisible by 3
4
Divisible by 5
Divisible by 3
7
8
Divisible by 3
Divisible by 5

AndyClaw · Answer 3 · 2013-02-01T20:51:08.360

-1

If you want a version with less characters,

for $i(1..100){print(($i%3&&$i%5?$i:"Divisible by ".($i%5?3:$i%15?5:15))."\n");}

Parenthesis after the print function means call the print function, not group the expression. Caused by the fact that parenthesis are optional for that function.

edited Feb 01 '13 at 20:51

answered Feb 01 '13 at 17:44

AndyClaw

740
8
15

1

I don't think `print (...)."\n"` does what you think it does. – mob Feb 01 '13 at 17:47
@mob: Yes, you are right. But how will I achieve the new line? – xan Feb 01 '13 at 17:58
@mob and xan I edited the answer to print the newline. I hope you are learning how perl apparently doesn't have the order of operations that you and I are guessing. – AndyClaw Feb 01 '13 at 20:47

Condition in ternary operator doesn't cause any change

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