Hi guys,

A friend of mine had just asked an interesting question about increment, decrement operators which led to my query.

What should be the output of 
perl -le '$a = 10; $a++ + $a--'

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It should be 21 instead of 20 because if I were to think how the calculation would be done - convert to postfix, push operands on stack till operator is encountered, when operator encountered, pop out 2 operands and perform operation and push result onto stack:

  1. Push value of $a - 10 on stack
  2. Increment $a by 1 - 11
  3. Push value of $a - 11 on stack
  4. Increment $a by 1 - 12
  5. Pop out 10, 11 perform addition

A similar explanation can be done for: 

perl -le '$a = 10; $a++ + $a--'

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perl -le '$a = 10; $a++ + $a-- + $a-- + $a--'

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Can you guys please tell me how would the postfix calculation progress in case of 

perl -le '$a = 10; print $a++ + ++$a;'

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and 
perl -le '$a = 10; print ++$a + $a++;'

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I don't know why the result of ++$a + $a++ turned out to be 23.

Thanks for going through,

In reply to Prefix and postfix increment/decrement operator query by saurabh.hirani

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