postincrement in printf function

Question

The following code prints 7. It increments value of a, but b remains unaffected. Post increment operator is not supposed to increment first and then use the value which it has done in this case. It should be the other way around. Why this change in behaviour? Also, and perhaps more importantly, why is b not pre-incremented?

int  main() {

  int a=5, b=2;
  printf("%d", a+++b);

}

Another (better?) duplicate is [What does the operation c=a+++b mean?](http://stackoverflow.com/questions/7485088/what-does-the-operation-c-ab-mean) — Yu Hao, Dec 18 '14 at 14:02

Gopi · Answer 1 · 2014-12-18T14:07:41.333

1

You have a well defined behavior. Where

a++ + b = 5 +2 = 7

If you have a pre-incrementor operator like

printf("%d\n",++a+b);

Then the output would be 8

The ++ operator has higher precendence than the + unary plus operator so the evaluation from left to right of

a+++b will happen

a++ + b which will fetch you 7

Post-incrementation is done by taking the value of your variable first which in this case is 5 so

a= 5 and b=2

edited Dec 18 '14 at 14:07

answered Dec 18 '14 at 13:59

Gopi

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how do you know it is not `a + ++b` ? – Ahmad Dec 18 '14 at 14:00
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@Ahmad because "++" is an operator too then parser must prefer "++" over "+" then it's a++ + b – Adriano Repetti Dec 18 '14 at 14:03
@Ahmad because, compiler is _greedy_. :-) – Sourav Ghosh Dec 18 '14 at 14:05
The compiler always knows because of the rules of precedence; as people are less likely to know, if you _have_ to write something like this, do it as `(a++) + b` or `a + (++b)` so it's obvious and you're not relying on arcane rules of precedence. – TripeHound Dec 18 '14 at 14:05

score 0 · Answer 2 · answered Dec 18 '14 at 13:59

0

In your code a is added to b and then it is incremented

answered Dec 18 '14 at 13:59

Ludovic Feltz

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Can you elaborate a bit? Otherwise, i'm pretty sure this answer will hit the low-quality review queue. :-) – Sourav Ghosh Dec 18 '14 at 14:06
gopi already have made a really good answer, i will delete mine... – Ludovic Feltz Dec 18 '14 at 14:09

postincrement in printf function

2 Answers2