C Increment Operator Explanation

Question

On executing this piece of C command, the output of num is 7. I was expecting it to be 6, can anyone explain why and how it turns out to be 7?

#include <stdio.h>

int main() {
    int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    int i = 0, num = 0;
    num = a[++i + a[++i]] + a[++i];
    printf("%d ", i);
    printf("%d ", num);
    return 0;
}

It's undefined behavior - you cannot modify same variable twice between in the same operation. — yeputons, Feb 17 '17 at 04:26
Didn't you ask the same question within the same hour? (Granted, you phrased it much better this time. :) ) — synchronizer, Feb 17 '17 at 04:26

score 2 · Answer 1 · answered Feb 17 '17 at 04:32

2

i is incremented 3 times within the same expression in num = a[++i + a[++i]] + a[++i];

This has undefined behavior in C. It is a moot point trying to explain why it outputs 7 or 6 or anything whatsoever. Undefined behavior can be anything.

answered Feb 17 '17 at 04:32

chqrlie

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score 0 · Answer 2 · answered Feb 17 '17 at 04:27

0

llvm on OS X gives me 6 and a warning:

wi.c:5:8: warning: multiple unsequenced modifications to 'i' [-Wunsequenced]

which suggests that we are looking at an undefined behavior. The exact nature of what it undefined here is not clear to me thus far, but feels a bit irrelevant.

answered Feb 17 '17 at 04:27

MK.

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[This SO answer](http://stackoverflow.com/a/19862240/4045754) explains and provides links for why this is undefined behavior and should be avoided. – yeniv Feb 17 '17 at 04:31

score 0 · Accepted Answer · answered Feb 17 '17 at 04:27

This is a bit tricky, the expression : a[++i+a[++i]], involves the increment of the variable i two times, and comes out to be a[i + 2 + a[i + 2]], which is a[0 + 2 + a[2]] = a[4] = 4, and the second operand, a[++i] becomes a[3], which is equal to 3, hence, the final answer is 7. In other words, this is undefined behaviour.

score -1 · Answer 4 · answered Feb 17 '17 at 04:36

-1

i = 0;
num = a[ ++i + a[++i]] + a[++i]

Will evaluate to

num = a[1+ a[2]] + a[3]
num = a[1 + 2] + a[3]
num = a[3] + a[3]
num = 3 + 3
num = 6

answered Feb 17 '17 at 04:36

Anshuman

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@Antti Haapala How come ? – Anshuman Feb 17 '17 at 11:22
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See the duplicate question and its answer. – Antti Haapala -- Слава Україні Feb 17 '17 at 11:34
@Antti Haapala Yes, you are right. Thankyou – Anshuman Feb 17 '17 at 14:00

C Increment Operator Explanation

4 Answers4