Arrays and Increment Operators in C?

Question

This is a sample code I have from someone and it runs giving the answers

3, 2, 15

Can someone please explain how this piece of code works and how it got to those outputs?

Code:

int a[5] = { 5, 1, 15, 20, 25 };
int i, j, m;
i = ++a[1];
j = a[1]++;
m = a[i++];
printf("%d, %d, %d", i, j, m);
_getch();

Also, [this operator precedence table](http://en.cppreference.com/w/c/language/operator_precedence) might help you understand it a little. — Some programmer dude, May 28 '15 at 12:06
@JoachimPileborg, his output is in the second line of the post `3, 2, 15`. He doesn't expect output, he would like to understand where it came from. — user1717828, May 28 '15 at 12:21
@user1717828 The output wasn't clear in the original post before the edit, and I'm sure the OP expected *something*. — Some programmer dude, May 28 '15 at 12:28

score 7 · Accepted Answer · edited May 28 '15 at 12:15

You should know about pre-increment (++var) and post-increment (var++).

Let's break down the code line by line, shall we?

i = ++a[1]; Here, a[1] is 1, (2nd element of array a), is pre-incremented to 2 and that value is stored into i.
j = a[1]++; same, the 2 is stored into j and then a[1] is post-incremented to the value 3.
m = a[i++]; a[2], i.e., 15 is stored into m, and i is post-incremented to 3.
printf("%d, %d, %d", i, j, m); -- Surprise!!

score 0 · Answer 2 · answered Jul 03 '15 at 18:26

Exactly so:

**Pre- ** Increment: increment the value in the variable, then use the incremented value.
** Post- ** Increment: grab the value and use it. After grabbing the not-yet-incremented value, increment the value that's stored in the variable. Proceed, using the value as it was before.

"Operator precedence" rules mean that, for example, ++a[1] means that the meaning of a[1] will be resolved first, then the ++ operator will be applied to that memory-location. The value of the number-one element (which is actually the second element ...) will be incremented, and the incremented value will be returned for use by the statement.

score -1 · Answer 3 · edited May 28 '15 at 13:04

-1

a[1] refers to the second element which is 1
In the step i=++a[1],a[1] is incremented to 2 and assigned to i
in the next step a[1] is incremented again and the value of a[1] is incremented again to 3
in the third step,m=a[i++], value of i is 2 from step 1 , but it is a post increment .so, m will remain m=a[2]

edited May 28 '15 at 13:04

mch

9,424
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answered May 28 '15 at 12:32

Pushkar Dsrj

11
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Arrays and Increment Operators in C?

3 Answers3