Conditional expressions as arguments to printf

Question

So i came across the following code in C Language

foo() {                     
    int v=10;
    printf("%d %d %d\n", v==10, v=25, v > 20);
}

and it returns 0 25 0 can anybody explain me how and why

possible duplicate of [Undefined behavior and sequence points](http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points) — phuclv, Jan 31 '15 at 07:53

Gopi · Accepted Answer · 2015-01-31T06:54:32.980

printf("%d %d %d\n", v==10, v=25, v > 20);

What you see is undefined behavior becuase the order of evalutaion within printf() is not defined.

The output can be explained as(Right to left evaluation)

v = 10 and hence v>20 is false so last `%d` prints 0
v = 25 Now v is 25 so second `printf()` prints out 25

Then you have

v ==10 which is false because v is 25 now. This is not a defined order of evaluation and might vary so this is UB

Thiyagu · Answer 2 · 2015-01-31T06:53:25.307

0

It is evaluated from right to left...

First it evaluates v > 20 its false so it prints 0

Next it sets v=25 an prints it

Next it check if v is 10. Its false so it prints 0 (the value of v is changed in the above step)

EDIT

This is the way your compiler evaluates it but the order of evalaution generally is undefined

edited Jan 31 '15 at 06:53

answered Jan 31 '15 at 06:38

Thiyagu

This seems to imply that function parameter evaluations have a defined order which I don't believe is the case in C. – tangrs Jan 31 '15 at 06:47

R Sahu · Answer 3 · 2015-01-31T06:50:31.803

0

Your code is subject to undefined behavior. Looks like in your platform,

v > 20 got evaluated first, followed by
v=25, followed by
v==10.

Which is perfectly standards compliant behavior.

Remember that those expressions could have been evaluated in any order and it would still be standards compliant behavior.

edited Jan 31 '15 at 06:50

answered Jan 31 '15 at 06:38

R Sahu

Unless I'm missing something obvious, I don't see a sequence point between the evaluations. Doesn't this make it undefined behaviour? – tangrs Jan 31 '15 at 06:46
@tangrs, It is undefined behavior. I was trying to explain the outcome reported by the OP. – R Sahu Jan 31 '15 at 06:48
You should probably indicate that in your last sentence since it could be misleading. – tangrs Jan 31 '15 at 06:50

score 0 · Answer 4 · answered Jan 31 '15 at 06:38

0

Your compiler seems to evaluate the function parameters from right to left.

So, v > 20 is evaluated first, then v=25 and then v==10.

Therefore you get the output 0 25 0

answered Jan 31 '15 at 06:38

kelsier

4 Answers4