Is using pre-increment in a function call in C wrong?

Question

void reverse(char *s,int j,int i) {
      printf("%d\t%d\n",i,j);
      if(i<j) {
        swap(s, i, j);
        reverse(s,--j,++i);
      }
    }

I read on Stack Overflow that using the postincrement operators in the function call can pose problems but does the use of the preincrement in function calls is also wrong??

Please help.

It is not wrong per se, i.e., it is possible to use it the way that you are using it. Where did you read that increment-operator in function calls can cause problems? In what context was that statement made? — Alexander Weinert, Jul 21 '13 at 12:54
wrong `if(i!=j)` Has it pass each other without being in the same — BLUEPIXY, Jul 21 '13 at 12:56
@AlexW someone said that using post operator in a function call may cause undefind behaviour...is it so?? — Coffee_lover, Jul 21 '13 at 12:57
@BLUEPIXY i am not getting it why is it wrong...can you please tell me a little more — Coffee_lover, Jul 21 '13 at 13:00
@BLUEPIXY thanx i got it now....and is it wrong to use preincrement and post increments in the function calls in c — Coffee_lover, Jul 21 '13 at 13:01
@BLUEPIXY can you give me an exaple where it might be a problem to use — Coffee_lover, Jul 21 '13 at 13:05
example Joni, `a[i]=i++`, `a[++i]=i`, `++i + i++` etc. There is a problem with that side effects occur in the middle of the expression. — BLUEPIXY, Jul 21 '13 at 13:08
@BLUEPIXY thanks for the response ,i got it now..it was my first question on stack overflow...and i am amazed..thanks again — Coffee_lover, Jul 21 '13 at 13:11

score 4 · Accepted Answer · answered Jul 21 '13 at 13:02

4

Increment operators cause undefined behavior if they modify the same variable in the same sequence point (statement). For example if you call a function like this: foo(++i, ++i) the C standard doesn't say in which order the increments should be performed, and you can't predict what will be passed to the function.

This is not the case here, so your program's semantics are well defined. That is, i and j are incremented and then their values are passed to the function.

answered Jul 21 '13 at 13:02

Joni

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And that applies equally to post- and pre- increment and decrement; for example, `foo(--i, i++)` also has undefined behavior. Note that this goes beyond just not knowing the order of evaluation; there are literally *no* constraints on the behavior of the program. – Keith Thompson Jul 22 '13 at 04:02

score 1 · Answer 2 · answered Jul 21 '13 at 14:07

The only problem I see with the function you have written is that you are using the condition i!=j and this can cause problems with strings of even length.
Consider the case of string length=4, you will have the function calls as reverse(s,3,0)-> reverse(s,2,1)->reverse(s,1,2).. and so you can see 'i' will never be equal to 'j' and the loop won't terminate, instead when j becomes negative and i becomes greater than the length of the string, you can possible have a run time error for accessing parts of memory you are not allowed to. Hope this made sense. :)

Is using pre-increment in a function call in C wrong?

2 Answers2