switch without break and i++ vs ++i

Question

i dont understand why those two,have different output

there are two simple c programms

int main()
{
    int i;
    for (i = 0;i<3; i++)
            {
              switch (i++)
              {
               case 0:
               {
                 printf("zero");
               }
               case 1:
               {
                printf("one");
               }

               case 2:
               {
               printf("two");
               break;
               }

              default:
               {               
                printf("end");
               }                
        }
     }
}

this give this output:zero one two two

in this case after switch when the value from variable i change 0 to 1?

int main()
{
    int i;
    for (i = 0;i<3; i++)
     {
         switch (++i)
         {
            case 0:
            {
                printf("zero");
            }
            case 1:
            {
                printf("one");
            }

            case 2:
            {
                printf("two");
                break;
            }

            default:
            {             
                printf("end");
            }   

        }
     }
}

this give this output:one two end

Grab paper and pencil. Even better, a debugger. Use your 1. C book, 2. brain. — , Jun 21 '13 at 21:49
`i++` returns `i` before `1` is added. `++i` returns `i` after `1` is added. I'm prettu sure I've seen this question more than once on SO and google — tay10r, Jun 21 '13 at 21:50
Try printing out `i` at the beginning and end of each loop, and read up on the difference between `i++` and `++i` (plenty of postings on this site). — Reinstate Monica -- notmaynard, Jun 21 '13 at 21:51
[duplicate](http://stackoverflow.com/questions/24853/what-is-the-difference-between-i-and-i) — tay10r, Jun 21 '13 at 21:54

Grady Player · Accepted Answer · 2013-06-22T04:18:25.393

4

switch (i++)
     {
     }

is the same as:

int j=i;
i++;
switch (j)
    {
    }

while

switch (++i)
     {
     }

is the same as:

i++;
switch (i)
    {
    }

edited Jun 22 '13 at 04:18

answered Jun 21 '13 at 22:04

Grady Player

14,399
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logically, not necessarily by compiler semantics... if anyone wants to complain – Grady Player Jun 21 '13 at 22:05
Really? With post-increment, all the case blocks are executed, and only then is i incremented? – David Heffernan Jun 21 '13 at 22:17
Not exactly. For the postfix, the value is incremented before the code inside the braces gets executed. I still upvoted though. Edit: ninja'd. – Oktalist Jun 21 '13 at 22:20
Not exactly?! Not in any way at all. Save your voting until the answer is accurate. This is very badly misleading. – David Heffernan Jun 21 '13 at 22:21
@DavidHeffernan, pretty frustrating to have an answer that is misleading and not really correct get the votes... but this is the first time that it has ever worked out in my favor. – Grady Player Jun 22 '13 at 04:21
Well done for editing the answer and removing the offending part. – David Heffernan Jun 22 '13 at 07:45

score 0 · Answer 2 · answered Jun 21 '13 at 21:51

0

In the first one you have switch(i++) this means that the program will add 1 to i after it has finished the switch case and the other program has ++i meaning that the program will add 1 before going into the switch case.

answered Jun 21 '13 at 21:51

user1681664

1,771
8
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score 0 · Answer 3 · answered Jun 21 '13 at 21:51

Using i++ compared to ++i is called postfix and prefix increment. The second case, i is given but incremented by 1 before going through the switch. Also, case 3: will occur because of this but since it doesn't exist default will occur on the final step of the loop.

David Heffernan · Answer 4 · 2013-06-21T22:15:12.363

Suppose that i has the value 0. Then

i++ evaluates to 0. This is the post-increment operator.
++i evaluates to 1. This is the pre-increment operator.

And the value of i is incremented by 1 in both cases.

The difference between post-increment and pre-increment operators is clearly explained in your text book.

Oh, I should also say that your main function is declared incorrectly. It should be int main(void).

You'd probably find the output of your programs easier to understand if you added in the missing break statements.

switch without break and i++ vs ++i

4 Answers4