switch case do while nesting

Question

#include<stdio.h>
int main()
{
    int n=0, i=2;
    switch(1)
    {
    case 0:do
           {
    case 1: n++;
    case 2: n++;
           }while(--i > 0);
    }
    printf("n = %d",n);
}

I was expecting output for above code to be 0, as case 1 and case 2 are inside a do while which is inside case 0. Switch is testing on value 1, so case 0 will never be executed and hence neither case 1 or 2.

value of n coming to be 4. Any explanation ?

Answer 1

Your code jumps into the middle of a loop. From that point on, it ignores the cases of the switch statement, so it does n++, n++, check condition, loop, etc. It helps if you think of a case as a label, and switch as a goto:

    int n=0, i=2;

    if (1 == 0)
        goto label0;
    else if (1 == 1)
        goto label1;
    else if (1 == 2)
        goto label2;

label0:
    do {
        puts("loop");
label1:
        n++;
label2:
        n++;
    } while (--i > 0);

Like your original code, this simply skips the part of the loop body before label1 (the puts), then continues as if it had entered the loop in the normal way.