How does 0+= VARIABLE result in VARIABLE+1?

I ran into a strange bug when updating a counter that is used to calculate the variable for a switch statement.

int iCount was assigned zero outside of the loop, and it is the counter used for the while loop.

To update the counter inside the loop, I wrote iCount+= packedCou开发者_开发百科nt, where packedCount was 7 in this case. In the debugger, however, 0+=packedCount resulted in packedCount+1, which was 8. That resulted in an array slot remaining unfilled throughout the loop.

When I changed the line to icount= packedCount+iCount, the proper value was returned.

So, is this behavior unique to C, as I do this regularly in Java with no strange effects.

EDIT- Code snippet added

#define SKIP   8
#define PACKED 7

int iCount;
iCount=0;

while (iCount < characters-1){  
    for (packedCount=iCount; packedCount< iCount+PACKED; packedCount++){ 
        //ASCII compressor logic goes here
    }
    //iCount+= packedCount; //this produces 8 for 0+packedCount

    //this works
    iCount= iCount+packedCount; //skip the next byte in array, since it was already packed
}

As far as the compilers are concerned

iCount += packedCount;
iCount = iCount + packedCount;

are identical. If they're producing different results, then something in your code is causing iCount to get trashed - a bad pointer reference, perhaps.

Try this, and other variations you can think of:

#define SKIP   8
#define PACKED 7

int iCount;
iCount=0;

while (iCount < characters-1){  
    for (packedCount=iCount; packedCount< iCount+PACKED; packedCount++){ 
        //ASCII compressor logic goes here
    }
    printf("iCount after the inner loop: %d\n", iCount);             /* DEBUG */
    printf("packedCount after the inner loop: %d\n", packedCount);   /* DEBUG */

    //iCount+= packedCount; //this produces 8 for 0+packedCount

    //this works
    iCount= iCount+packedCount; //skip the next byte in array, since it was already packed

    printf("iCount after update: %d\n", iCount);                     /* DEBUG */
}