Using postfix increment in an L-value

Question

Possible Duplicate:
In what order does evaluation of post-increment operator happen?

Consider the following snippet(in C):

uint8_t index = 10;
uint8_t arr[20];

arr[index++] = index;

When I compile this with gcc, it sets arr[10] to 10, which means that the postfix increment isn't being applied until after the entire assignment expression. I found this somewhat surprising, as I was expecting the increment to return the original value(10) and then increment to 11, thereby setting arr[10] to 11.

I've seen lots of other posts about increment operators in RValues, but not in LValue expressions.

Thanks.

Answer 1

The line

arr[index++] = index;

causes undefined behaviour. Read the C standard for more details.

The essence you should know is: You should never read and change a variable in the same statement.

Answer 2

Assignment operation works from right to left, i.e. right part of expression calculated first and then it is assigned to the left part.