Question

a[i] = i++;

Why does the above code not work?

What is wrong with above code? I am asking this question to improve my knowledge.

Answer 1

Because the ISO standard says that you are not allowed to change a variable more than once (or change and use one) without an intervening sequence point.

There is no sequence point between the use of i in a[i] and the change of i in i++.

The list of sequence points from C11 (not really changed that much since C99) are described in Annex C:

• Between the evaluations of the function designator and actual arguments in a function call and the actual call.
• Between the evaluations of the first and second operands of the following operators: logical AND &&; logical OR ||; comma ,.
• Between the evaluations of the first operand of the conditional ?: operator and whichever of the second and third operands is evaluated.
• The end of a full declarator: declarators;
• Between the evaluation of a full expression and the next full expression to be evaluated. The following are full expressions: an initializer; the expression in an expression statement; the controlling expression of a selection statement (if or switch); the controlling expression of a while or do statement; each of the expressions of a for statement; the expression in a return statement.
• Immediately before a library function returns.
• After the actions associated with each formatted input/output function conversion specifier.
• Immediately before and immediately after each call to a comparison function, and also between any call to a comparison function and any movement of the objects passed as arguments to that call.

and 5.1.2.3 Program execution states:

Evaluations A and B are indeterminately sequenced when A is sequenced either before or after B, but it is unspecified which.

The presence of a sequence point between the evaluation of expressions A and B implies that every value computation and side effect associated with A is sequenced before every value computation and side effect associated with B.

Section 6.5 Expressions pretty much covers your exact case:

If there are multiple allowable orderings of the subexpressions of an expression, the behavior is undefined if such an unsequenced side effect occurs in any of the orderings.

This paragraph renders undefined statement expressions such as i = ++i + 1; and a[i++] = i; while allowing i = i + 1; and a[i] = i;.