Why does this C code work? (It shouldn't)

Question

I am checking a student's homework. The assignment is to print the amount of English letters to the console. For some reason, what he did works (7th line):

 int main(void)
 {
     char first = 'A';
     char last = 'Z';
     int amount = 0;

     amount = ("%d - %d", last - first + 1);
     printf("The amount of letters in the English alphabet is %d\n", amount); 
     return(0);
 }

After seeing it, I tried putting other things in the brackets instead of "%d - %d". No matter what I put there and how many commas were there, it'd only take what's after the last comma (which is the correct sentence).

What is actually happening there?

Answer 1

This is one of the examples of usage of comma operator. In case of

 ("%d - %d", last - first + 1);

the LHS operand of the comma operator ("%d - %d") is evaluated, result is discarded, then RHS (last - first + 1) is evaluated and returned as the result. The result, is then assigned to amount and thus, you have the amount holding the result of the operation last - first + 1.

Quoting C11, chapter §6.5.17, comma operator

The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.

FWIW, in this case, "%d - %d" is just another string literal, it does not carry any special meaning.