In a test exam, we were told to find the value of some expressions.
All but 1 were clear, which was "20"[1]. I thought it was the 1st index of the number, so 0, but testing with a computer it prints 48.
What exactly does that 'function' do?
It's not a function, it's just indexing an array.
"20" here is a character array, and we're taking the value at index 1 - which is '0' - the character '0'.
This is the same as
char chArr[] = "20"; // using a variable to hold the array
printf ("%d", chArr[1]); // use indexing on the variable, decimal value 48
printf ("%c", chArr[1]); // same as above, will print character representation, 0
The decimal value of '0' is 48, according to ASCII encoding, the most common encoding around these days.