1It seems that s is a character array or a pointer to the first
2element of a character array. And element s[i] contains a character
3that represents a digit as for example '5' . To convert this character
4that for example in ASCII has internal code equal to 53
5(while internal code of character '0' is 48) there is used expression
6
7s[i] -'0'
8that is equivalent to
9
1053 - 48
11and equal to number 5
1 '0' - 48
2 '1' - 49
3 '2' - 50
4 '3' - 51
5 '4' - 52
6 '5' - 53
7 '6' - 54
8 '7' - 55
9 '8' - 56
10 '9' - 57