Use of s[i] - '0' [duplicate]

Answer 1

Firstly, in your question title

the use of '0'

should be written as

the use of s[i] - '0'

That said, by subtracting the ASCII value of char 0 (represented as '0'), we get the int value of the digit represented in char format (ASCII value, mostly.)

Answer 2

The reason for the subtraction is that s[i] is, for example, '6'. The value '6' evaluates to 54 (the ascii code). The ASCII code of '0' is 48. So '6' - '0' = 6, which is the char value expressed as an int.

Answer 3

It's because the value inside s[i] is a char type.

To convert the char '1' into an integer, you do '1' - '0' to get 1. This is determined by position in the ASCII table, char '0' is 48, while char '1' is 49.