Check out example codes for "str[i] - '0'". It will help you in understanding the concepts better.
Code Example 1
The part str[i] - '0' takes the ASCII character of the corresponding digit which are sequentially "0123456789" and subtracts the code for '0' from the current character. This leaves a number in the range 0..9 as to which digit is in that place in the string.
Learn ReactJs, React Native from akashmittal.com