C :: Convert Strings From Hex Values To Decimal Equivalents

I understand most of program below. Essentially, we have strings that we want to convert from hex values to decimal equivalents. We check if first two characters of string are 0x or 0X, which signifies hex format. If our hex string consists of solely digits like 0x25, then the processing is simple. We take the digit assign it to answer variable, and for each additional position in the hex base-16 system, we multiply the digit by 16.

Now if the hex string is something like 0x2A, then for 'A', the hexalpha_to_int() function is called, since we are able to find 'A' in the hexalpha string, we take the value of 'A', which is ascii 65 divide it by 2 and add 10 to it: 65/2+10=42.5. This doesn't make sense. What is the purpose of this logic right here: 10 + (i / 2).

Code:
#include <stdio.h>
#include <stdlib.h>

int hexalpha_to_int(int c){
char hexalpha[] = "aAbBcCdDeEfF";

[Code] ....