Math operation trouble

Question

This might seem like a very simple solution to some people but I have been trying to figure it out for a while now and its bugging me.

int sum = (p[i] - 'A') + (s2[i % keyLen] - 'A');
 char c = 'A' + sum%26;

I am having trouble coming up with the solution math has never been a strong point of mine. the value of char c should equal 79 at the end of it all if p[i]=72 or 'H' and s2[i]=5.

This operation works wonderfully but I am trying to understand what is going on so I have tried to do it with a normal calculator and keep coming up with the wrong answer. What exactly is the correct order of operations and what should the values be?

Here is the complete source code that I have so far that works:

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    char s2[25];
    strcpy(s2, argv[1]);
    int keyLen = strlen(s2);
    printf("Please enter a string of text to be encrypted!\n");
    string p = GetString();
    for (int i = 0, n = strlen(p); i < n; i++)
    {
        if (isupper(p[i])){
        int sum = (p[i] - 'A') + (s2[i % keyLen] - 'A');
        char c = 'A' + sum%26;
        printf("%c", c);
        }
    }
    printf("\n");
    printf("%d\n", keyLen);

}

The way I am doing this operation is:

int sum = (p[i] - 'A') + (s2[i % keyLen] - 'A');
//int sum = (72-65) + (72 % 5) - 65) sum= -56
char c = 'A' + sum%26;
//char c = 65 + -56 %26 char c = -8

I obviously know my math is off but I thought I was doing it right when I run the operation through the debugger sum = 14 and char c = 79 or 'O' like it should.

Answer 1

int sum = (p[i] - 'A') + (s2[i % keyLen] - 'A');
//int sum = (72-65) + (72 % 5) - 65) sum= -56

Your misunderstanding is here.

What the code is doing

(s2[i % keyLen] - 'A')

means

(char_from_s2 - 'A')

where char_from_s2 is the character in s2 at index i % keyLen, i.e. the character at position i wrapped around to fit in the length of the string.

What your calculation shows

Your calculation is doing something different:

(char_from_p % keyLen) - 'A'

You're taking the character from p instead of from s2, and applying the % operator to the character value instead of the index.