Yamaha32088 Yamaha32088 - 2 months ago 5x
C Question

Math operation trouble

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'
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. If someone could walk me through exactly the order of operations and what the values would be I would appreciate it. 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("%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
char c = 79 or 'O'
like it should.

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')


(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.