CS50 Caesar - Converting from ASCII number to character - c

Question

Going through the CS50 exercise Caesar, and I'm still very new to C. My code to validate the key is all working, but as i try to convert from plaintext to the cipher text I keep getting caught up with the conversion form the ASCII number to the char. For example if I run the plaintext with the argv[1] as 27 I get /023/024. I recognize the math is off and if anyone has any pointers about that that would be great, but the main thing I'm curious about is why when I try to assign f into input[i] I get this "/000" type format.
string input = get_string("plaintext: ");
int len = strlen(input);
int i = 0;
while (input[i] != '\0')
{
int mod = atoi(argv[1]);
int t = input[i] + mod;
int f = t % 26;
input[i] = f;
i++;
}
printf("ciphertext: %s\n", input);

Answer 1

Your implementation of doing the %26 is wrong because when you do t % 26 you will get a letter but you have forgotten to add the ASCII value if it is a small of a larger one.
you must change your code to
Here is my code which I had used,
It is correct and hope it helps you...
#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
int main(int argc, string argv[])
{
if (argc != 2) //checking if there are 2 arguments while calling the main method the first one here will be "./caesar" and the second will be the input given
{
printf("Wrong command\n");
return 1;
}
int k = atoi(argv[1]);//converting string input to integer
string a = argv[1];
for (int i = 0; i < strlen(a); i++)//checking if the input is a number for each character
{
if (a[i] < '0' || a[i] > '9')
{
printf("False\n");
return 1;
}
}
if (k < 0)
{
printf("Wrong command\n");
return 1;
}
else
{
//if everything is successful we ask for the plaintext
string code = get_string("plaintext: ");
//we just print 'ciphertext' and not the 'ciphered text'
printf("ciphertext: ");
for (int i = 0, n = strlen(code); i < n; i++)
{
if (islower(code[i]))
{
printf("%c", (((code[i] + k) - 97) % 26) + 97);
}
else if (isupper(code[i]))
{
printf("%c", (((code[i] + k) - 65) % 26) + 65);
}
else
{
printf("%c", code[i]);
}
}
printf("\n");
return 0;
}
}
Your mistake is that, you are not checking if it is an upper case or lower case and you are not subtracting 97 or 65 before doing the modulus...
If any other doubts, you are free to ask.
Here is the result after running check50 for the 2021 version
:) caesar.c exists.
:) caesar.c compiles.
:) encrypts "a" as "b" using 1 as key
:) encrypts "barfoo" as "yxocll" using 23 as key
:) encrypts "BARFOO" as "EDUIRR" using 3 as key
:) encrypts "BaRFoo" as "FeVJss" using 4 as key
:) encrypts "barfoo" as "onesbb" using 65 as key
:) encrypts "world, say hello!" as "iadxp, emk tqxxa!" using 12 as key
:) handles lack of argv[1]
All successful