C language - Caesar Encryption Program

Question

I want to make a program that takes letters and uses a Caesar encryption to shift them from a up 1 value to b. It must use a string in order to do this.

My problem is my program will not take the user's input into the string. (I attempted to put guy[10] in the scanf, but this only caused the program to crash-- so I willingly put the incorrect guy there so the program may compile).

#include <stdio.h>


int main(){
int i=0; //setting the individual slot number for the array-- later used in the while loop
char guy[10];
printf("Enter Plain Text:");
scanf("%s",&guy); //takes user's input-- such as "abc" and puts it into its respective slot in the array guy[10] r-right?

while (guy[10] != '\0'){ //while loop that runs until it reaches the end of the string
    if ((guy[i]) >= 'A' && (guy[i]<= 'Z')){ //moves capital letter values up 1
        guy[i]=guy[i]++; //makes the current "slot" number go up 1 value. Example: a = 97 + 1 -> b = 98
        }
    if ((guy[i]) >= 'a' && (guy[i]) <= 'z'){// moves lower case letter values up 1
        guy[i]=guy[i]++;
    }
    i++; //moves the array's interval up to the next "slot"

}
printf("Encrypted text is: %s",guy);
}

Answer 1

Your first issue is this line:

scanf("%s",&guy);

as guy is an array so we don't need to get a pointer to it, it's name is treated as a pointer in this context. Simply do:

(void) scanf("%s", guy);

Your second issue is this line:

while (guy[10] != '\0')

as WhozCraig noted int his comment -- this should use index i, not 10

The third issue is that this statement makes little sense:

guy[i]=guy[i]++;

Reasonable alternatives include:

guy[i] = guy[i] + 1;
guy[i]++;
guy[i] += 1;

The fourth issue is you've not dealt with wrap-around. E.g. what does 'Z' map to in your code? It looks like it will come out as "[" instead of "A".

The fifth issue is that scanf() can overflow the array guy as it's input size is unlimited. For guy[10], we need to do something like:

scanf("%9s", guy);

To limit the input to nine characters with room for the final '\0'. Using fgets() would be a better choice in this situation as it's safer and we don't need the parsing power of scanf():

fgets(guy, 10, stdin);

Here's a rework that addresses these five issues:

#include <stdio.h>

int main() {
    char text[10];

    printf("Enter Plain Text: ");
    (void) fgets(text, 10, stdin); // takes user's input -- such as "abc" and put it into its respective slots in the array

    int i = 0; // slot index for the array

    while (text[i] != '\0') { // loop until reach end of string

        if (text[i] >= 'A' && text[i] <= 'Z') { // move capital letter values up 1
            // make the letter go up 1 modulo 26. Example: A = 65 + 1 -> B = 66; Z = 90 + 1 -> A = 65
            text[i] = ((text[i] - 'A' + 1) % ('Z' - 'A' + 1)) + 'A';
        } else if (text[i] >= 'a' && text[i] <= 'z') { // move lower case letter values up 1
            text[i] = ((text[i] - 'a' + 1) % ('z' - 'a' + 1)) + 'a';
        }

        i++; // move the array's index up to the next "slot"
    }

    printf("Encrypted text is: %s\n", text);
}