String concatenation with variables and pointers in 'C'

Question

I am working on Caesar's cipher for an online course and I have a problem with the original value being in the final output, and I cannot seem to get rid of it. My suspicion is that it is due to

strcpy(str1, &final_val);
strcat(str2, str1);

being called in a wrong way, so when I run

make test && ./test 1

This provides my program with the argument 1 and provides the key to shift the letters and encode the message. I expect to see

plaintext: asd
ciphertext: bcd 

Instead I get

plaintext: asd
ciphertext: bacbdc

If you want to try out the code, you will need to do it inside of this sanbox , because it has the required CS50 library.

Code

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

bool input_valid(int count, string arg);
bool in_alphabet(int count, string arg);

int main(int argc, string argv[]) {
    int key;
    int ascii_val;
    char final_val;
    string string;

    char str1[80];
    char str2[80];

    // check input again if validation fails
    if (!input_valid(argc, argv[1])) {
        printf("Invalid input!\nUSAGE: ./caesar key\n");
        return 1;
    }

    string = get_string("plaintext: ");

    // get integer from string input
    key = strtol(argv[1], NULL, 10);

    for (int i = 0; i < strlen(string); i++) {
        ascii_val = (int)string[i];

        bool valid_lower_case = (ascii_val + key) >= 97 && (ascii_val + key) <= 122;
        bool valid_upper_case = (ascii_val + key) >= 65 && (ascii_val + key) <= 90;

        // check if value is a letter
        if (isalpha(string[i])) {
            // check if value is in the valid alphabet range
            if (valid_lower_case || valid_upper_case) {
                final_val = ascii_val + key;
            } else {
                // for lowercase: wrap around if the letter passes 'z'
                final_val = 97 + (key - (122 - (ascii_val - 1)));
            }
        } else {
            final_val = ascii_val;
        }

        strcpy(str1, &final_val);
        strcat(str2, str1);
    }
    for (int i = 0; i < 5; i++) {
        printf("%i\n", str2[i]);
    }

    printf("ciphertext: %s\n", str2);
}

bool input_valid(int count, string arg) {
    // input has more args than just the file name
    // input is an integer
    return count > 1 && isdigit(arg[0]);
}

Answer 1

strcpy(str1, &final_val); is undefined behavior. strcpy expects both parameters to be pointers to null-terminated strings. However, since arrays decay to pointers when passed to functions, strcpy doesn't know the difference between a pointer to an array of characters and the address of a single char variable.

It will try to copy memory beginning at &final_val into str1 , only stopping when it encounters a null terminator elsewhere in your process' memory, if there is one. To copy a single character to a string, just use str[x] = ch and str[x + 1] = '\\0' .

Answer 2

Replaced strcopy() and strcat() . Got it to work by appending to str2 like this:

str2[i] = final_val;