Unable to get output even though my logic is working

Question

I have created a program in C. The program is a version substitution string. I have analyzed the program carefully by using the printf statements. My program logic is working correctly. However I am not able to add or concatenate strings.

I am not getting the output from encryptKey function. Why is that? I have earlier being programming in JS and now trying my hand on C.

Here is my code: -

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

string encryptKey(string message);

//stores the cipher word for each alphabet
//used for encryption purposes
char cipher[26];

int main(int argc, string argv[])
{

    //Exit the program if no key is added in commandline
    if(!argv[1] || argc >= 3)
    {
        printf("Incorrect or missing command line arguments!\n");
        return 1;
    }

    //checks if there are 26 characters in the decipher key
    if(strlen(argv[1]) != 26)
    {
        printf("Need 26 characters to decipher the key\n");
        return 1;
    }

    //check all the individual characters of the key
    for(int i = 0; i < 26; i++)
    {
        //check if the characters are letters only
        if(tolower(argv[1][i]) < 'a' && tolower(argv[1][i]) > 'z')
        {
            return 1;
            printf("All characters should be letters!\n");
        }

        //checks if each letter exists once

        //load the secret key to an array
        cipher[i] = argv[1][i];
    }


    //Asks user for message
    string message = get_string("plaintext:");
    string secretMessage = encryptKey(message);

    //outputs secret message
    printf("ciphertext: %s\n", secretMessage);
}

//encrypt the message
string encryptKey(string message)
{
    string output = "";

    //loop through each alphabet
    for(int i = 0; i < strlen(message); i++)
    {
        char lower = tolower(message[i]);

        if(lower >= 'a' && lower <= 'z')
        {
            //small case starts from location 97
            int tempLocation = lower - 97;


            if(isupper(message[i]))
            {
                output += toupper(cipher[tempLocation]);
            }
            else if(islower(message[i]))
            {
                output +=  tolower(cipher[tempLocation]);
            }
            else
            {
                output += message[i];
            }
        }
    }

    return output;
}

I even tried this to join strings: - strcat(output, toupper(cipher[tempLocation]));

I get this error: -

error: incompatible integer to pointer conversion passing 'int' to parameter of type 'const char *' [-Werror,-Wint-conversion]
                strcat(output, toupper(cipher[tempLocation]));
                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/string.h:130:70: note: passing argument to parameter '__src' here
extern char *strcat (char *__restrict __dest, const char *__restrict __src)
                                                                     ^

Answer 1

As mentioned by chux - Reinstate Monica: Strings working different in C.

Here are some hints:

• Give output some memory:

char output[512] = { 0 };

• Assign string value correct (be sure that output is 0 terminated string \0 at last index) f.e.:

char ch = toupper((unsigned char) cipher[tempLocation]);
strncat(output, &ch, 1);

• Change signatures of your methods like:

// message: is a string (pointer to char[] which is a string)
// enrypted: copy created string (output) at end of the method to encrypted instead of return output
// int is the return value - 0 success, != 0 failure
int encryptKey(char *message, char *enrypted)

• copy result to given char memory:

instead of:

string secretMessage = encryptKey(message);

do:

char secretMessage[512] = { 0 };

// no & needed ... a array is a pointer
if (encryptKey(message, secretMessage) != 0)
{
  printf("\n Something failed");
}

and at the end of your method:

strcpy(encrypted, output);

Like others commented doing the whole thing trial & error will get you nowhere.

Hope the hints are enough for you to figure out the rest.