Why my returned value is not printed as a string but it is printed as a char?

Question

I have a function (called encrypt) that will return a string (called ciphertext). Inside this function, I created an array char[] (called cptxt_arr) and after running a loop and creating the ciphertext from the plaintext (got it from the user), I stored each letter inside the char[], then I assigned the char[] to the string and returned the string.

Here's my question: In the main(void) function, how come, when I print the encrypt function as a string (%s) I don't get anything from the CLI, but if I print the same function as char (%c) I do get the letters?

PS: From what I've read, I know I can't return arrays in C, which is why I'm assigning the char[] to a string variable and returning that.

I just want to know why is this happening because If I print the string ciphertext inside the encrypt function it works meaning that the code works fine.

NOTE: For the sake of the question, use positive integers for the "key" variable. I cleaned a bit the code, so the important part could be seen more easily.

Here's the code:

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

string encrypt(string plaintext, int key);

int main(void)
{
    int key;
    string plaintext;

    // Get the key.
    key = get_int("Key: ");

    // Get plaintext from the user.
    plaintext = get_string("plaintext:  ");

    // Print encrypted text.
    printf("ciphertext: %s\n", encrypt(plaintext, key));

    // Same but with the formatted char (%c).
    // printf("ciphertext: %c\n", encrypt(plaintext, key)[0]);
}

// Encrypt text.
string encrypt(string plaintext, int key)
{
    // Get plaintext length.
    int length = strlen(plaintext);

    char cptxt_arr[length];
    string ciphertext;

    for (int i = 0; i < length; i++)
    {
        if (plaintext[i] >= 'A' && plaintext[i] <= 'Z')
        {
            cptxt_arr[i] = ((plaintext[i]) + key);

        }
        else if (plaintext[i] >= 'a' && plaintext[i] <= 'z')
        {
            cptxt_arr[i] = ((plaintext[i]) + key);
        }
    }

    // Assign cptxt_arr[] to string and returning it.
    ciphertext = cptxt_arr;

    return ciphertext;
}

I have (I don't if it could matter) created a variable in the main(void) function and assigned the encrypt function to it, which doesn't work either.

Answer 1

The CS50 data type string is nothing more than a reference to an array which contains a sequence of characters. It is not an array itself.

These references are called "pointers" in C and you will learn all about them in week 4 of CS50.

In the line

ciphertext = cptxt_arr;

you are not making a copy of the characters contained in the array cptxt_arr. Instead, you are making ciphertext refer to the array cptext_arr.

In the line

return ciphertext;

the function encrypt will pass this reference to cpttxt_arr to the function main. However, as soon as the function encrypt returns, all of its local variables will cease to exist. This includes the array cptxt_arr. This means that the function main cannot do anything with the returned reference, because the reference is invalid, as it refers to an array that no longer exists.

In week 2 of CS50, you are supposed to solve the exercises by overwriting the array that was created and returned by get_string.

In week 4 of CS50, you will learn how to use the functions strcpy and malloc, with which you can copy character arrays and allocate memory for these copies in such a way that the copies do not cease to exist when the function returns.

Answer 2

Why my returned value is not printed as a string but it is printed as a char?

Code attempts to return the address of a local object which leads to undefined behavior (UB).

char cptxt_arr[length];
string ciphertext;
...
ciphertext = cptxt_arr;
return ciphertext; // Bad

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Certainly a dupe someplace.