Encountering "variable is uninitialized when used here" error, when attempting to code a ciphertext program

Question

I am trying to make a cipher program, and I am getting this error nearing the final steps of my cipher function (Really sorry if I'm not being any more clear, this is my first time asking/posting a question here, about a bloated code of mine)

Here is the code:

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

string ptoc(string ptext);

//Declaration of validated key as global so that Caesar(ptoc) function can access it
long int ckey;

int main(int argc, string argv[])
{
    string *c;
    string ctext;
    string ptext;
    if (argc != 2)
{
    printf("Only enter the key\n");
    return 1;
}
else if (argc == 2)
{
    //Converting entered CL argument into integer
    c = &argv[2];
    long int key = strtol(argv[1], c, 10);
    if (key == 0) //
    {
        printf("Enter a valid key, a positive integer\n");
        return 1;
    }
    else // Assigning to another variable if valid
    {
        ckey = key;
    }
    printf("%ld\n", ckey);
}
ptext = get_string("Enter the plaintext: \n");
ctext = ptoc(ptext);
}

//Cipher function
string ptoc(string ptext)
{
string collect;
for (int i = 0, len = strlen(ptext); i < len; i++)
{

    char c = ptext[i];
    if (isalpha(c))
    {
        if (c == '.' || c == ',' || c == '!')
        {
            i++;
        }
        else if (isupper(c))
        {
            if (c >= 'A' && c <= 'Y')
            {
                 sprintf(collect, "%i", c + 1);
            }
            else if (c == 'Z')
            {
                sprintf(collect, "%i", c - 24);
            }
        }
        else if (islower(c))
        {
            if (c >= 'a' && c <= 'y')
            {
                sprintf(collect, "%i", c + 1);
            }
            else if (c == 'z')
            {
                sprintf(collect, "%i", c - 24);
            }
        }
    }
}
return collect;
}

The error the compiler gives me is this:

Cae1.c:59:30: error: variable 'collect' is uninitialized when used here [-Werror,-Wuninitialized]
                     sprintf(collect, "%i", c + 1);
                             ^~~~~~~
Cae1.c:44:19: note: initialize the variable 'collect' to silence this warning
    string collect;
                  ^
                   = NULL

What I don't get is why is the compiler saying the variable collect(which is a string) is uninitialized when I have initialized it within the right scope?

Sorry for the bloated mess again.

Answer 1

This declaration

string collect;

is equivalent to that declaration

char *collect;

due to the typedef

typedef char * string;

That is this declaration declares an uninitialized pointer that has an indeterminate value. And this pointer is used in calls like this

sprintf(collect, "%i", c + 1);

that results in undefined behavior.

As you are returning this pointer from the function ptoc then you need to allocate dynamically a memory of an appropriate size to which the pointer will point to.

Something like

string collect = malloc( an_appropriate_size );