Variable is purged when print statement is called

Question

I have written a small program to convert vowels in a word to 'leet', for an assignment for the Harvard CS50 course I'm taking online. NB: this is relevant because we are using CS50 headers which give us access to a string data type. I'm not a student at Harvard, just taking the free version through edx.org.

My main function runs an if-else to validate the input, and then call the converter function if input is correct. In the else block, a string variable is assigned the value returned from my replace() function, and then the variable is (supposed to be) printed. Main will then return 0 and exit.

Problem: when I run this program, the leeted variable is purged when the printf statement is called in main. When I run our course debugger, I can see that the leeted variable is correctly assigned, but the second the debugger runs the printf line in main, the variable becomes empty. I'm absolutely baffled.

I have added another printf line at the end of the replace() function. When this print statement is commented out, the above problem occurs as described. However, when I activate the print statement at the end of the replace() function, the print statement in main() works.

Can anyone tell me why is is happening?

code:

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

// function declarations
bool argcheck (int argc, string argv[]);
string replace(string input);

// #######################################
// MAIN
int main(int argc, string argv[])
{
    if (argcheck(argc, argv) == false)
    {
        return 1;
    }
    else
    {
        string leeted = replace(argv[1]);
        printf("from main(): %s\n", leeted);
    }
    return 0;
}

// #######################################
// FUNCTIONS

// input validation
bool argcheck (int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./no-vowels word\n");
        return false;
    }
    return true;
}

// converter implementation
string replace(string input)
{
    char output[strlen(input)];

    int i = 0;
    while (input[i] != '\0')
    {
        switch(input[i])
        {
            case 'a':
                output[i] = (char) '6';
                break;

            case 'e':
                output[i] = (char) '3';
                break;

            case 'i':
                output[i] = (char) '1';
                break;

            case 'o':
                output[i] = (char) '0';
                break;

            default:
                output[i] = input[i];
                break;
        }
        i++;
    }
    string finished = (string) output;
    // printf("from replace(): %s\n", finished);
    return finished;
}