How to make a conditional loop from "Yes, no" question by comparing String with pointer initiation? [C Programming]

Question

int main(int argc, char* argv[]) {
    char* string; //local variable for a character
    string = (char*)malloc(sizeof(char));
    char* yes = "yes";

    printf("Do you want to play (y/n)?");
    scanf_s("%s", string);
    if (strcmp(yes, string) == 0) {
        printf("Hello welcome...");
}

Above is my code. Essentially, I want to make a loop asking for the user to input yes, y to continue; n, no to discontinue. But I simplify it for the sake of simple codes. The program just output the question, I press enter yes and enter then it stops. I cant figure out a way to do it using array syntax( char string[] way, although array and malloc are basically the same) so I use pointer and malloc instead. I'm going mad because this is bugging me so much. The practice assignment only asks to input character 'y' 'n' using %c but i want to do it the %s. Really appreciate any help, im really stuck now. Thank you so much

Answer 1

Your code has two significant problems. First, if you want to input a string of characters, your malloc call needs to allocate space for more than one character; you should allocate the maximum number of characters you think the user's input will contain plus one - strings in C have a zero (nul) character at the end, to mark the end of the string).

Second, when you use the scanf_s function to read in a string (using the %s format specifier, as you have done), then you need to add additional parameters (the size of the target string buffer) after each string argument.

Here's a modified version of your code with these (and a few other) corrections:

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

int main(int argc, char* argv[])
{
    char* string = malloc(5); //local variable for a character string (up to 5 chars)
    char* yes = "yes";
    printf("Do you want to play (y/n)?");
    scanf_s("%s", string, 5);
    if (strcmp(yes, string) == 0) {
        printf("Hello welcome...");
    }
    free(string); // Don't forget to release the memory!
    return 0; // Conventionally, return 0 for success or non-zero on error
}

Note 1: Each and every call to malloc (or calloc) should be paired with a call to free to release the allocated memory, or you will end up with memory leaks.

Note 2: Please read this post: Do I cast the result of malloc?

Note 3: Although most (all?) C compilers will not insist on it, it is good practice to explicitly add a return 0; (for success) statement at the end of the main function.

Please feel free to ask for any further clarification and/or explanation.