After changing the value of a variable, how to keep it from changing back when repeated in a loop (inside a function) in C

Question

I'm a student and, as you can see, I'm currently studying C. I've written a code that is meant to print a slider that can be controlled by a and d. b is the variable I use to move the slider.

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

int b = 2;
void slider();

int main() {
    //int Ar[5] = {};


    while (true) {
        
        slider();

    }

}


void slider() { 
    int Ar[5] = {};
    
    switch (_getch()) {
    case'a':
        Ar[b] = 0;
        b--;
        if (b <= 0)
            b = 0;
        Ar[b] = 1;
        break;
    case'd':
        Ar[b] = 0;
        b++;
        if (b >= 4)
            b = 4;
        Ar[b] = 1;
        break;
    }
    for (int i = 0; i < 5; i++) {
        if (Ar[i] == 1)
            printf("|");
        else
            printf("-");
    }
    printf("\n");
    
}

The problem is that when I use this code without a global variable or inside a function and use that inside the while loop, I cannot get it to work as b will return to the value of 2 after each repetition.

What I exactly need is for it to work just like it does in the current form of the code but without using a global variable and keeping the code inside a separate function. Global variables are banned in this project (so please tell me if anything else in that code except int b=2; counts as a global variable), and the slider is part of a bigger project, so it will be called inside a function.

Answer 1

The problem is scope of the variable. Variables declared and initialized in the function will be re-initialized with the same value every time the function is called. The variables will no longer be valid after the function returns and nothing is tracked across function calls.

There are a few options depending on what you are allowed to use:

1. Use a static function variable. In the function, declare b as static and this will keep the variables value in scope across function calls. This is technically not a global variable so it might be allowed based on your stated criteria. Static variables will not be on the stack like other function variables, but will be in the same memory location as other global variables are created in the actual application, but only the function that declares it will have access to it.
2. Not sure if you have done pointers yet, but another option is to track the value in main and pass the value in as a pointer to the function that modifies it. This would allow for the value to be tracked externally from the function call and be modified in the function. If doing this way the slider function would have to be modified every where the value of b is checked or modified to use *b (i.e. (*b)++, *b<=0, etc.) Something like :

int main() {
    int b = 2;
    while (true) {
        slider(&b);
    }
}


void slider(int *b) {
// Modify references to b to become *b
} 

3. Another option that doesn't use static or pointers is to have main declare and track b, but rather than passing in a pointer return the new value of b so that main can update it. Something like:

int main() {
    int b = 2;
    while (true) {
        b = slider(b);
    }
}


int slider(int b) {
// Function unchanged
...
    return b; //return at end of last line so that main can track current value
}