Thank you all for your kind, patient and educative responses when I obnoxiously post amateur questions! 💙 While I cannot make any promises because of how my brain works, I am almost ready to continue reading The C Programming Language, 2nd Edition. I just want to experiment a little bit with error handling, specifically how to handle wrong input (char VS. int, etc.) and also to learn to indentify code that runs the risk of overflow/underflow.

Question: what errors do you recommend checking for and handling?

Meanwhile, thank you all! 🥰

#include <stdio.h>  

//Function declarations  
int newPin();  
int checkPin(int i);  

//Program that prompts for, verifies and saves pins temporarily into an array  
int main() {  

    //New pin  
    int pin = 0;  

    //History  
    int history[10] = {0,0,0,0,0,0,0,0,0,0};  
    int history_limit = 10;  
    int history_index = 0;  

    printf("Hello there! What would you like to do? (V)iew your saved pins, (S)ave a new pin or (E)xit: ");  
    int choice = 0;  
    while ((choice = getchar()) != EOF) {  
        switch (choice) {  
            case ('V'): { //Display saved pins  
                printf("\nYour saved pins are:\n\n");  
                for (int i = 0; i < history_limit; i++) printf("%d\n", history[i]);  
                printf("\nWhat would you like to do next? (V)iew your saved pins, (S)ave a new pin or (E)xit: ");  
                break;  
            }  
            case('S'): { //Prompt for and verify newly entered pin  
                pin = newPin();  
                if (checkPin(pin) == pin) {  
                    history[history_index] = pin;  
                    history_index++;  
                    if (history_index >= history_limit) history_index = 0;  
                }  
                break;  
            }  
            case ('E'): goto EXIT; //Terminate program  
        }  
    }  
EXIT:	printf("\nGoodbye!\n");  
    return 0;  
}  

//Function definitions  
//Prompt user to enter a new pin  
int newPin() {  
    
    int pin = 0;  
    
    printf("This enter your pin: ");  
    scanf("%d", &pin);  
    getchar();  

    return pin;  
}  

//Verify newly entered pin  
int checkPin (int i) {  

    int check = 0;  
    
    printf("Confirm your new pin: ");  
    while((scanf("%d", &check)) != EOF) {  
        if (check != i) printf("Mismatch! Confirm your new pin: ");  
        else if (check == i) { 
            printf("Success! Your new pin is %d. What would you like to do next? (V)iew your saved pins, (S)ave a new pin or (E)xit: ", i);  
            goto EXIT;  
        }  
    }  
EXIT:	return i;  
}  

//TODO  
//Error handling (overflow, input data type, other?)