Input validation of multiple conditions C Program-CodePudding

I am creating a bank program where the user enters a card number and pin and I am trying to validate their input. It worked for validating one card number and pin but it doesn't work for multiple card numbers and pins.

The code is as follows:

do {
    printf("Card number: ");
    scanf("%d", &cardNum);

    printf("PIN: ");
    scanf("%d", &pin);

    if ( cardNum != 1111 && pin != 2222 || cardNum != 3333 && pin != 4444 ) {
        printf("Error! Card number or PIN is invalid.\n");
    }

} while ( cardNum != 1111 && pin != 2222 || cardNum != 3333 && pin != 4444 );

Any help is appreciated. Thanks!

CodePudding user response：

You're not combining the conditions correctly.

It's generally easier to describe the conditions that you want to match, and then invert the entire thing with a single !(...) around it.

do {
    printf("Card number: ");
    scanf("%d", &cardNum);

    printf("PIN: ");
    scanf("%d", &pin);

    if (!(cardNum == 1111 && pin == 2222 || cardNum == 3333 && pin == 4444)) {
        printf("Error! Card number or PIN is invalid.\n");
    }

} while (!(cardNum == 1111 && pin == 2222 || cardNum == 3333 && pin == 4444));

If you want to rewrite it without the wrapping !(...), see de Morgan's Laws. You'll need to apply it multiple times to each level of condition nesting. The result would be:

if ((cardNum != 1111 || pin != 2222) && (cardNUm != 1111 || pin != 4444))

It's not easy to tell what this is actually trying to match when written like this.

And to avoid writing the condition twice, use a while(1) loop instead of do-while, and break out when the input is valid.

while (1) {
    printf("Card number: ");
    scanf("%d", &cardNum);

    printf("PIN: ");
    scanf("%d", &pin);

    if (cardNum == 1111 && pin == 2222 || cardNum == 3333 && pin == 4444) {
        break;
    }
    printf("Error! Card number or PIN is invalid.\n");
}