Input Validation

Input Validation

Users type mistakes. A solid program checks its input before believing it.

Check the scanf return

scanf returns the number of items it successfully read:

int num;
if (scanf("%d", &num) == 1) {
    printf("Got: %d", num);
} else {
    printf("That wasn't a number!");
}

Range checks

Ask again if the value is out of range:

int age;
printf("Enter an age: ");
scanf("%d", &age);
while (age < 0 || age > 150) {
    printf("That age is impossible!\n");
    scanf("%d", &age);
}

Clearing bad input

If a scanf fails, the bad characters stay in the buffer. You may need to clean it up before the next read.

TL;DR

• Check scanf's return value to catch failures.
• Loop until input is in range.
• Bad reads leave junk in the buffer - clear it.
• Never trust user input.