为什么通过scanf（）输入意外的值类型会导致该程序进入无限循环

Question

Consider the following code snippet:

int n;
int a[100];
int main()
{
    printf("\nThis program will sort a given list of between 1 and 100 integers.\n\n");
    int ready = 0;
    while(!ready)
    {

        printf("How many integers are in your list? ");
        scanf("%d",&n);

        if(n>100)
        {
            printf("\n\nError:\tToo many integers.\n\tThis program can only handle up to 100 integers.\n\n\n"); 
        }
        else if (n<1)
        {
            printf("\n\nError:\tNot enough integers.\n\tThis program requires at least 1 integer to sort.\n\n\n");
        }
        else ready=1;
    }
}

It works as expected if you enter any integer at the prompt, but If you enter a character, it starts to continually output:

How many integers are in your list?

Error: Too many integers.
       This program can only handle up to 100 integers.

...
...
recurse over and over

Obviously it has something to do with the scanf() function, but I would like to know what goes on under the hood that causes this abstraction to leak the way it does.

I am used to languages with floaties and life jackets and I am trying to get used to swimming in the deeper end of the swimming pool with C.

Answer 1

If you enter a character then scanf() fails and the results are not defined after that and also the input is not consumed and stays in the buffer recursively fetching the same value causing your scanf() to fail repeatedly.

So you should do

if(scanf("%d",&n) == 1)
// Do your stuff

Answer 2

Because the scanf function will only extract the input if it's correct. If the input is not correct, the input will still be in the input buffer when the loop iterates, and the next call to scanf will read the exact same input.

You might want to either use the return value of scanf to determine if you should exit the loop, or use eg fgets to read and extract the complete line, and then use eg strtol (or sscanf ) to get the value.