scanf 忽略，无限循环

Question

int flag = 0;
int price = 0;
while (flag==0)
{
    printf("\nEnter Product price: ");
    scanf("%d",&price);
    if (price==0) 
        printf("input not valid\n"); 
    else 
        flag=1;
}

当我输入一个有效数字时，循环按预期结束。但是如果我输入的不是数字，比如hello，那么代码就会进入无限循环。它只是继续打印Enter Product price:和input not valid。但它不会等我输入一个新号码。这是为什么？

Answer 1

When you enter something that isn't a number, scanf will fail and will leave those characters on the input. So if you enter hello, scanf will see the h, reject it as not valid for a decimal number, and leave it on the input. The next time through the loop, scanf will see the h again, so it just keeps looping forever.

One solution to this problem is to read an entire line of input with fgets and then parse the line with sscanf. That way, if the sscanf fails, nothing is left on the input. The user will have to enter a new line for fgets to read.

Something along these lines:

char buffer[STRING_SIZE];
...
while(...) {
    ...
    fgets(buffer, STRING_SIZE, stdin);
    if ( sscanf(buffer, "%d", &price) == 1 )
        break;   // sscanf succeeded, end the loop
    ...
}

If you just do a getchar as suggested in another answer, then you might miss the \n character in case the user types something after the number (e.g. a whitespace, possibly followed by other characters).

You should always test the return value of sscanf. It returns the number of conversions assigned, so if the return value isn't the same as the number of conversions requested, it means that the parsing has failed. In this example, there is 1 conversion requested, so sscanf returns 1 when it's successful.