C，不同的GCC，fflush（）无法正常工作？

Question

I'm a beginner programmer. I have a function that doesn't let float numbers or characters to be inputted. It was working fine with gcc 3.4.2, but now I updated to 4.7.1 and it isn't working properly. It only works now with the first input a[0]. If I input lets say 'x', it will show "Wrong input", however if I input for example '1' for a[0], and then 'x' for a[1], it will still say Input OK and assign '1' to a[1]; How can I fix this? Thank you!

void initArray(unsigned int a[]) {

double q;
int x, c;

for ( x = 0; x < SIZE; x++){
    printf("a[%d] ", x);
    printf("Enter number: ");

    scanf("%lf", &q);

    if (q == (unsigned int) q) {
        printf("Input OK.\n");
        a[x] = q;
        fflush(stdin);
    }
    else {
        printf("Wrong Input\n");
        fflush(stdin);
        x--;
    }
}
printf("\n");
} 

Answer 1

You should check the return value of scanf . It returns the number of items it managed to "scan", which will be zero if it failed to scan anything, for example when you input 'x' :

if (scanf("%lf", &q) == 1)
{
    printf("Input OK.\n");
    a[x] = q;
}
else
{
    printf("Wrong Input\n");
    x--;
}

Answer 2

I suggest to you to replace

scanf("%lf", &q);

with

while ((c=(scanf(" %lf%c", &q, &tmp) !=2 || !isspace(tmp)))
       || (q != (unsigned int) q)) {
    printf("Your input is invalid please enter again: ");
    if(c) scanf("%*[^\n]");
}

The scanf("%*[^\\n]"); clean your stdin so no need any more for fflush(stdin) in your code

So your code could be:

void initArray(unsigned int a[]) {

double q;
int x, c=0;
char tmp;


for ( x = 0; x < SIZE; x++){
    printf("a[%d] ", x);
    printf("Enter number: ");

    while ( (c=(scanf(" %lf%c", &q, &tmp) !=2 || !isspace(tmp)))
               || (q != (unsigned int) q)) {
       printf("Your input is invalid please enter again: ");
       if(c) scanf("%*[^\n]");
    }

    a[x] = q;
}
printf("\n");
}