How to make sure input is a double in the C programming language

Question

How do I make sure I've got a double and not something else?

int main() {
    int flagOk = 0;
    double number;
    while(!flagOk) {
     printf("Put in a double");
     scanf("%lf", &number);
     if(number == "%lf"); //this want make sure
     flagOk = 1;
    }
}

Answer 1

Check the return value from scanf(); it tells you how many conversions were successful.

In the context, if the conversion fails, you get 0; if it succeeds, you get 1.

Error recovery needs to be thought about. I usually find it easier to read a line of data using fgets() - and never gets()! - and then process it with sscanf(). It is easy to discard the erroneous data if the conversion fails.

Answer 2

You probably want a code fragment more like this:

double number;
do {
    printf("Enter a double: ");
    scanf("%*c"); // burn stdin so as not to buffer up responses
} while (1 != scanf("%lf", &number));

However as Jonathan has pointed out, some better line-by-line parsing may be better. Scanning directly from stdin this way isn't intuitive to the user.