How to check if user inputs are double in C?

Question

Normally I would use something like this:

    double value;
if (scanf("%lf", &value) == 1)
    printf("It's float: %f\n", value);
else
    printf("It's NOT float ... \n");

But this time I need to read two numbers at once

scanf("%lf %lf", &x, &y);

How do I check that?

If you want to use [`scanf`](https://en.cppreference.com/w/c/io/fscanf), you check it by *still* checking what the [`scanf`](https://en.cppreference.com/w/c/io/fscanf) function returns. — Some programmer dude, Oct 28 '18 at 00:12

score 1 · Accepted Answer · answered Oct 28 '18 at 00:31

As @SRhm mentioned in the comment section, you simply can use:

scanf("%lf %lf", &x, &y) == 2

to get two numbers from the user input. A quote from scanf - C++ Reference explain the return value of the function:

On success, the function returns the number of items of the argument list successfully filled.

scanf will return an integer representing the number of variables successfully read from the user input.

score 0 · Answer 2 · edited Oct 28 '18 at 13:56

0

Some programmer dude has a good point. Unfortunately, this technique won't work if only the second input is a double but you want to distinguish that from the case where both are not doubles. In order to distinguish different cases, you might want to take in the input as a string by using the fgets() function, and then parse the string into different parts separately using sscanf() on the string multiple times.

Study the C standard library in detail and you'll find many wonderful library functions that can help you do many things.

edited Oct 28 '18 at 13:56

Jonathan Leffler

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answered Oct 28 '18 at 00:34

ComputerNerd

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Or just insert two `scanf` calls. – Ajay Brahmakshatriya Oct 28 '18 at 05:30
1

Please: never ever suggest the use of `gets()` on SO (or anywhere else) because [the `gets()` function is too dangerous to be used — ever!](https://stackoverflow.com/questions/1694036/why-is-the-gets-function-dangerous-why-should-it-not-be-used) – Jonathan Leffler Oct 28 '18 at 13:53
you have a valid point there. I was thinking strictly function right here, but thanks for pointing that out. – ComputerNerd Oct 29 '18 at 10:52

How to check if user inputs are double in C?

2 Answers2