C :: Create Program Which Requires The Input?

I would like to create a C program which requires the input in this form: number1 operator number2, where number1 and number2 are numbers of type double and operator is one of the characters +,-,*,/.

There is my attempt to write this code:

Code:

#include <stdio.h>
double main() {
char operator;

[Code]....

Now I have to solve these problems: This code above doesn't work and I don't know why.I don't know how to fix the case when some user enters the input in this form:

Code: 1.5896 *5 or
Code: 7 / 5

I mean how the program knows that

Code: 1.5896 *5 is the same as
Code: 1.5896*5

I don't know how to fix the case when the user enters the input in the incorrect form for example

Code: 3