cannot convert 'double(_cdecl*)()' to 'double' - c++

Question

As an assignment I have to write a code that takes user inputs, performs an operation with them, then prints them to the screen. However, I keep getting an error on line 18 where I call FunctionMultiply saying that the function cannot convert 'double(_cdecl*)()' to 'double'. I searched for this type of problem but it seems like all of them have to do with arrays which aren't in my code. How can I fix this?
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <ctype.h>
int GetInt(void);
double GetDouble();
char GetLetter(void);
double FunctionMultiply(int, double);
int FunctionCharacter(char);
int main()
{
GetInt();
GetDouble();
GetLetter();
FunctionMultiply(GetInt, GetDouble);
FunctionCharacter(GetLetter);
printf("%f", FunctionMultiply);
printf("%c", FunctionCharacter);
return 0;
}
int GetInt(void)
{
int integer;
printf("Enter an integer\n");
scanf("%i", &integer);
return integer;
}
double GetDouble()
{
double dub;
printf("Enter a floating point number\n");
scanf(" %lf", &dub);
return dub;
}
char GetLetter(void)
{
char letter;
printf("Enter a letter\n");
scanf(" %c", &letter);
return letter;
}
double FunctionMultiply(int arg1, double arg2)
{
double product = arg1 * arg2;
return product;
}
int FunctionCharacter(char letter)
{
if (toupper(letter) <= 'M')
{
return 0;
}
else
{
return 1;
}
}

Answer 1

I think you are confusing function identifiers with storage. You have just called a bunch of functions at the beginning and not stored their results in anything.
It appears that you expect using the function identifier on its own will give you the result of the last call to that function. But it does not.
Here is how you would store the return values and use them later:
int my_int = GetInt();
double my_double = GetDouble();
char my_char = GetLetter();
double multiply_result = FunctionMultiply( my_int, my_double );
char char_result = FunctionCharacter( my_char );
printf( "%f", multiply_result );
printf( "%c", char_result );

Answer 2

Modify your main() like this:
int main()
{
int i = GetInt();
double d = GetDouble();
char c = GetLetter();
double a = FunctionMultiply(i, d);
char c = FunctionCharacter(c);
printf("%f", a);
printf("%c", c);
return 0;
}
Your problem is that you are passing function names rather than calling them. i.e. GetInt instead of GetInt().

Answer 3

It looks like you weren't paying attention to the lessons or examples showing how to use functions.
GetInt();
This calls the GetInt function, and ignores its return value.
GetDouble();
This calls the GetDouble function, and ignores its return value.
GetLetter();
This calls the GetLetter function, and ... you know the score by now.
FunctionMultiply(GetInt, GetDouble);
This is just nonsense. You're trying to call the FunctionMultiply function, passing the functions GetInt and GetDouble as arguments. You need to pass it an int and double, but you don't have an int and a double because you didn't store the results of GetInt and GetDouble anywhere.
You should have done this:
int i = GetInt();
double d = GetDouble();
char l = GetLetter();
Now you have variables i, d and l that hold the results of those function calls, so you can pass them in to other functions:
FunctionCharacter(i, d);

Answer 4

You seem to be under the impression that the name of a function magically changes to the result of the call, after you've called the function once.
It doesn't. The function call expression itself is the result of the call.
Instead of
ReturnADouble();
// call ^ and value v somehow separated? Why did you ever think that?
double result = ReturnADouble;
But according to the language rules, ReturnADouble is still the name of a function, and the compiler righteously complains when you give the name of a function when you should be giving a numeric value.
Your code should read more like
double result = ReturnADouble();
// ^ this call results in a value