error: a parameter list without types is only allowed in a function definition

Question

I'm learning how to create and use functions. My code doesn't compile but I can't seem to figure out what I'm doing wrong.

Here is my code:

#include <stdio.h>
#include <cs50.h>
#include <stdbool.h>

bool tri (double x, double y, double z);

int main(void)
{
    double x = get_double("Give:");
    double y = get_double("Another:");
    double z = get_double("Now:");

    bool tri (x, y, z);
}

bool tri (double x, double y, double z)
{
    if (x<1 || y<1 || z<1)
    {
        return false;
    }

    if (x+y > z && y+z > x && z+x > y)
    {
        return true;
    }

    else
    {
        return false;
    }
}

When I try to compile this code, I get the following error:

error: a parameter list without types is only allowed in a function definition
bool tri (x, y, z);
          ^

Solution 1

Your problem

What you wrote is recognized by your compiler as a function prototype. So it tells you that you should have something like this, with types:

bool tri(double x, double y, double z);

Solution

Here, you don't want a function prototype, instead, you want to call your function. You don't need to specify the return type when calling a function.

So simply do this:

tri(x, y, z);

...instead of bool tri(x, y, z);.

Solution 2

A couple of small changes are suggested below - most notably you put the return value of the function into a bool called val declared at the top and then print out at the end the values of x,y,z and val which is the true or false returned by your function.

Note you need to use the function you have written and something that returns a values which is why you need something like val = tri (x, y, z); to capture the returned value, which you can then print out...

#include <stdio.h>
#include <cs50.h>
#include <stdbool.h>

bool tri (double x, double y, double z);

int main(void)
{
    bool val;
    double x = get_double("Give:");
    double y = get_double("Another:");
    double z = get_double("Now:");

    val = tri (x, y, z);
    printf("x=%g, y=%g, z=%g\n",x,y,z);
    printf("%s", val ? "true" : "false");
    return 0;
}

bool tri (double x, double y, double z)
{
    if (x<1 || y<1 || z<1)
    {
        return false;
    }

    if (x+y > z && y+z > x && z+x > y)
    {
        return true;
    }

    else
    {
        return false;
    }
}

Comments

[callmeanythingyouwant]

Perfect. I understand it now. Thank you!

[callmeanythingyouwant]

I realise I don't understand the usage of bool very well. I had no idea I could declare it like you declare an integer. Thank you so much!

[tom]

@callmeanythingyouwant - no problem, but note you can only use bool because you include the stdbool.h header at the top of your code.