Question

Learning C here, and I’m pretty confused on how to use function prototypes.

I’m mainly having a problem calling the function into main. I’m sure I have something messed up in here because all this does is print whatever’s in memory.

Thank you for the help.

#include <stdio.h>


double source_volt(double vs);


int main()
{
    double source_volt(double vs);
    double vs;
    printf("%lf", vs);
    return 0;
}


double source_volt(double vs)
{

    int valid = 0;
    do
    {
        printf("Enter source voltage Vs in volts: ");
        scanf("%lf", &vs);

        if (vs < 0 || vs > 100)
        {
            valid = 0;
            printf("Please enter a number between 0 and 100.\n");
        }
        else
            valid = 1;
    }while(!valid);
    return vs;
}

Answer 1

This is what is not working for you:

int main()
{
    double source_volt(double vs); //You already declared the function, 
                                   //you dont need the doubles
    double vs;
    printf("%lf", vs);
    return 0;
}

Instead:

int main()
{
    double vs;
    vs = double source_volt(vs); //The double keyword is gone
    printf("%lf", vs);
    return 0;
}

But really, you don’t need an argument at all in source volt.

You could rewrite that function to:

double source_volt(void)
{
    double vs;
    int valid = 0;
    do
    {
        printf("Enter source voltage Vs in volts: ");
        scanf("%lf", &vs);

        if (vs < 0 || vs > 100)
        {
            valid = 0;
            printf("Please enter a number between 0 and 100.\n");
        }
        else
            valid = 1;
    }while(!valid);
    return vs;
}