Help with C error 'conflicting types'

[smdawson]

I am trying to round money to the nearest cent. When I change calcStartBalance to return type int it works, but not with double. I keep getting the error 'conflicting types for function calcStartBalance'. help?

This is the problem piece of the code I have:


#define HUNDRED 100.00
#define POINT005 0.005

double roundToHundreths(double a)
{
double b = 0.0;
b = a + POINT005;
b *= HUNDRED;
b = (int)b;
b /= HUNDRED;
return b;
}

double calcStartBalance(double start_b, double down_p)
{
return (roundToHundreths(start_b - down_p));
}

[dwhitney67]

The code you posted works fine on my system; this is what I used:


#include <stdio.h>

#define HUNDRED 100.00
#define POINT005 0.005

double roundToHundreths(double a)
{
double b = 0.0;
b = a + POINT005;
b *= HUNDRED;
b = (int)b;
b /= HUNDRED;
return b;
}

double calcStartBalance(double start_b, double down_p)
{
return (roundToHundreths(start_b - down_p));
}

int main()
{
printf("%5.2lf\n", calcStartBalance(101, 5.43333));
return 0;
}


P.S. This works too:


double roundToHundreths(double a)
{
return (int)((a + POINT005) * HUNDRED) / HUNDRED;
}

[smdawson]

I was able to get yours to compile, but mine still will not..Here is all the code I am trying to compile. I even broke it up smaller and tried to compile a segment the size of yours and that did not work either.


#include <stdio.h>
#define HUNDRED 100.00
#define POINT005 0.005


int main(void)
{
double start_balance = 0.0;
double down_payment = 0.0;
double air = 0.0;
double month_payment = 0.0;
double new_balance = 0.0;

printf("Enter the selling price: ");
scanf("%lf", &start_balance);

printf("Enter the down payment: ");
scanf("%lf", &down_payment);

printf("Enter the anual interest rate: ");
scanf("%lf", &air);

printf("Enter the monthly payment: ");
scanf("%lf", &month_payment);

new_balance = calcStartBalance(start_balance, down_payment);
printf("The loan amount: %5.2lf\n", new_balance);

}

double roundToHundreths(double a)
{
double b = 0.0;
b = a + POINT005;
b *= HUNDRED;
b = (int)b;
b /= HUNDRED;
return b;
}

double calcStartBalance(double start_b, double down_p)
{
return (roundToHundreths(start_b - down_p));
}

double canBePaid(double month_p, double interest)
{
return (roundToHundreths(month_p - interest));
}

double calcInterest(double air, double new_b)
{
return (roundToHundreths(air * new_b));
}

double calcNewBalance(double old_b, double to_b)
{
return (roundToHundreths(old_b - to_b));
}


p.s. - any criticisms you have about good coding practices I am not following are welcome :)

[Some Penguin]

It's probably 'problem unrelated', but your main() method declares an int return type without actually having a return statement. It also probably wouldn't hurt to add a function prototype (pref. in a header file) to avoid the compiler producing any implicit declarations.

[dwhitney67]

... It also probably wouldn't hurt to add a function prototype (pref. in a header file) to avoid the compiler producing any implicit declarations.
That's the ticket! The OP presented one thing in his opening post, then something different later.

@ the OP: When you compile your source code, use the -Wall option and correct any/all warnings.

[smdawson]

Sorry guys, I didn't know about prototyping the functions. After I did that it got rid of the errors.

Also, should I use pointers for everything in my functions?
like this example I found here (http://www.java2s.com/Tutorial/C/0160__Function/Functionisdefinedaftermainprototypethefunction.htm )


#include <stdio.h>

f1(int *k);

main ( ){
int i;

i = 0;
printf (" The value of i before call %d \n", i);
f1 (&i);
printf (" The value of i after call %d \n", i);
}

f1(int *k)
{
*k = *k + 10;
}

[worseisworser]

Nope, F1 could just be passed values, then return its result as a value too, then the code after the call to F1 could assign, or not; thus using a functional style of programming as opposed to the imperative style seen in the example.

(..I'm typing this on my phone; someone can probably show you a code example..)

[dwhitney67]

dawson,

always show full prototypes; don't leave off the 'int' simply because it is implied by the compiler.

Using a pointer for the example you provided is not esthetically pleasing. The following would have been better:


#include <stdio.h>

int f1(int k);

int main (){
int i = 0; // always initialize your variables

printf (" The value of i before call %d \n", i);
i = f1 (i);
printf (" The value of i after call %d \n", i);

return 0;
}

int f1(int k)
{
return k * 10;
}

[smdawson]

Ok. So always write full prototypes and I don't need to use pointers in the functions; I can just pass values. Thank you for all the help.

[dwhitney67]

Ok. So always write full prototypes and I don't need to use pointers in the functions; I can just pass values. Thank you for all the help.

Sometimes you will have to pass a pointer to a function; it just so happens that in the case you presented, it was not necessary.

In other cases, such as when calling the stat() C library function, it is necessary. For example:


...

struct stat info;

int result = stat("/some/file", &info);

if (result == 0)
{
// structure 'info' has relevant information about the
// file /some/file.
}

[smdawson]

Sometimes you will have to pass a pointer to a function; it just so happens that in the case you presented, it was not necessary.

Ok, so I can just check the documentation of functions I use and that should let me know if parameters need to be a pointer or not? Or is there a rule-of-thumb about using or not using pointers?

[dwhitney67]

Ok, so I can just check the documentation of functions I use and that should let me know if parameters need to be a pointer or not? Or is there a rule-of-thumb about using or not using pointers?

As with most things in life, you will learn when to do something, and when not too.

Presenting your code online for a peer review was a good choice. Software development companies often times require their developers to have in-house peer reviews.

Good luck with your learning.