Smallest number of bills program in C function-pointer issue - c

Question

so C-programming noob here. I'm working on some practice questions, and I can't seem to figure out where I'm making a mistake on this one.
I'm pretty sure there is an error in how the pointers are being grabbed by the main function, but I've tried everything I could think of/read up on and have no idea how to resolve my problem.
Some more information about the question - the change calculation has to be a function, and I made up a program to get input from the user to then go through the function and spit out the smallest number of bills/coins used. No small change (quarters,dimes,nickels, pennies), so only int number are required.
#include <stdio.h>
#include <math.h>
int main(void)
{
/* local variable definition of enter amount*/
int dollars, *twenties, *tens, *fives, *toonies, *loonies;
printf("enter amount: ");
scanf("%d", &dollars);
printf("\nChange for $%d is:\n", dollars);
/* Calling pay_amount function to get smallest bills*/
printf("$20s: %d\n", &twenties);
printf("$10s: %d\n", &tens);
printf("$5s: %d\n", &fives);
printf("$2s: %d\n", &toonies);
printf("$1s: %d\n", &loonies);
return;
}
/*Function pay_amount declaration */
void pay_amount(int dollars, int *twenties, int *tens, int *fives, int *toonies, int *loonies)
{
while (dollars>=0);
*twenties = (dollars/20);
*tens = ((dollars%20)/10);
*fives = (((dollars%20)%10)/5);
*toonies = ((((dollars%20)%10)%5)/2);
*loonies = (((((dollars%20)%10)%5)%2));
}
unwanted result example:
enter amount: 120
Change for $120 is:
$20s: -4196336
$10s: -4196340
$5s: -4196344
$2s: -4196348
$1s: -4196352

Answer 1

There are several issues with your program. Here are a few of them.
First, you don't want your actual variables to be pointers, but simple ints for you to point to:
int dollars, twenties, tens, fives, toonies, loonies;
Second, you need to pass the actual variable values to printf, not their addresses:
printf("$20s: %d\n", twenties);
printf("$10s: %d\n", tens);
printf("$5s: %d\n", fives);
printf("$2s: %d\n", toonies);
printf("$1s: %d\n", loonies);
Third, you don't actually call your pay_amount function.
Fourth, if you were to call it, it would loop indefinitely due to this completely extraneous loop that you should just remove:
while (dollars>=0);
Fifth; while this isn't actually a bug (it won't stop your program from working in any way), the additional reminder operations in pay_amount are redundant:
*twenties = (dollars/20);
*tens = ((dollars%20)/10);
*fives = ((dollars%10)/5);
*toonies = ((dollars%5)/2);
*loonies = ((dollars%2));
Sixth, as a note on terminology, this has nothing to do with "function pointers", which signify pointers that point to functions, rather than pointers that are passed to functions.

Answer 2

you need change:
/* local variable definition of enter amount*/
int dollars, *twenties, *tens, *fives, *toonies, *loonies;
to
/* local variable definition of enter amount*/
int dollars, twenties, tens, fives, toonies, loonies;
function pay_amount() like this:
pay_amount(int *dollars,int *twenties,int *tens,int *fives,int *toonies,int *loonies)
call function like this: pay_amount(&dollars, &twenties, etc);
and inside the pay_amount() like this:
while (*dollars>=0);
*dollars/20;
*tens = ((*dollars%20)/10);