Bug in While-Loop Keeps Counting Beyond Condition - c

Question

For school I am to write a C program that takes some amount of cash and returns the smallest number of coins it would take to reach that amount. I don't know what I am doing wrong. I've been tweaking and trying all sorts of different things but I cannot seem to totally debug the program.
The program gives correct answers to some inputs but overstates the amount of coins needed for many inputs.
Here is what I have so far.
#include <stdio.h>
int main()
{
float cash;
int n;
int counter=0;
int quarters=0;
int dimes=0;
int nickels=0;
int pennies=0;
for (;;)
{
printf("Enter change amount: ");
scanf("%f",&cash);
if (cash > 0)
{
break;
}
}
n = cash * 100;
counter = 0;
while (n > 0)
{
while (n >= 25)
{
counter ++;
n = n - 25;
quarters ++;
printf("%i\n",n);
}
while (n >= 10 && n < 25)
{
counter ++;
n = n - 10;
dimes ++;
printf("%i\n",n);
}
while (n >= 5 && n < 10)
{
counter ++;
n = n - 1;
nickels++;
printf("%i\n",n);
}
while (n > 0 && n < 5)
{
counter ++;
n = n - 1;
pennies ++;
printf("%i\n",n);
}
}
printf("%d\n",counter + n);
printf("%i quarters, %i dimes, %i nickels, %i pennies\n",
quarters, dimes, nickels, pennies);
return 0;
}

Answer 1

I'm a bit surprised they're wanting you to use break to exit a loop, as you usually want loops to conclude "naturally" (and you usually save breaks for switch statements). Something like this should work, using integer division and the modulus operator (edit note: I'm using two ints instead of a single float because of inaccuracy with the latter. If someone more knowledgeable wants to show how to do it with float, would be interesting.):
#include <stdio.h>
int main() {
int dollar, cent;
int q = 0;
int d = 0;
int n = 0;
int p = 0;
int re;
printf("Enter amount: ");
scanf(" %d.%d", &dollar, &cent);
q = dollar * 4;
re = cent;
q = q + (re / 25);
re = re % 25;
d = re / 10;
re = re % 10;
n = re / 5;
re = re % 5;
p = re;
printf("q %d d %d n %d p %d\n", q, d, n, p);
return 0;
}
This approach also works if, for example, you're given the seconds and want to find the min:sec from that. If you're given 65 seconds, you do 65 / 60 for the minutes portion (which is 1), and the seconds portion is just the remainder after you divide by 60, or 65 % 60 (which is 5).

Answer 2

Here is a more complete answer that fits with the code that you gave us to start with. I changed the loops to subtraction/multiplication and fixed the bug where you were treating nickels as pennies. You don't need the counter variable anymore, but I left it in.
#include <stdio.h>
int main()
{
float cash;
int n;
int counter=0;
int quarters=0;
int dimes=0;
int nickels=0;
int pennies=0;
for (;;)
{
printf("Enter change amount: ");
scanf("%f",&cash);
if (cash > 0)
{
break;
}
}
n = cash * 100;
counter = 0;
if (n > 0)
{
quarters = (int)floor(n / 25);
n -= quarters*25;
printf( "%i\n", n );
dimes = (int)floor(n / 10);
n -= dimes*10;
printf("%i\n",n);
nickels = (int)floor(n / 5);
n -= nickels*5;
printf("%i\n",n);
pennies = n;
printf("%i\n",n);
}
printf("%i quarters, %i dimes, %i nickels, %i pennies\n",
quarters, dimes, nickels, pennies);
return 0;
}

Answer 3

First of all, try start with abstracting out how to handle one type of coin:
int coinsNeeded( int amount, int coinAmount )
{
return (int) floor( amount / coinAmount );
}
Then, handle each coin separately:
quarters = coinsNeeded( cash, 25 );
cash -= (quarters * 25);
Just repeat that for each type of coins you want to consider, and then print out the information at the end. There is some disagreement about whether you want to use floating points or not. Floating points do have rounding errors that you want to avoid when using money. what you actually need is a fixed point data type, but I digress. You can get close enough by doing it the way you're doing it (multiplying by 100 and just dealing with pennies).