Why am I getting garbage when I use pointers in this program? C Language

Question

I am a C++ programmer trying to get a grasp of the C language syntax and I do not understand why I'm getting garbage values when I use pointers in this function.

#include<stdio.h>
#include <math.h>

void separate(double, double *dollar, double *quarter, double *dime, double *nickle,  double *penny);
int
main(void)
{
double amount = 38.39, change, paid = 40.0, dollar, quarter, dime, nickle, penny;

change = paid - amount;
separate(change, &dollar, &quarter, &dime, &nickle, &penny);

printf("Your total change is: $%d\n", change);
printf("Dollars: %d\n", dollar);
printf("Quarters: %d\n", quarter);
printf("Dimes: %d\n", dime);
printf("Nickles: %d\n", nickle);
printf("Pennies: %d\n", penny);

getchar();
return (0);
}

void separate(double change, double *dollar, double *quarter, double *dime, double *nickle, double *penny)
{
double coins;
coins = change - floor(change);
*dollar = floor(change);
*quarter = coins / 25;
coins = coins - (*quarter * 25);
*dime = coins / 10;
coins = coins - (*dime * 10);
*nickle = coins / 5;
*penny = coins - (*nickle * 5);
}

Getting trash in all of your variables, or just change? – Skeen Mar 21 '13 at 01:43 — Skeen, Mar 21 '13 at 01:43
What is 'garbage' and what output are you expecting? – Mike D Mar 21 '13 at 01:45 — Mike D, Mar 21 '13 at 01:45
%d is a specifier for integers. – UncleO Mar 21 '13 at 01:51 — UncleO, Mar 21 '13 at 01:51

score 2 · Answer 1 · answered Mar 21 '13 at 01:50

you are printing using %d for doubles! try %f

printf("Your total change is: $%f\n", change);
printf("Dollars: %f\n", dollar);
printf("Quarters: %f\n", quarter);
printf("Dimes: %f\n", dime);
printf("Nickles: %f\n", nickle);
printf("Pennies: %f\n", penny);

score 1 · Answer 2 · answered Mar 21 '13 at 01:52

1

Indeed. Rather unintuitive, but %d in a printf statement is for integers.Try printing with %f for floating point numbers.

I also recommend you take a look at this:

answered Mar 21 '13 at 01:52

Sinkingpoint

7,449
2
29
45

score 1 · Answer 3 · 2013-03-21T02:07:23.843

1

You're trying to print double variables with %d, printf then will take the memory of double (or part of it) as memory of integer variable, since printf use va_list as a pointer to arguments, if you assign a inappropriate format, the type of arg point will go wrong too.

Replace all %d with %f.

edited Mar 21 '13 at 02:07

answered Mar 21 '13 at 01:52

score 1 · Accepted Answer · answered Mar 21 '13 at 01:54

I suspect that your problem is that you're not doing division correctly.

Take a look:

void separate(double change, double *dollar, double *quarter, double *dime, double *nickle, double *penny)
{
    // for your example, change is initially 1.61
    double coins;
    coins = change - floor(change);  // .61
    *dollar = floor(change);         // 1
    *quarter = coins / 25;           // .0244
    coins = coins - (*quarter * 25); // 0
    *dime = coins / 10;              // 0
    coins = coins - (*dime * 10);    // 0
    *nickle = coins / 5;             // 0
    *penny = coins - (*nickle * 5);  // 0
}

Integer division and floating point division don't work the same way. You should instead multiply by 100 to get the number of cents, divide by 25 to get the number of quarters, then floor to make it a whole number.

*quarters = floor(coins * (100 / 25)); // floor(2.44) = 2
coins = coins - *quarters * 25;        // .11

Repeat as needed to get quantities of other coins.

Also, everyone else has mentioned that you're using %d to print doubles instead of %f (%d is an integer format string) so I'll mention it too, because it's part of the problem.

Also, you're sooner or later bound to encounter weird behavior with this program due to floating point precision issues. It's almost universally agreed upon that using integral types for currency is safer and easier. — Wug, Mar 21 '13 at 01:57

Why am I getting garbage when I use pointers in this program? C Language

4 Answers4