Need some help
I am learning C++, working through the books recommended by this website. I'm trying to code Exercise 6-3 of Practical C++ Programming. Here's the problem:
Given an amount less than $1.00, compute the number of quarters, dimes, nickels, and pennies needed.
Here's my code:
For some reason, it's not calculating the right amount. For example, if I run the program and put in 0.35, when it gets to this part
float money; // amount of money
int quarters, dimes, nickels, pennies; // Stores the amount of each coin needed.
std::cout<< "Enter a non-negative amount of money " // Prompt user for money
<< "less than $1.00, or -1 to stop: "; // amount
std::cin >> money;
if (money == -1) // Loop until user enters -1
if (money < -1 || money >= 1.0) // Start loop over if money
continue; // amount is invalid.
std::cout << "$" << money << " in change is ";
quarters = (money / 0.25); // Stores the amount of quarters needed, then subtracts
money -= (quarters * 0.25); // that amount in cents from the money total.
dimes = (money / 0.1); // Stores the amount of dimes needed, then subtracts
money -= (dimes * 0.1); // that amount in cents from the money total.
nickels = (money / 0.05); // Stores the amount of nickels needed, then subtracts
money -= (nickels * 0.05); // that amount in cents from the money total.
pennies = (money * 100); // Money is in cents, so it is multiplied by 100 to convert
// it to the amount of pennies needed.
std::cout << quarters << " quarters, " << dimes << " dimes, "
<< nickels << " nickels, " << " and " << pennies << " pennies.\n";
, I know money is equal to 0.1 at that point, but dimes is not being set equal to 1. I put in
dimes = (money / 0.1)
just to make sure that money equals 0.1 at that point, which it does. I also tried
std::cout << money;
to see that money, when it is 0.1, divided by 0.1 does indeed equal 1, which it does. But it won't set dimes equal to 1. If I run the program at 0.36, it calculates everything right. What am I doing wrong?
std::cout << (money / 0.1)
It has to do with the fact that computers cant handle floating point ("real numbers") perfectly. To see this, do:
Since "money" can be considered a discrete quantity, you could just work with pennies, as "ints", and after you get the money as a float just multiply it by 100, so you only work with pennies (discrete quantities). Later just divide by 100. This will ensure correct comparisons, since your working with ints.
float foo = 0.1;
if ( foo == 0.1)
std::cout << "you would think its equal" << std::endl;
std::cout << "but its not equal" << std::endl;
Ok, I tried working with pennies. I did this after getting money as a float from user:
It stores the wrong amount sometimes. If I put in 0.35, it stores 34 in pennies. Some numbers work, such as 0.36 or 0.37 (0.38 stores as 37).
pennies = (money * 100); // stores money as pennies
So if you do
it prints "34"? Or even by doing "cin >> f" instead of hardcoding it, you get 34?
float f = 0.35;
int i = 100*f;
cout << i;
EDIT: Compilers are probably a factor too, what are you using?
Yeah. Using exactly this code:
I got this:
f = 0.35;
i = f * 100;
std::cout << i << std::endl;
Press any key to continue
I'm using Microsoft Visual Studio .NET 2003.
Try forcing an int by casting i.e.:I get 34 when its interpreted as a float, i.e.:and forcing it to be an int I get 35.
I tried that, and it still didn't work. However, I was getting the warning "truncation from double to float" on this lineI tried changing it to a double, and it works now. Given that the double f equals 0.35, (f * 100) stored into an integer gives me 35.
if you do "f = (float) 0.35;", and leave f as a float, do you later get integer 35?
No, it's still 34. I tried this though
This prints out 35.
f = 0.35;
f *= 100;
i = f;
std::cout << i << std::endl;
Prefer double over float.
To create a float, you must do x.yf. Notice the "f" at the end. That will make the compiler treat the number as a float rather than a double.
Nevertheless, I would still urge you to use integer math.