While循环会忽略比较两个浮点变量的布尔表达式

Question

Edit:
I solved the issue by first multiplying the float value by 100, then rounding it with roundf() function, then casting it to an integer to be stored in an integer variable. I did the remaining operations with integer values from there on and everything worked. Even though the solution offered by @JacobBoertjes actually worked, my assignment requiered me to use get_float() from the cs50.h library, so I didn't implement it. Here's the final code:

// Get user input as a positive float value
float f_change;
do {
    printf("Change owed: ");
    f_change = get_float();
} while(f_change < 0);

// Round & cast
int int_change = (int) roundf(f_change * 100);

---

My program accepts an amount of money, say $4.20, and figures out the least amount of coins with which it can represent this value. For example, desired output from the program with $4.20 as an input would be: 16 quarters ($4.00), 2 dimes ($0.20).

My program successfully calculates the number of quarters, but fails to do so while working on dimes. The cause of this failure is the second for loop in the code. 0.10 >= 0.10 does not evaluate to true, so the last iteration of the loop never happens. What am I doing wrong?

Here is the code. I provided test print statements with their outputs written as comments.

#include <stdio.h>
#include <cs50.h>

int main(void) {

  // Fake user input
  float owed_coin = 4.2f;

  // Initialize coin variables
  int coin_count = 0;

  float quarters = 0.25f,
        dimes = 0.10f;

  // Calculate quarters
  while(owed_coin >= quarters) {
    owed_coin -= quarters;
    coin_count += 1;
  }
  printf("owed_coin: %.2f\ncoin_count: %d\n\n", owed_coin, coin_count);
  // Prints  owed_coin: 0.20
  //         coin_count: 16

  // Calculate dimes
  while(owed_coin >= dimes) {
    owed_coin -= dimes;
    coin_count += 1;
  }
  printf("owed_coin: %.2f\ncoin_count: %d\n\n", owed_coin, coin_count);
  // Prints  owed_coin: 0.10
  //         coin_count: 17

}

Answer 1

Floating point comparison is generally a bad idea because floats often become non-exact and thus will not be exactly equal. As @bruceg mentioned, a good solution is to keep your monetary values in terms of cents, so that you avoid using a float.

You could replace float owed_coin = 4.2f; with int owed_coin = 420;

In terms of gathering user input into this number, here is my suggestion using scanf

int n1, n2;

printf("Please enter amount:");
scanf("%d.%2d", &n1, &n2);

owed_coin = n1*100 + n2;

---

Another solution allows you you keep your variables as floats, and just compare for a very small difference between the two. It can be found here: What's wrong with using == to compare floats in Java?

It uses the Java Math library, but a similar solution could look something like this in C:

while((owed_coin - dimes) >= -0.001) {
    owed_coin -= dimes;
    coin_count += 1;
}

If you want to learn more about why floating point numbers suffer small innacuracies then check this out: https://en.wikipedia.org/wiki/Floating-point_arithmetic#Accuracy_problems