将货币价值作为浮动进行比较不起作用 [英] Comparing currency values as floats does not work

问题描述

如果我输入 0.01 或 0.02，程序会立即退出.我该怎么办?

If I type 0.01 or 0.02 the the program just exits immediately. What do I do?

#include <stdio.h>

char line[100];
float amount;

int main()
{
printf("Enter the amount of money: ");
fgets(line, sizeof(line), stdin);
sscanf(line, "%f", &amount);

if (amount == 0.0)
    printf("Quarters: 0
Dimes: 0
Nickels: 0
Pennies: 0");

if (amount == 0.01)
   printf("Quarters: 0
Dimes: 0
Nickels: 0
Pennies: 1");


if (amount == 0.02)
   printf("Quarters: 0
Dimes: 0
Nickels: 0
Pennies: 2");

return (0);
}

推荐答案

对这个答案的反对票是当之无愧的，它错误地提倡简单的平等测试.虽然这是未定义的行为，不应该被鼓励，但即使如此，它也会有时起作用.

The downvotes on this answer are well deserved, it was wrongly advocating for a simple equality test. While this is undefined behaviour and should not be encouraged, even so it happens to work sometimes.

您永远不应该测试浮动值的相等性.而是使用 epsilon 值根据范围测试您的变量，因为 this answer 建议这样做.

You should never test a floating value for equality. Instead use an epsilon value to test your variable against a range as this answer is suggesting it.

例如以这个例子:

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

#define  EPSILON 0.00001f

int main() {
    float a = 3.234324333f;
    float b = a;
    b += 1;
    b -= 1;
    
    printf("a value is %f
", a);
    printf("b value is %f
", b);

    printf("
a == b ?
");
    if (a == b) {
        printf("true
");
    } else {
        printf("false
");
    }

    printf("a ~= b with epsilon = %f ?
", EPSILON);
    if (fabs(a - b) <= EPSILON) {
        printf("true
");
    } else {
        printf("false
");
    }
    return 0;
}

当运行时:

a value is 3.234324
b value is 3.234324

a == b ?
false
a ~= b with epsilon = 0.000010 ?
true

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