如预期浮点数不工作 [英] Floating point numbers do not work as expected

问题描述

在下面当我给输入作为110 2 1 2 2，总和打印为52和sum3为31.200001了code，而它SHLD已经31.200000

  INT的main（）{    INT T，N，I，A [2000]中，M，J，F;
    scanf函数（％d个，＆amp; T公司）;
    而（T  - ）{
        scanf函数（％d个，＆安培; N）;
        scanf函数（％d个，＆安培; F）;
        对于（i = 0; I＆LT; F;我++）{
            scanf函数（％d个，＆安培; A []）;
        }
        scanf函数（％d个，＆安培; M）;
        如果（N！= 0）{
            INT总和= N *（N + 1）/ 2;
            INT SUM2 = 0;
            为（J = 0; J＆LT; I; J ++）{
                SUM2 + = A [J]。
            }
            sum- = SUM2;
            的printf（％d个\\ N，总和）;
            浮sum3;
            如果（N％2 == 0）sum3 =（1.0-2.0 * M / N）*总和;
            别的sum3 =（1.0-2.0 *米/（N + 1））*总和;
            的printf（％F \\ N，sum3）;
        }
        其他的printf（0.0000 \\ n）;
    }
    返回0;
}
 

解决方案

浮点指南：

  

为什么不是我的号码，像0.1 + 0.2加起来一个漂亮的圆0.3，
  而是我得到一个奇怪的结果，像0.30000000000000004？

  
  

由于内部，电脑使用的格式（二进制浮点）
  不能准确地重新present一个像0.1，0.2或0.3的。

  
  

当code编译或间preTED，你的0.1已经是
  圆形到该格式的最接近的数字，这导致一个小
  舍入误差的计算甚至发生之前。

In the code below when i give input as 1 10 2 1 2 2, sum is printed as 52 and sum3 as 31.200001 whereas it shld have been 31.200000

int main(){

    int t,n,i,a[2000],m,j,f;
    scanf("%d",&t);
    while(t--){
        scanf("%d",&n);
        scanf("%d",&f);
        for(i=0;i<f;i++){
            scanf("%d",&a[i]);
        }
        scanf("%d",&m);
        if(n!=0){
            int sum=n*(n+1)/2;
            int sum2=0;
            for(j=0;j<i;j++){
                sum2+=a[j];
            }
            sum-=sum2;
            printf("%d\n",sum);
            float sum3;
            if(n%2==0) sum3=(1.0-2.0*m/n)*sum;
            else sum3=(1.0-2.0*m/(n+1))*sum;
            printf("%f\n",sum3);
        }
        else printf("0.0000\n");
    }
    return 0;
}

解决方案

From the Floating-Point Guide:

Why don’t my numbers, like 0.1 + 0.2 add up to a nice round 0.3, and instead I get a weird result like 0.30000000000000004?

Because internally, computers use a format (binary floating-point) that cannot accurately represent a number like 0.1, 0.2 or 0.3 at all.

When the code is compiled or interpreted, your "0.1" is already rounded to the nearest number in that format, which results in a small rounding error even before the calculation happens.

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