C++ 设置小数点后的位数
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/3923202/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Set the digits after decimal point
提问by user474401
I have a float number for example 12.12123 Is there a function which would display only number with 2 digits after decimal point 12.12 ?
我有一个浮点数，例如 12.12123 是否有一个函数可以只显示小数点后 2 位的数字 12.12 ？
Here is the code:
这是代码：
y1 = ( c1 - (a1 * x)) / b1;
y2 = ( c2 - a2 * x) / b2;
if (y1 == y2)
cout << "The same";
so if the y1 = 1.001 and the y2 = 1.002 they do not appear as the same.
所以如果 y1 = 1.001 和 y2 = 1.002 它们看起来不一样。
I tried to add. cout.setf(ios::fixed, ios::floatfield); cout.precision(2);
我试着补充。cout.setf(ios::fixed, ios::floatfield); cout.precision(2);
but it does not seem to help.
但它似乎没有帮助。
回答by Matteo Italia
/* The C way */
#include <stdio.h>
...
float f = 12.12123f;
printf("%.2f",f);
// The C++ way
#include <iostream>
...
float f = 12.12123f;
std::cout.setf(std::ios_base::fixed, std::ios_base::floatfield);
std::cout.precision(2);
std::cout << f;
// The alternative C++ way
#include <iostream>
#include <iomanip>
...
float f = 12.12123f;
std::cout << std::fixed << std::setprecision(2) << f;
In C, the 0 padding is added automatically to the right if there are not enough digits to print. In the C++ examples, instead, this is disabled; to enable this behavior, you should enable the fixed mode on the stream with std::fixed(or enabling the relevant stream flags with std::ios_base::setf()).
在 C 中，如果没有足够的数字可打印，则 0 填充会自动添加到右侧。在 C++ 示例中，这是禁用的；要启用此行为，您应该在流上启用固定模式std::fixed（或启用相关流标志std::ios_base::setf()）。
Edit: I remembered wrong; if fixedis not set, the precisionsetting says to the stream the totalnumber of digits to display, including also the ones before the decimal point. So, in this case I think that the only way is to use the fixedmode (examples fixed), which will yield the same behavior of printf.
编辑：我记错了；如果fixed未设置，则precision设置会向流说明要显示的总位数，包括小数点之前的位数。因此，在这种情况下，我认为唯一的方法是使用fixed模式（示例已修复），这将产生printf.
Links:
链接：
- std::ios_base::precision()reference;
- std::setprecisionreference;
- printfreference.
回答by Frédéric Hamidi
You're looking for printf("%.2f", 12.12123);or:
您正在寻找printf("%.2f", 12.12123);或：
#include <iostream>
#include <iomanip>
using namespace std;
cout << fixed << setprecision(2) << 12.12123;
EDIT:Question changed, so does the answer.
编辑：问题改变了，答案也改变了。
You never want to use direct equality with floating point, you always compare within epsilontolerance. Your epsilonis just quite large.
您永远不想使用浮点直接相等，您总是在epsilon容差内进行比较。你epsilon只是相当大。
Replace if (y1 == y2)with if (abs(y1 - y2) < 0.01).
替换if (y1 == y2)为if (abs(y1 - y2) < 0.01)。
回答by patentfox
double num = 1.4567;
printf("%.2f",num);
This would print 1.46
这将打印 1.46
回答by dutt
cout.precision(2);
cout <<f;
回答by Armen Tsirunyan
#include <iostream>
#include <iomanip>
int main ()
{
double d = 1.242354345;
using namespace std;
cout << setiosflags(ios::fixed | ios::showpoint)
<< setprecision(2)
<< d;
}
回答by patentfox
You are probably asking wrong question. Try following:
你可能问错了问题。尝试以下操作：
double diff = fabs(y1-y2);
if(diff < 0.005)
cout << "Almost same";
回答by Ruel
cout.setf(ios::fixed, ios::floatfield);
cout.precision(2);
float al = 42.645; //sample
cout << al;
