如何使用stl更改打印的精度？ [英] How can I change the precision of printing with the stl?

问题描述

我想使用带有小数位数的stl打印数字到文件，而不是整体精度。

所以，如果我这样做：

  int precision = 16 ; 
 std :: vector< double> thePoint（3）; 
 thePoint [0] = 86.3671436; 
 thePoint [1] = -334.8866574; 
 thePoint [2] = 24.2814; 
 ofstream file1（tempFileName，ios :: trunc）; 
 file1<< std :: setprecision（precision）
<< thePoint [0]<< \\
<< thePoint [1]<< \\
<< thePoint [2]< \\; 
  

我会得到这样的数字：

  86.36714359999999\-334.8866574\24.28140258789063 
  

希望是这样：

  86.37\-334.89\24.28 
  

换句话说，截断两个小数点。如果我设置精度为4，那么我会得到

  86.37\-334.9\24.28 
  
 
 
 
即第二个数字被不正确地截断。
 
 
 
不想明确操作每个数字，以获得截断，特别是因为我似乎得到偶尔的9重复或0000000001或类似留下的东西。
 
 
 
我肯定有一些明显的，像使用printf（％。2f）或类似的东西，但我不确定如何混合stl< and ofstream。
解决方案
使用std :: fixed可以为你工作。
  file1<< std :: fixed<< std :: setprecision（precision）
< thePoint [0]<< \\
<< thePoint [1]<< \\
<< thePoint [2]< \\; 
  
 
I want to print numbers to a file using the stl with the number of decimal places, rather than overall precision. 

So, if I do this:
int precision = 16;
std::vector<double> thePoint(3);
thePoint[0] = 86.3671436;
thePoint[1] = -334.8866574;
thePoint[2] = 24.2814;
ofstream file1(tempFileName, ios::trunc);
file1 << std::setprecision(precision)
    << thePoint[0]  << "\\"
    << thePoint[1]  << "\\"
    << thePoint[2] << "\\";
I'll get numbers like this:
86.36714359999999\-334.8866574\24.28140258789063
What I want is this:
86.37\-334.89\24.28
In other words, truncating at two decimal points.  If I set precision to be 4, then I'll get
86.37\-334.9\24.28
ie, the second number is improperly truncated.

I do not want to have to manipulate each number explicitly to get the truncation, especially because I seem to be getting the occasional 9 repeating or 0000000001 or something like that that's left behind.

I'm sure there's something obvious, like using the printf(%.2f) or something like that, but I'm unsure how to mix that with the stl << and ofstream.
解决方案
Use std::fixed , this should work for you.
 file1 << std::fixed << std::setprecision(precision)
     << thePoint[0]  << "\\"
     << thePoint[1]  << "\\"
     << thePoint[2] << "\\";


                        
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