c++ - 从文本文件读入 vector

Question

我很难理解为什么我的代码不起作用。有问题的部分是当我将值从我的文本文件传递到我的成绩变量时。我不明白为什么这是错误的。除此以外，我的代码的其他一切都很好，但无论如何我都包含了它。

string fileName;

cout << "Program that takes data from file and calculate\nmean and standard deviation and put it in file out.txt\n";
cout << "Enter the input file name ";
cin  >> fileName;
  double grade;
ifstream inData;

inData.open(fileName.c_str());

//  Declare vector<double> vecx
vector<double> vecx;
//  read data from input file to vector vecx,
while(inData >> grade)
{
    vecx.push_back(grade);
}


inData.close(); 
//  keep track how many elements you read from file
//  When you done with reading from file, close stream inData

如果您有兴趣，这里是完整的代码。

#include <iostream>
#include <fstream>
#include <vector>
#include <cmath>

using namespace std;

int main()
{
    string fileName;

    cout << "Program that takes data from file and calculate\nmean and standard deviation and put it in file out.txt\n";
    cout << "Enter the input file name ";
    cin  >> fileName;
      double grade;
    ifstream inData;

    inData.open(fileName.c_str());

    //  Declare vector<double> vecx
    vector<double> vecx;
    //  read data from input file to vector vecx,
    while(inData >> grade)
    {
        vecx.push_back(grade);
    }


    inData.close(); 
    //  keep track how many elements you read from file
    //  When you done with reading from file, close stream inData

    // read element by element  in vector vecx  and calculate sum;
    double sum=0;
    double average;
    for (int i=0; i < vecx.size(); i++)
        sum=sum+vecx[i];

    average=sum/(vecx.size());
    // sum divide by number of elements to find mean(average)

    //again read element by element and calculate
    //square of difference between element and mean
    //calculate sum of squares of differences
    //divide by number of elements and
    //take square root - you got the standard deviation

    sum=0;
    double variance, stdev;
    for (int i=0; i < vecx.size(); i++)
        sum=sum+(vecx[i]-average*vecx[i]-average);

    variance=sum/(vecx.size());
    stdev=sqrt(variance);

    //open output stream
    ofstream outData;
    outData.open("out.txt");
    //output  mean and standard deviation

    cout << "Average is " << average << endl;
    cout << "Standard deviation is " << stdev << endl;

    //close stream
    outData.close();
}

我觉得自己像个白痴，因为我不明白为什么这不起作用......我应该能够弄清楚，但我没有。

Answer 1

C++ 遵循先例的正常数学顺序。

这一行:

sum=sum+(vecx[i]-average*vecx[i]-average);

看起来不正确。乘法发生在减法之前，所以它正在计算:

sum=sum+((vecx[i]-(average*vecx[i]))-average);

但大概你的意思是:

sum=sum+((vecx[i]-average)*(vecx[i]-average));

您可能有兴趣查看 the full list of precedence ordering .