Read from a text file into a vector

Question

I'm having a hard time seeing why my code doesn't work. The part that's a problem is when I'm passing values from my text file into my grade variable. I don't see why this is wrong. Everything else with my code is fine except for that, but I included it all anyway.

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

And here is the full code in case you were interested.

#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();
}

I feel like an idiot for not seeing why this isn't working...I should be able to figure this out but I haven't.

Answer 1

C++ follows the normal mathematical orders of precedent.

This line:

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

looks incorrect. Multiplication happens before subtraction so it's calculating this:

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

but presumably you meant this:

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

You might be interested in seeing the full list of precedence ordering .

Answer 2

You are never writing data out to your file. Use your ofstream outData instead of std::cout . Otherwise, you are only sending your output to the console.

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

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

//close stream
outData.close();