Calc standard deviation

[samyulg]

I need to calculate the mean and standard deviation of 16 house prices. The data is being read in from a text file. The program compiles, but it does not correctly calculate the mean and standard deviation. Could this be a read in mishap or is something wrong in my calculation? Please help

#include <iostream>
#include <cmath>
#include <fstream>
using namespace std;

void instruct();

void readin(double num[], int& n);

double calc(double num[]);

int main()
{

//declarations
 int n;
 double num[100], mean, stdDev;

//input
instruct();

readin(num, n);

//processing
calc(num);

//output
cout << "Standard Deviation: " << stdDev << endl;
cout << "Mean: " << mean << endl;
}

 void instruct()
{
cout << "This program calculates the mean and standard deviation of house prices. " << endl;
}

void readin(double num[], int& n)
{
  ifstream infile;
  infile.open("prices.txt");
  n = 0;
  while (infile >> num[n]) n++;
}

double calc(double num[])
{
 double sum = 0.0, mean = 0.0, stdDev = 0.0;
 int i;

  for(i = 0; i < 16; i++)
  {
    sum += num[i];
  }

  mean = sum/16;

  return mean;

  for(i = 0; i < 16; i++)
  {
    stdDev += pow(num[i] - mean, 2);
  }

  return sqrt(stdDev / 16);
}

[Ganado]

First, check if your infile is opening successfully:

ifstream infile("prices.txt"); // line 39
if (! infile)
{
    cout << "Error: Could not open prices.txt!\n";
}

Second, a function can only return one thing.

Your calc function ends on line 57, after you return mean. The code beyond it is dead code (unreachable).

[samyulg]

I made sure the file was opening properly and it was. I also made 2 separate functions to calculate the mean and standard deviation but I'm still getting weird numbers for my mean and standard deviation. When I run it I get:
This program calculates the mean and standard deviation of house prices.
Standard Deviation: 6.94649e-310
Mean: 6.94649e-310

#include <iostream>
#include <cmath>
#include <fstream>
using namespace std;

void instruct();

void readin(double num[], int& n);

double calcMean(double num[]);

double calcDev(double num[], double mean);

int main()
{

//declarations
 int n;
 double num[100], mean, stdDev;

//input
instruct();

readin(num, n);

//processing
calcMean(num);

calcDev(num, mean);
//output
cout << "Standard Deviation: " << stdDev << endl;
cout << "Mean: " << mean << endl;
}

void instruct()
{
cout << "This program calculates the mean and standard deviation of house prices. " << endl;
}

void readin(double num[], int& n)
{
  ifstream infile;
  infile.open("prices.txt");
  n = 0;
  while (infile >> num[n]) n++;
}
double calcMean(double num[])
{
 double sum = 0.0, mean = 0.0;
 int i;

  for(i = 0; i < 16; i++)
  {
    sum += num[i];
  }

  mean = sum/16;

  return mean;
}

double calcDev(double num[], double mean)
{
int i;
double stdDev = 0.0;
  for(i = 0; i < 16; i++)
  {
    stdDev += pow(num[i] - mean, 2);
  }

  return sqrt(stdDev / 16);
}

[Ganado]

You are never using the return value of your calcMean function.
The mean variable in main is never set to anything.

mean = calcMean(num);

Same thing for stdDev. You are never assigning a proper value to it. You should turn on compiler warnings in your IDE or compiler, because they would flag things like this as "uninitialized variable" warnings.

stdDev = calcDev(num, mean);

Also, you can declare and initialize variables on the same line.

So, for example, instead of doing

double mean;

// ...

mean = calcMean(num);

You can just do:

double mean = calcMean(num);

And i can be declared inside the for loop.

for (int i = 0; i < 16; i++)
    ...