Help with program

[joemacconnell]

I have to write a program to calculate sum, mean, variance, and standard deviation and I was able to do the sum and mean but I don't understand the variance and standard deviation.

If the size of the set is 7 and the set is represented by X[0], X[1], X[2], X[3], X[4], X[5], X[6], then the (biased) variance may be computed as:


variance = [(X[0]-mean)2 + (X[1]-mean)2 + ... + (X[6]-mean)2]/7 and the standard deviation is the square root of the variance (use sqrt() in <math.h> to calculate square root of a value)

Run your program two times, using two different sets of data:

· Run I: 7 numbers --- 12, 22, 34, 43, 45, 54, 99
Output:
sum: 309
average: 44.1429
variance: 676.408 <--
standard deviation: 26.0078 <--
*last two don't compute correctly when running the program
· Run II: 8 numbers -- 1.0, 1.5, 2.5, -9.3, 0, -1.0, 3.0, 6.4


Can anyone help? Here is my code so far:

// This program calculates the sum, average, variance, and standard deviation of any numbers.
// Using the for structure
using namespace std;
#include <iostream>
#include <math.h>
 
int main()
{
   int n, count;
   double x, sum, avg, var, sd;
 
   sum = 0;
   cout << "How many numbers?  ";
   cin >> n;
   int array[n];
   
   for(count=0; count<n; count++){
       cout << "Enter Number:  "; //Changed prompt so it was more user friendly.
         cin >> array[n];
        sum = sum + array[n];
   }  //end for

   var = 0;
   for (count=0; count<n; count++){
       var += pow(array[n]-avg,2);
   }
   var /= n;

   avg = sum / n;
   sd= sqrt(var);
   cout << "The sum is " << sum << endl;
   cout << "The average is  " << avg << endl;
   cout << "The variance is  " << var << endl;
   cout << "The standard deviation is  " << sd << endl;
   return 0;   //successful termination

 }//end main 

[JewelCpp]

in line 26:
it should be var+=pow(array[count]-avg,2);
other than that you did actually understood variance and standard deviation

[ne555]

http://www.cplusplus.com/forum/general/112111/

$ g++ foo -W{all,extra,pedantic}
foo.cpp: In function ‘int main()’:
foo.cpp:15:15: warning: ISO C++ forbids variable length array ‘array’ [-Wvla]
foo.cpp:10:11: warning: unused variable ‘x’ [-Wunused-variable]
foo.cpp:25:33: warning: ‘avg’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Compute the average before the variance.