DaniWeb Forum Index > Software Development > C++

variance calc incorrect why?

Please support our C++ advertiser: Programming Forums - DaniWeb Sister Site

Thread Solved

•

Join Date: Aug 2006

Posts: 9

Reputation: Sashar400 is an unknown quantity at this point

Solved Threads: 0

Sashar400

Offline

Newbie Poster

variance calc incorrect why?

Sep 27th, 2006

#include <iostream>
#include<iomanip>
#include <cmath>
usingnamespace std;
int main()
{

char choice;
const int arraysize=20;
int num[arraysize];
do
{

cout<<"I will give you the Sum,Mean,Var & the Std Dev of any series of numbers?Y/N:"<<endl;
cin>>choice;
if(choice =='Y'||choice =='y')
{

cout<<"How many numbers will you enter? (up to 20)?"; cin>>num[arraysize];
for (int j=0;j<num[arraysize];j++)
{
cout<<"Enter Number"<<j+1<<":"; cin>> num[j];
}

cout << " You have entered the following:"<<endl;
for (j = 0; j < num[arraysize]; j++)
{

cout << num[j]<<" ";
}
int sum=0;
for (j=0;j<num[arraysize];j++)
sum+= num[j];
cout<<"\nThe Sum is "<<sum<<endl;
float mean= (sum/num[arraysize]);
cout << showpoint << fixed << setprecision (2);
cout<<"The Mean is "<<mean<<endl;
float var=((num[0]-mean)*pow(num[0]-mean,2) + (num[1]-mean)*pow(num[1]-mean,2) + + (num[2]-mean)*pow(num[2]-mean,2))/ (num[arraysize-1]);
cout << showpoint << fixed << setprecision (2);
cout<<"The Varience is " <<var<<endl;
float sqrtvar=sqrt(var);
cout << showpoint << fixed << setprecision (2);
cout<<"The Standard deviation is "<<sqrtvar<<endl;
}
}while(choice!='N'&& choice!='n');
return 0;

}

•

Join Date: Jun 2005

Posts: 275

Reputation: andor has a spectacular aura about

Solved Threads: 29

andor

Offline

Posting Whiz in Training

Re: variance calc incorrect why?

Sep 27th, 2006

And the question is?

If you want to win, you must not loose (Alan Ford)

•

Join Date: Jun 2006

Posts: 7,651

Reputation: ~s.o.s~ has much to be proud of

Solved Threads: 474

~s.o.s~

Offline

Failure as a human

Re: variance calc incorrect why?

Sep 27th, 2006

•

Originally Posted by Sashar400

Help with Code Tags

(Toggle Plain Text)
#include <iostream>
#include<iomanip>
#include <cmath>
usingnamespace std;
int main()
{ 

char choice;
int total_numbers  = 0 ;
const int arraysize=20;
int num[arraysize]; 
do
{

cout<<"I will give you the Sum,Mean,Var & the Std Dev of any series of numbers?Y/N:"<<endl;
cin>>choice; 
if(choice =='Y'||choice =='y')
{ 

cout<<"How many numbers will you enter? (up to 20)?";
// cin>>num[arraysize];  this is wrong it means entering value in 20th element
cin >> total_numbers ;  // this is correct way of accpeting single number
 for (int j=0; j < total_numbers; j++)
{
cout<<"Enter Number"<<j+1<<":"; cin>> num[j];
}

cout << " You have entered the following:"<<endl;
for (j = 0;  j < total_numbers; j++)
{
cout << num[j]<<" ";
}
int sum=0;
for (j=0; j <  total_numbers; j++)
sum+= num[j];
cout<<"\nThe Sum is "<<sum<<endl;
float mean= (sum/ total_numbers);
cout << showpoint << fixed << setprecision (2);
cout<<"The Mean is "<<mean<<endl;
float var=((num[0]-mean)*pow(num[0]-mean,2) + (num[1]-mean)*pow(num[1]-mean,2) + + (num[2]-mean)*pow(num[2]-mean,2))/ (num[arraysize-1]); 
cout << showpoint << fixed << setprecision (2);
cout<<"The Varience is " <<var<<endl;
float sqrtvar=sqrt(var);
cout << showpoint << fixed << setprecision (2);
cout<<"The Standard deviation is "<<sqrtvar<<endl;
}
}while(choice!='N'&& choice!='n');
return 0;

}
#include <iostream> #include<iomanip> #include <cmath> usingnamespace std; int main() { char choice; int total_numbers = 0 ; const int arraysize=20; int num[arraysize]; do { cout<<"I will give you the Sum,Mean,Var & the Std Dev of any series of numbers?Y/N:"<<endl; cin>>choice; if(choice =='Y'||choice =='y') { cout<<"How many numbers will you enter? (up to 20)?"; // cin>>num[arraysize]; this is wrong it means entering value in 20th element cin >> total_numbers ; // this is correct way of accpeting single number for (int j=0; j < total_numbers; j++) { cout<<"Enter Number"<<j+1<<":"; cin>> num[j]; } cout << " You have entered the following:"<<endl; for (j = 0; j < total_numbers; j++) { cout << num[j]<<" "; } int sum=0; for (j=0; j < total_numbers; j++) sum+= num[j]; cout<<"\nThe Sum is "<<sum<<endl; float mean= (sum/ total_numbers); cout << showpoint << fixed << setprecision (2); cout<<"The Mean is "<<mean<<endl; float var=((num[0]-mean)*pow(num[0]-mean,2) + (num[1]-mean)*pow(num[1]-mean,2) + + (num[2]-mean)*pow(num[2]-mean,2))/ (num[arraysize-1]); cout << showpoint << fixed << setprecision (2); cout<<"The Varience is " <<var<<endl; float sqrtvar=sqrt(var); cout << showpoint << fixed << setprecision (2); cout<<"The Standard deviation is "<<sqrtvar<<endl; } }while(choice!='N'&& choice!='n'); return 0; }

I think you gettting the fundamentals of programming wrong. Why you taking in the number of elements entered by the user into an array. Why not just use a simple int variable like "total_numbers".

I have made some changes in teh program, try running it now and see what you get. And also format your code a bit so it can be read by other people and put the code in the [code] tags.

Hope it helped ,bye.

Last edited by ~s.o.s~; Sep 27th, 2006 at 12:45 pm.

I don't accept change; I don't deserve to live.

Jo Tujhe Jagaaye, Nindein Teri Udaaye Khwaab Hai Sachcha Wahi.
Nindon Mein Jo Aaye Jise To Bhul Jaaye Khawab Woh Sachcha Nahi.
Khwaab Ko Raag De, Nind Ko Aag De

•

Join Date: Aug 2006

Posts: 9

Reputation: Sashar400 is an unknown quantity at this point

Solved Threads: 0

Sashar400

Offline

Newbie Poster

Re: variance calc incorrect why?

Sep 27th, 2006

same result

•

Join Date: Dec 2005

Posts: 5,850

Reputation: Salem has a reputation beyond repute

Solved Threads: 751

Salem

Offline

Void main'ers are DOOMed

Re: variance calc incorrect why?

Sep 27th, 2006

See also http://www.daniweb.com/techtalkforums/thread56253.html
> usingnamespace std;
Now I don't know whether this is an endemic problem with your coding, but it should be using namespace std;, that is, there is a space in there.

Now maybe it's you or maybe it's some damn fool code colouring script you're running which is bugged to hell and back. Either way, it's not helpful to have to second guess what the difference is between what you posted and what we see.

Just post your code between tags, direct from your source code editor. We don't need colouring in a variable width font if the price is mangled code. Keep it nice and simple in a monospaced font.

•

Join Date: Jun 2006

Posts: 7,651

Reputation: ~s.o.s~ has much to be proud of

Solved Threads: 474

~s.o.s~

Offline

Failure as a human

Re: variance calc incorrect why?

Sep 27th, 2006

I think you are getting the formulas for deviation and variace wrong.

See this code for reference;

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include<iomanip>
#include <cmath>
using namespace std;
 
int main()
{
    float deviation, var ;
    char choice;
    int total_numbers  = 0 ;
    const int arraysize=20;
    int num[arraysize];
    do
    {
        cout<<"I will give you the Sum,Mean,Var & the Std Dev of any series of numbers?Y/N:"<<endl;
        cin>>choice;
        if(choice =='Y'||choice =='y')
        {
            cout<<"How many numbers will you enter? (up to 20)?";
            cin >> total_numbers ;  // this is correct way of accpeting single number
            for (int j=0; j < total_numbers; j++)
            {
                cout<<"Enter Number"<<j+1<<":"; cin>> num[j];
            }
            cout << " You have entered the following:"<<endl;
            for ( int j = 0;  j < total_numbers; j++)
            {
                cout << num[j]<<" ";
            }
 
            int sum=0;
            for (int j=0; j <  total_numbers; j++)
                sum+= num[j];
            cout<<"\nThe Sum is "<<sum<<endl;
            float mean= (sum/ total_numbers);
            cout << showpoint << fixed << setprecision (2);
            cout<<"The Mean is "<<mean<<endl;
 
            if (total_numbers > 1)
            {
                for (int i = 0; i < total_numbers; ++i )
                {
                    var += ((num [i] - mean) * (num [i] - mean)) ;
                }
                var /= (total_numbers - 1) ;
                deviation =sqrt(var);
            }
 
            else
            {
                var = 0.0 ;
                deviation = 0.0 ;
            }
            cout << showpoint << fixed << setprecision (2);
            cout<<"The Varience is " <<var<<endl;
            cout << showpoint << fixed << setprecision (2);
            cout<<"The Standard deviation is "<<deviation<<endl;
        }
    }
    while(choice!='N'&& choice!='n');
    return 0;
 
}
#include <iostream>
#include<iomanip>
#include <cmath>
using namespace std;

int main()
{
    float deviation, var ;
    char choice;
    int total_numbers  = 0 ;
    const int arraysize=20;
    int num[arraysize];
    do
    {
        cout<<"I will give you the Sum,Mean,Var & the Std Dev of any series of numbers?Y/N:"<<endl;
        cin>>choice;
        if(choice =='Y'||choice =='y')
        {
            cout<<"How many numbers will you enter? (up to 20)?";
            cin >> total_numbers ;  // this is correct way of accpeting single number
            for (int j=0; j < total_numbers; j++)
            {
                cout<<"Enter Number"<<j+1<<":"; cin>> num[j];
            }
            cout << " You have entered the following:"<<endl;
            for ( int j = 0;  j < total_numbers; j++)
            {
                cout << num[j]<<" ";
            }

            int sum=0;
            for (int j=0; j <  total_numbers; j++)
                sum+= num[j];
            cout<<"\nThe Sum is "<<sum<<endl;
            float mean= (sum/ total_numbers);
            cout << showpoint << fixed << setprecision (2);
            cout<<"The Mean is "<<mean<<endl;

            if (total_numbers > 1)
            {
                for (int i = 0; i < total_numbers; ++i )
                {
                    var += ((num [i] - mean) * (num [i] - mean)) ;
                }
                var /= (total_numbers - 1) ;
                deviation =sqrt(var);
            }

            else
            {
                var = 0.0 ;
                deviation = 0.0 ;
            }
            cout << showpoint << fixed << setprecision (2);
            cout<<"The Varience is " <<var<<endl;
            cout << showpoint << fixed << setprecision (2);
            cout<<"The Standard deviation is "<<deviation<<endl;
        }
    }
    while(choice!='N'&& choice!='n');
    return 0;

}

Maybe for the formula you should look here;

http://www.cs.mtu.edu/~shene/COURSES/cs201/NOTES/chap08/means-8.html

Hope it helped, bye.

•

Join Date: Sep 2006

Posts: 84

Reputation: rinoa04 is on a distinguished road

Solved Threads: 4

rinoa04

Offline

Junior Poster in Training

Re: variance calc incorrect why?

Sep 28th, 2006

Like what ~S.O.S~ have said. The formula for variance in your coding is wrong. The formula of a variance is like this. Take the example that user input 3 numbers 1,2,3 and the mean is 2

(1-2)^2+(2-2)^2+(3-2)^2)/3. Your variance formula seems to be fixed up to 3 numbers. Therefore, the answer will be incorrect no matter how many numbers are input. Below is one of the way to correct the calculation.

Help with Code Tags

(Toggle Plain Text)

float var=0.0;
for (j=0;j<num[arraysize];j++)
{
var += (float)pow(num[j]-mean,2);
}
cout << "array=" << num[arraysize] << " Var=" << var << endl;
var /= num[arraysize];

Hope it helps.

Last edited by rinoa04; Sep 28th, 2006 at 6:01 am.