Thread: functions

I tried different things.. so I returned that to what the professor put originally .... I know that won't work...

Actually if you are looking for variance ... the difference between the smallest and largest numbers you can simply do this...

Code:

double CalcVariance(int numberPts, double vals[]) 
  {    return FindBiggest(numberPts,vals) - FindSmallest(numberPts,vals); }

OR... since you already have biggest and smallest stored away at that point you can to this...

Code:

variance = biggest - smallest;

... you don't even need the function.

Surely you must have learned (retained?) something from writing those other functions???

I don't think that is what the user is talking about when they say variance. The calculation is a bit more involved than subtracting the smallest from the biggest.

From Wikipedia entry on variance:

The variance of a random variable or distribution is the expectation, or mean, of the squared deviation of that variable from its expected value or mean. Thus the variance is a measure of the amount of variation of the values of that variable, taking account of all possible values and their probabilities or weightings (not just the extremes which give the range).

For example, a perfect die, when thrown, has expected value of (1 + 2 + 3 + 4 + 5 + 6) / 6 = 3.5. Its expected absolute deviation — the mean of the equally likely absolute deviations from the mean (3.5 − 1, 3.5 − 2, 3.5 − 3, 4 − 3.5, 5 − 3.5, 6 − 3.5) — is (2.5 + 1.5 + 0.5 + 0.5 + 1.5 + 2.5) / 6 = 1.5. But its expected squared deviation —its variance (the mean of the equally likely squared deviations) — is (2.52 + 1.52 + 0.52 + 0.52 + 1.52 + 2.52) / 6 = 17.5/6 ≈ 2.9.

I know this isn't right... but I'm a little confused by what you said so if you can guide me on my errors it would be great. Its always says zero no matter what value I give it... This is what I have so far...
/*d)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

Code:

double CalcVariance(int numberPts, double vals[], double mean)
{
    double diff, sumsq = vals [0];
    int i;
    for (i = 0; i < numberPts; i++) 
        {
        vals [i] = mean - vals [i];
        sumsq = diff*diff + sumsq;
        // calculate the difference between mean and the current item in vals
        // add the square of the difference to sumsq
        }
   return sumsq/i;
    // divide the sum squared difference by the number of data points and return it
}

in case you need it all the code I have so far...

Code:

/* Melissa Schwager Homework 4
Write a program that will accept the number of points 
and then read in the points that are to be stored in an array.
The program will then call functions that
a) find the average of the values in the array 
b) find the largest value in the array 
c) find the smallest value in the array 
d) find the variance of the values */

#include <stdio.h>
#include <stdlib.h>

/*a)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
double findAve(int numberPts, double vals[])
{
    double sum = 0;
    int i;
    for (i = 0; i < numberPts; i++)
        sum = sum + vals[i];
    return sum/numberPts;
}
/*b)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
double findBiggest(int numberPts, double vals[])
{
    double biggest = vals[0];
    int i;
    for (i = 0; i < numberPts; i++)
        if (vals[i] > biggest)
        biggest = vals[i];
    return biggest;
}
/*c)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
double findSmallest(int numberPts, double vals[])
{
    double smallest = 0;
    int i;
    double sum;
    for (i = 0; i > numberPts; i++)
        if (vals[i] < smallest)
        smallest = vals[i]; /* Need code similar to findBiggest */
    return smallest;
}
/*d)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

double CalcVariance(int numberPts, double vals[], double mean)
{
    double diff, sumsq = vals [0];
    int i;
    for (i = 0; i < numberPts; i++) 
        {
        vals [i] = mean - vals [i];
        sumsq = diff*diff + sumsq;
        // calculate the difference between mean and the current item in vals
        // add the square of the difference to sumsq
        }
   return sumsq/i;
    // divide the sum squared difference by the number of data points and return it
}

int main()
{
    double values[20];
    int numpts, i;
    double mean;
    double biggest = -1000000; /*really negative */
    double smallest = -100000;
    double variance =  -100000;
    printf("Enter the number of points from 2 to 20: ");
    scanf("%d", &numpts);
    if (numpts <= 1 || numpts > 20)
    {
        printf("Only 2 to 20 points can be entered, please correct amount of values and retry\n");
        return 0;
    }
    for (i = 0; i < numpts; i++)
    {
        printf("Enter a value: ");
        scanf("%lf", &values[i]);
    }
    mean = findAve(numpts, values);
    printf("Average = %lf\n", mean);

    biggest = findBiggest(numpts, values);
    printf("Biggest = %lf\n", biggest);

    smallest = findSmallest(numpts, values);
    printf("Smallest = %lf\n", smallest);
 
    variance = CalcVariance(numpts, values, mean);
    printf("variance = %lf\n", variance);
   
    return 0;
}

thanks for any help!!!

Code:

 double diff, sumsq = vals [0];
    int i;
    for (i = 0; i < numberPts; i++)
        {
        vals [i] = mean - vals [i];
        sumsq = diff*diff + sumsq;

What does diff have for a value?


Quzah.

I really should post the 1st code, and not try and mess with it cuz I only seem to make it worst.
this is what I had after he informed me of what to do, before I started to mess with it and screw it up. Sorry about that.

Code:

double CalcVariance(int numberPts, double vals[], double mean)
{
    double diff, sumsq = 0;

    int i;
    for (i = 0; i < numberPts; i++) 
        {

        diff = mean - i;

        sumsq = diff*diff + sumsq;
        // calculate the difference between mean and the current item in vals
        // add the square of the difference to sumsq
        }
   return sumsq/i;

    // divide the sum squared difference by the number of data points and return it
}