trouble with calculations

[lindag1783]

Hi, I really appreciate your help here. The variance is worked out like the first example that you redirected my browser to regarding variance (the website about it). Example: Numberofscores(N)=2, score1=2, score2=5, all the other scores are input as 0 (seeing there is no scores). The mean is worked out as 2 + 5 / 2 = 3.5 (the program will do this calculation). Variance is worked out like this: (score1 - Mean) to the power of 2 + (score2 - Mean) to the power of 2 / 2 (N). I did not know what power was in C++ so I times it by itself. I think maybe there is something wrong with the compiler I am using as it will not recognise some commands. It does not return any errors but the calculations are wrong for the variance. Here is my code: #include using namespace std; int main(void) { double M, N, total, score1, score2, score3, score4, score5, score6, score7, score8, V1, V2, V3, V4, V5, V6, V7, V8, s1, s2, s3, s4, s5, s6, s7, s8; cout << "Input the number of people in sample "; cin >> N; cout << "input the 1st score "; cin >> score1; cout << "input the 2nd score "; cin >> score2; cout << "input the 3rd score "; cin >> score3; cout << "input the 4th score "; cin >> score4; cout << "input the 5th score "; cin >> score5; cout << "input the 6th score "; cin >> score6; cout << "input the 7th score "; cin >> score7; cout << "input the 8th score "; cin >> score8; total = score1 + score2 + score3 + score4 + score5 + score6 + score7 + score8; M = total / N; cout << "the mean for these scores is: " << M << "\n"; s1 = score1 - M; V1 = s1 * s1; s2 = score2 - M; V2 = s2 * s2; s3 = score3 - M; V3 = s3 * s3; s4 = score4 - M; V4 = s4 * s4; s5 = score5 - M; V5 = s5 * s5; s6 = score6 - M; V6 = s6 * s6; s7 = score7 - M; V7 = s7 * s7; s8 = score8 - M; V8 = s8 * s8; cout << "the variance for these scores is: " << (double) (V1 + V2 + V3 + V4 + V5 + V6 + V7 + V8) / N << "\n"; return 0; } linda

[Justin Tay]

Notice that if score3 to score8 is 0, the values for V3 to V8 is (0 - 3.5)^2 and you use these in your calculation for variance.

#include <iostream>
#include <vector>

using namespace std;

int main(void)
{
vector<double> scores;
double score;
double mean;
double total = 0;
double variance = 0;
int numberOfPeople;

cout << "Input the number of people in sample: ";
cin >> numberOfPeople;

for(int n = 0; n < numberOfPeople; n++)
{
	cout << "Input the score for person " << n+1 << ": ";
	cin >> score;
	scores.push\_back(score);
	total += score;
}

mean = total / numberOfPeople;
cout << "The mean for these scores is: " << mean << "\\n";

for(int n = 0; n < numberOfPeople; n++)
{
	double temp = scores\[n\] - mean;
	variance += temp \* temp;
}

variance /= numberOfPeople;
cout << "The variance for these scores is: " << variance << "\\n";
return 0;

}

[lindag1783]

Hi there Thanks for that I appreciate your code, it is good. I think I need to study some more before attempting to write calculations like this:rolleyes: :-) linda

[Gary R Wheeler]

1. You should initialize your variables score1 through score8 to 0.0. 2. Even though you let the user enter N, the number of samples, your calculation for the mean M and the variance assumes that all 8 scores are used. This means that the user must enter 0.0 for any scores greater than or equal to N for the calculations to work correctly.

---

Software Zen: delete this;

Fold With Us![^]

[lindag1783]

Hi Thanks for your help. How do I initilize these variables, I am very new to this. So, this will only work for certain scores? Like, if I input a score that is higher then the mean then the variance will not work? (is that what you meant at the end?). How would I get it to work for all scores then, is there any special way or code I can use to fix this? Thanks once again. Much appreciated.:-D linda

[Gary R Wheeler]

using namespace std;
int main(void)

{

double M, N, total, score1, score2, score3, score4, score5, score6, score7, score8,
V1, V2, V3, V4, V5, V6, V7, V8, s1, s2, s3, s4, s5, s6, s7, s8, Variance;

score1 = 0.0;
score2 = 0.0;
score3 = 0.0;
score4 = 0.0;
score5 = 0.0;
score6 = 0.0;
score7 = 0.0;
score8 = 0.0;

cout << "Input the number of people in sample ";
cin >> N;
cout << "input the 1st score ";
cin >> score1;
cout << "input the 2nd score ";
cin >> score2;
cout << "input the 3rd score ";
cin >> score3;
cout << "input the 4th score ";
cin >> score4;
cout << "input the 5th score ";
cin >> score5;
cout << "input the 6th score ";
cin >> score6;
cout << "input the 7th score ";
cin >> score7;
cout << "input the 8th score ";
cin >> score8;
total = score1 + score2 + score3 + score4 + score5 + score6 + score7 + score8;
M = total / N;
cout << "the mean for these scores is: " << M << "\n";
s1 = score1 - M;
V1 = s1 * s1;
s2 = score2 - M;
V2 = s2 * s2;
s3 = score3 - M;
V3 = s3 * s3;
s4 = score4 - M;
V4 = s4 * s4;
s5 = score5 - M;
V5 = s5 * s5;
s6 = score6 - M;
V6 = s6 * s6;
s7 = score7 - M;
V7 = s7 * s7;
s8 = score8 - M;
V8 = s8 * s8;
Variance = (V1 + V2 + V3 + V4 + V5 + V6 + V7 + V8) / N;
cout << "the variance for these scores is: " << Variance << "\n";
return 0;

}

My modifications are highlighted in **bold**. To give you another view, here is how I would do this:

int main(void)
{
int N = 0;
double score[8];
for (int i = 0; i < 8; i++) score[i] = 0.0;
cout << "Enter number of people in sample [1-8] ";
cin >> N;
if ((N > 0) && (N <= 8)) {
double total = 0;
for (int i = 0; i < N; i++) {
cout << "Enter score " << (i + 1) << " ";
cin >> score[i];
total = total + score[i];
}
double mean = total / (double)N;
double variance = 0.0;
for (int i = 0; i < N; i++) {
variance = variance + ((score[i] - mean) * (score[i] - mean));
}
variance = variance / (double)N;
cout << "Variance is " << variance;
}
else {
cout << "Incorrect number of samples.";
}
}

First, instead of having 8 discrete variables score1 through score8, I use an array. This makes it easier to step through each value using a for loop. Next, I initialize the score array, even though the logic doesn't strictly require it; it's

[Saurabh Garg]

Oh I got it now, your code is correct but you are using it incorrectly. You have written code for computing variance of 8 numbers but you are trying to use it for only two numbers. Let me explain. Code for mean is M = (s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8) / N; when you give N =2, s1 = 2, s2 = 5 and rest equal to zero. mean becomes M = (s1 + s2 + 0 + 0 + 0 + 0 + 0 + 0) / N = (s1 + s2) = N which is correct so mean is computed correctly. Code for variance is [(s1-M)*(s2-M) + (s1-M)*(s2-M) + ... + (s8-M)*(s8-M)] / N. Now when you give N =2, s1 = 2, s2 = 5 and rest equal to zero. Variance becomes [(s1-M)*(s2-M) + (s1-M)*(s2-M) + (0-M)*(0-M) + ... + (0-M)*(0-M)] / N which is not what you want. Correct formula for variance of two numbers is [(s1-M)(s1-M) + (s2-M)(s2-M)]/2. I hope this helps. -Saurabh

[lindag1783]

Hi Thanks so much for all this help, it is much appreciated. This is my first program, good guess! I would like to eventually work out some more things but I think I must hit the books about it before that will happen, very much lacking in knowledge. Thank you all for all your help regarding this problem. :-> linda

[lindag1783]

Just one more question. This 0.0 declaration works well now it will work out the variance but only if all scores are input, what if you only have 2 scores, how do you leave the other fields blank. Thanks linda

[Gary R Wheeler]

My alternative handles this case. You could place an 'if' around each prompt, something like this:

if (N >= 3) {
cout << "Enter score 3 ";
cin >> score3;
}
if (N >= 4) {
cout << "Enter score 4 ";
cin >> score4;
}

Doing that, you only prompt for and read in the number of scores that you need.

---

Software Zen: delete this;

Fold With Us![^]