Questions

[controlstructure]

Hi guys, I have two codes, both which perform the same function.

The first code is

/*This program was created to calculate the mean and standard deviation of four user entered values*/

//Beginning of Code
#include <iostream>
#include <cmath>
using namespace std;

int main ()
{
	//Declare the variables
	float number1, number2, number3, number4;

	//Enter a value for the first number
	cout << "Please enter a value for the first number:";
	cin >> number1;

	//Enter a value for the second number
	cout << "Please enter a value for the second number:";
	cin >> number2;

	//Enter a value for the third number
	cout << "Please enter a value for the third number:";
	cin >> number3;

	//Enter a value for the fourth number
	cout << "Please enter a value for the value number four:";
	cin >> number4;

	//The formula to calculate the mean of the four user entered integers
	double average = (number1 + number2 + number3 + number4)/4.0;

	//Output of the mean to the screen
	cout << "The average value is \n" << (number1 + number2 + number3 + number4)/4.0 << "\n";

	//The formula to calculate the numerator that will be used in the standard deviation equation
	double numerator = pow(number1 - average, 2.0) + pow(number2 - average, 2.0) + pow(number3 - average, 2.0) + pow(number4 - average, 2.0);

	//Output of the standard deviation to the screen using the numerator calculated above
	cout << "The standard deviation is \n" <<sqrt((numerator)/3.0) << "\n";

	
return 0;

}

The second code is

#include <iostream>
#include <cmath>

using namespace std;

int main()
{
	double a, b, c, d, sum, stdev, n;

	cout << "Please enter your four values" << endl;
	cin >> a>> b>> c>> d;

	cout << "Please enter n (the number of values)" << endl;
	cin >> n;

	sum = a + b + c + d;
	double avg = sum/n;
	double numerator = pow(a - avg, 2.0) + pow(b - avg, 2.0) + pow(c - avg, 2.0) + pow(d - avg, 2.0);

	cout << "The mean value is \n" << avg<< endl;
	cout << "The standard deviation is \n" <<sqrt(numerator/3) << endl;

	return 0;

}

Now my question arises in, when I tried to calculate the standard deviation straight out, WITHOUT the numerator, I kept getting an error. Why do I need to seperate it out into a numerator to calculate the std deviation?

My second question was why can I not include average in the first double used, where a , b, c, d etc are stored?

It's not for homework purposes, it is something that Ihave already done and submitted. I merely want to learn. Thanks!

[Peter87]

You shouldn't get an error. Maybe you didn't put parenthesis at the right positions.

sqrt((pow(a - avg, 2.0) + pow(b - avg, 2.0) + pow(c - avg, 2.0) + pow(d - avg, 2.0)) / 3)


You say this doesn't work? It should work
double a, b, c, d, sum, stdev, n, avg;

[Caprico]

You might have tried to initialize avg to sum/n... that won't work... Otherwise your programs are fine.... But what I don't get is, if you're calculating average of four numbers, why do you take input for n? Shouldn't it be 4, user independent?

[controlstructure]

I'm not sure I understand your question Caprico. I just put the variable n in the program to remind the user that it was present.

[Caprico]

I mean, consider this:

Average of two numbers: (a+b)/2
Average of three numbers: (a+b+c)/3
Average of four numbers: (a+b+c+d)/4

The denominator is automatically decided by the number of values you're calculating average for... So why take an input for it? Just makes your program vulnerable to incorrect results...