C++ calculate average and standard deviation

Question

I have this program where it asks a user how many values the user will enter in, asks for the values and puts them into a vector. Then I need to calculate the average and standard deviation. I have the average part but it looks like something is wrong and I get the wrong average. And im not too sure how to get started on the standard deviation part.

#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

int main()
{
    // Create vector
    vector<double> values;

    // Declare variables
    double count;
    double value;
    double average;
    double stdDev;

    // Prompt user and get the number of values
    cout << "How many values will you enter?\n";
    cin >> count;

    // Get the values from user and calculate average
    for( double i = 1; i<= count; i++)
    {
        cin >> value;
        values.push_back(value);
        average+=values[i];
    }
    cout << average/count << '\n';

    //Calculate standard deviation


    return 0;
}

any help is appreciated.

double average; is uninitialized. It has indeterminate value. — LogicStuff
double average; => double average = 0;. local standard datatypes are not 0 initialized. — NathanOliver
After the first push_back, values[0] is defined but values[1] is not. And so on. Your counting should start at 0. — Mark Ransom

Shafi Shafi · Accepted Answer · 2016-02-12T22:21:00

Use double average = 0; instead of double average;
And use values[i-1] instead of values[i] inside the loop.

Why?

You declared average and didn't initialized it. For this it has a value that can't be known before the code executes. When you are adding value[i] with average that value of average is in it. For this you are getting wrong output. You need to add only all value[i]. So initialize average with 0.
Vector indexing is 0 based. So the n-th element inserted to a vector will be found at [n-1]-th position.

C++ calculate average and standard deviation

1 Answers