I tried coding my own simple moving average in C++ - c++

Question

I want a function that works.
I believe my logic is correct, thus my (vector out of range error) must be coming from the lack of familiarity and using the code correctly.
I do know that there is long code out there for this fairly simple algorithm.
Please help if you can.
Basically, I take the length as the "moving" window as it loops through j to the end of the size of the vector. This vector is filled with stock prices.
If the length equaled 2 for a 2 day moving average for numbers 1 2 3 4. I should be able to output 1.5, 2.5, and 3.5. However, I get an out of range error.
The logic is shown in the code. If an expert could help me with this simple moving average function that I am trying to create that would be great! Thanks.
void Analysis::SMA()
{
double length;
cout << "Enter number days for your Simple Moving Average:" << endl;
cin >> length;
double sum = 0;
double a;
while (length >= 2){
vector<double>::iterator it;
for (int j = 0; j < close.size(); j++){
sum = vector1[length + j - 1] + vector1[length + j - 2];
a = sum / length;
vector2.push_back(a);
vector<double>::iterator g;
for (g = vector2.begin(); g != vector2.end(); ++g){
cout << "Your SMA: " << *g;
}
}
}
}

Answer 1

You don't need 3 loops to calculate a moving average over an array of data, you only need 1. You iterate over the array and keep track of the sum of the last n items, and then just adjust it for each new value, adding one value and removing one each time.
For example suppose you have a data set:
4 8 1 6 9
and you want to calculate a moving average with a window size of 3, then you keep a running total like this:
iteration add subtract running-total output average
0 4 - 4 - (not enough values yet)
1 8 - 12 -
2 1 - 13 13 / 3
3 6 4 15 15 / 3
4 9 8 16 16 / 3
Notice that we add each time, we start subtracting at iteration 3 (for a window size of 3) and start outputting the average at iteration 2 (window size minus 1).
So the code will be something like this:
double runningTotal = 0.0;
int windowSize = 3;
for(int i = 0; i < length; i++)
{
runningTotal += array[i]; // add
if(i >= windowSize)
runningTotal -= array[i - windowSize]; // subtract
if(i >= (windowSize - 1)) // output moving average
cout << "Your SMA: " << runningTotal / (double)windowSize;
}
You can adapt this to use your vector data structure.

Answer 2

Within your outermost while loop you never change length so your function will run forever.
Then, notice that if length is two and closes.size() is four, length + j - 1 will be 5, so my psychic debugging skills tell me your vector1 is too short and you index off the end.

Answer 3

This question has been answered but I thought I'd post complete code for people in the future seeking information.
#include <iostream>
#include <vector>
using namespace std;
int main() {
vector<double> vector1 { 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 };
double length;
cout << "Enter number days for your Simple Moving Average:" << endl;
cin >> length;
double sum = 0;
int cnt = 0;
for (int i = 0; i < vector1.size(); i++) {
sum += vector1[i];
cnt++;
if (cnt >= length) {
cout << "Your SMA: " << (sum / (double) length) << endl;
sum -= vector1[cnt - length];
}
}
return 0;
}
This is slightly different than the answer. A 'cnt' variable in introduced to avoid an additional if statement.