Data structure for moving average

type sma struct {
	size uint
	data []float32
	sum  float64
}  

Here -
size: stores window size N
data: is a go slice type that stores last N data points
sum: stores sum of last N data points stored in data

Interface for moving average

type smaOps interface {
	addData(val float32)
	getAvg() float64
	toString() string
}

This is a simple interface that defines operations on the data structure.

Here -
addData - updates sum, stores new val in data and adjusts data so that only last N data points are stored in data
getAvg - computes average of last N data points stored in data
toString - provides access to the elements in data slice by returning string representation of last N data points stored in data

Additional

Basic error handling has been added in case of input processing and in case there is not enough data in the data slice.

Output from sample run

Enter window size: 3
Enter numbers for calculating average - 
1
Not enough data points!

3
Not enough data points!

5

 Average: 3.000000 (1.000000 3.000000 5.000000)

6

 Average: 4.666667 (3.000000 5.000000 6.000000)

8

 Average: 6.333333 (5.000000 6.000000 8.000000)