How to calculate weighted average of non-zero values?

I am new to Go, I am doing the following:

I have a slice:

valueList := []int16{500, 400, 0, 300}

and a weight slice:

weightList := []float64{0.1, 0.2, 0.3, 0.4}

Now I want to do this:

res := make([]float64, 4)
for i, value := range valueList {
    res[i] = float64(value) * weightList[i]
}

But as you can see, I get a 0 in valueList, now I want to average the weight to the others if value is not 0, so in this example, it should be:

500 * (0.1 + 0.3/3) + 400 * (0.2 + 0.3/3) + 300 * (0.4 + 0.3/3)

If another 0 shows up, average the weight again.

How can I do this simply?

You can calculate the sum weight of non-zero values while iterating over them, and do the division by the sum weight at the end:

vs := []int{500, 400, 0, 300}
ws := []float64{0.1, 0.2, 0.3, 0.4}

sumWeight, avg := 0.0, 0.0
for i, v := range vs {
	if v == 0 {
		continue
	}
	sumWeight += ws[i]
	avg += float64(v) * ws[i]
}
avg /= sumWeight

fmt.Println("Average:", avg)

Output (try it on the Go Playground):

Average: 357.1428571428571

Note:

This solution does not require weights to sum up to 1.0 neither to be in the range of 0..1 (but they can't be negative).

Note #2:

If the input slices are empty (either by having len == 0 or all values being 0) or sumWeight of non-zero values is 0.0, the result will be NaN due to dividing by sumWeight which is 0.0. You might want to check this case and do something else:

if sumWeight > 0.0 {
	avg /= sumWeight
}