Is there a method for adding a tail to an array that is more effective than append in F#?

Question

I am currently trying to build a program where I have a recursive function that for every loop appends one new element to the array it is building. I didn't want to use the append function that many times, because my function is supposed to do a large number of loops, and I've come to learn from previous experiences that the append function in general takes a lot of time. I've tried to look everywhere for a function that simply adds one element to the tail of the array, but I've found nothing of such sort. So I was thinking I would ask here. So my question is basically: "Is there a more effective way of adding one element to the back of an array than using append?" Updated with a new question regarding the previous one So I used a list instead, inserting each new element as a head, and reverted the list when the function was finished. This made the function about 70 times faster. But the problem remains as I have another function that does pretty much the same, which became about 4 ti…

Accepted Answer

ResizeArray is a more efficient data structure for this task, for example:

```
let filterRange predicate (i, j) =
    let results = ResizeArray(j-i+1)
    for k = i to j do
        if predicate k then results.Add(k)
    results.ToArray()
```

ResizeArray module from F# PowerPack provides high-order functions to manipulate `ResizeArray` in a functional way. However, beware that these functions create new `ResizeArray` instances which make them less efficient than .NET methods.

A purely functional alternative is to use a list as an accumulator, prepend elements to the accumulator, reverse it (if the order matters) and convert to Array in the end:

```
let filterRange predicate (i, j) =
    let rec loop k acc =
        if k = j+1 then acc
        elif predicate k then loop (k+1) (k::acc)
        else loop (k+1) acc
    loop i [] |> List.rev |> Array.ofList
```

Is there a method for adding a tail to an array that is more effective than append in F#?

Solution

Problem