Filter

In F#, the filter function allows you to remove elements from a sequence that do not match a specified condition. The resulting sequence only contains elements that satisfy the condition.

The syntax for filter is as follows:

filter (predicate: 'a -> bool) (list: 'a list) : 'a list

where:

• predicate is a function that takes an element of type generic - we’ll see that - and returns a boolean value.
• list is a list of elements of type of passed generic type.
• The return value is a new list of elements of the type generic that pass the predicate test.

Suppose we have a list of integers:

let numbers = [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]

We can use filter to remove all even numbers from the list:

let odds = filter (fun x -> x % 2 <> 0) numbers

In this example, fun x -> x % 2 <> 0 is the predicate function that checks if a given element is odd. The resulting list odds contains only the odd numbers in the original list:

[1; 3; 5; 7; 9]

The filter function can also be used with arrays. Here’s an example:

let numbers = [|1; 2; 3; 4; 5; 6; 7; 8; 9; 10|]
let odds = filter (fun x -> x % 2 <> 0) numbers

The resulting array odds contains only the odd numbers in the original array:

[|1; 3; 5; 7; 9|]

The filter function is a powerful tool in F# that allows you to remove elements from a sequence that do not match a specified condition. It can be used with both lists and arrays, and is a great tool for data manipulation and processing.