IndexOf

A string is contained in another. A character may also be present. With IndexOf, and its friends LastIndexOf, IndexOfAny and LastIndexOfAny we search for chars and strings.

For the most part, F# string methods are the same as C# ones. The calling syntax is different: there are no parentheses required, and the "let" keyword is used.

Initial example

Here we use IndexOf. We have a string containing 3 words—fish, frog and dog. We use IndexOf to search for "frog" and "bird." For "frog" we get the index 5.

let words = "fish frog dog"

// Call IndexOf on a string.
// ... The word "frog" exists but "bird" does not.
let frogPosition = words.IndexOf("frog")
let birdPosition = words.IndexOf("bird")

// Print our results.
printfn "%d" frogPosition
printfn "%d" birdPosition
5
-1

LastIndexOf

With this function we search from the right to the left (backwards in order). So LastIndexOf returns the last position of the substring. This is the opposite of IndexOf.

let codes = "abc def abc def"

// Use IndexOf and LastIndexOf to search from the left and right.
let firstAbc = codes.IndexOf "abc"
let lastAbc = codes.LastIndexOf "abc"

// The substring "abc" was located in different places.
printfn "%d" firstAbc
printfn "%d" lastAbc
0
8

Contains

This is a simple method. It returns true if the substring is contained in the string, and false if not. It is a simplification of IndexOf.

let title = "Areopagitica"

// The string contains this substring.
let test1 = title.Contains "ca"

// But this substring is not found.
let test2 = title.Contains "cat"

// Print the results as booleans.
printfn "%b" test1
printfn "%b" test2
true
false

IndexOfAny

This method (and its friend LastIndexOfAny) receives a char array argument. We use a special F# syntax with vertical bars to specify a char array.

let colors = "green orange red blue"

// Find first index of any of the letters in the char array.
let firstOfSet1 = colors.IndexOfAny [|'o'; 'b'|]
printfn "First o or b: %d" firstOfSet1
printfn "%s" (colors.Substring firstOfSet1)

// Use another char array.
let firstOfSet2 = colors.IndexOfAny [|'b'; 'g'|]
printfn "First b or g: %d" firstOfSet2
printfn "%s" (colors.Substring firstOfSet2)

// Use LastIndexOfAny.
let lastOfSet1 = colors.LastIndexOfAny [|'b'; 'l'; 'u'|]
printfn "Last b or l or u: %d" lastOfSet1
printfn "%s" (colors.Substring lastOfSet1)
First o or b: 6
orange red blue
First b or g: 0
green orange red blue
Last b or l or u: 19
ue

In F# we can use a for-loop to search for substrings in a string. But IndexOf is simpler. With IndexOf and its related methods, we must handle -1 when nothing is found.