String IndexOf Examples
This page was last reviewed on Sep 25, 2022.
Dot Net Perls
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.
But For "bird" we do not find a match. So the IndexOf method returns -1. This is a special value meaning "not found."
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.
Tip We can check against a return value of -1 with IndexOf for the same functionality as Contains.
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.
Result These methods return the first occurrence of any of the characters in the set. LastIndexOfAny searches in reverse.
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
A summary. 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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Sep 25, 2022 (edit).
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