Loop, `string` chars

While F# programs can use many advanced features, often they need to perform simpler tasks like looping over a string. This can be done in many ways in F#.

For the least complex looping requirements, a for-in loop is best. But if we need the index of each character, a for-to loop is a better choice.

Example

For clarity, we use a string with the value "abc". Then we iterate over its characters, in forward and reverse order, in different ways.

Version 1 This is the easiest way to loop over the characters. But it does not give us the indexes of each char.

Version 2 With for-in, we can implement a reverse loop by passing the string to Seq.rev first.

Version 3 We can iterate over indexes, and then access each char by index, with a for-to loop.

Version 4 It is possible to access the characters from the string with the Chars accessor.

Version 5 For a reverse loop with indexes, we can use for downto—this is a decrementing index loop.

let value = "abc"

// Version 1: use for-in.
for c in value do
    printfn $"For-in: {c}"

// Version 2: use for-in with Seq.rev for reverse loop.
for c in Seq.rev value do
    printfn $"Seq.rev: {c}"

// Version 3: use for-to loop.
for i = 0 to value.Length - 1 do
    let c = value[i]
    printfn $"For-to []: {c}"

// Version 4: Use for-to loop with Chars.
for i = 0 to value.Length - 1 do
    let c = value.Chars(i)
    printfn $"For-to Chars: {c}"

// Version 5: use for-downto loop for reverse loop.
for i = value.Length - 1 downto 0 do
    let c = value[i]
    printfn $"For-downto: {c}"For-in: a
For-in: b
For-in: c
Seq.rev: c
Seq.rev: b
Seq.rev: a
For-to []: a
For-to []: b
For-to []: c
For-to Chars: a
For-to Chars: b
For-to Chars: c
For-downto: c
For-downto: b
For-downto: a

Looping is often done in F#, and looping over strings is a common requirement as well. With for-in and for-to, we have a variety of powerful loops to choose from.

Loop, string chars

Example

Loop, `string` chars