C# and F# — numbering a list — a non-variable approach

From time to time, you want to create an index for your list in order to make some calculations or simply to print elements with numeric order information.

A standard way in almost every language is to use a separate variable which, in this case, is supposed to be our counter. Like here, for C#:

List<string> listToIterate = new List<string>() { "one", "two", "three", "four", "five" };

int i = 1;
foreach (string item in listToIterate)
{
    System.Console.WriteLine($"A list index: {i}, a list element: {item}");
    i += 1;
}

And F# here:

let listToIterate = [ "one"; "two"; "three"; "four"; "five" ]

let mutable i = 0
for item in listToIterate do
    printfn $"A list index: {i}, a list element: {item}"
    i <- i + 1

Both C# and F# code output is:

There is an easy way in both instances to make code shorter and more concise.

For C# we can use LINQ:

List<string> listToIterate = new List<string>() { "one", "two", "three", "four", "five" };

foreach (var item in listToIterate.Select((listEmlement, i) => (listEmlement, i)))
{
    Console.WriteLine($"A list index: {item.i}, a list element: {item.listEmlement}");
}

For F#, we use a pipeline that indexes a list at the first step, then iterates through a new list and prints every element.

let listToIterate = [ "one"; "two"; "three"; "four"; "five" ]

listToIterate
|> List.indexed
|> List.iter (fun (i, item) -> printfn "A list index: %d, a list element: %s" i item)