If, `elif`

In programs we usually use ifs to execute statements inside the blocks. But in F# we can use an if-construct to return a value. It is part of an expression.

By turning an if-statement into an expression, we can use "if" in more ways. We can assign to the result of an if.

An example

This program uses an if, elif, else construct. It first creates a string. Then it tests the length of this string in the if-statement.

Then We must use the "then" keyword after the if-condition. A "then" is also required for an elif, but not an else.

Info We use a single equals sign to test for equality in an expression. The expression of an if-statement must be in parentheses.

let animal = "bird"

// Test the length of the string.
if (animal.Length = 1) then
    // Not reached.
    printfn "A"
elif (animal.Length = 2) then
    // Not reached.
    printfn "B"
else
    // This statement is reached.
    printfn "C"
C

If inside let

An if-statement can be used to return a value (as an expression). We can embed an if in more complex statements. Here we assign the value of "result" with an if.

Info The program sets the value of result to 1 if count is equal to or greater than 200. It also has two other conditions.

let count = 50

// Use an if, elif, else construct within a variable assignment.
let result =
    if count >= 200 then 1
    elif count <= 100 then 2
    else 3

// Write results.
printfn "%A" count
printfn "%A" result50
2

Not

There is no "!=" operator for ints in F#. To see if an int does not equal a value, we use the equals operator and then surround that expression with the "not" operator.

let code = 10

// Use an if-not statement to test a variable.
if not (code = 5) then
    printfn "Not five!"
Not five!

`Match` versus if

We can write a logical test with a match or an if-expression. The syntax for match is closer to a "switch" in C-like languages. A match may be easier to use an expression.

Info The "let" statement creates a function that tests its argument "v" and prints a statement based on its value. It uses "match."

Next The second "let" does the same thing but uses an if-statement. You can see this version looks more like C# or C code.

// We can use a match to handle the argument.
let testPrint v =
    match v with
    | 0 | 1 | 2 -> printfn "[MATCH] branch A: %A" v
    | _ -> printfn "[MATCH] branch B: %A" v

// We can use an if-else to handle the argument.
let testPrintIf v =
    if v = 0 || v = 1 || v = 2 then
        printfn "[IF] branch A: %A" v
    else
        printfn "[IF] branch B: %A" v

// Test functions.
testPrint 0
testPrint 9
testPrintIf 0
testPrintIf 9[MATCH] branch A: 0
[MATCH] branch B: 9
[IF] branch A: 0
[IF] branch B: 9

Convert `bool` to `int`

F# offers no C-like ternary operator. Instead, we use inline if-expressions. With an if-expression we can convert a bool to an int.

An if-block operates in F# much like in other languages. But it has a special feature. It can evaluate an if as an expression, which can return values or be used in assignments.

If, elif

An example

If inside let

Not

Match versus if

Convert bool to int

If, `elif`

`Match` versus if

Convert `bool` to `int`