The if...then and if...then...else are conditional control or decision-making expressions. Using these expressions, the program can behave differently based on a defined condition checked during the expression's execution.
if...then ExpressionThe basic format of the if...then expressions is as follows.
let age = 15
if age < 16
then Console.WriteLine("You are only {0} years old and already learning F#? Wow!", age)
In the above example, the expression runs when the Boolean expression evaluates to true.
if...then...else ExpressionThe if...then...else expression runs different branches of code and also evaluates to a different value depending on the Boolean expression given.
The following example shows how to use the if...then...else expression.
let num1 = 12
let num2 = 19
if num1 < num2 then Console.WriteLine("{0} is less than {1}", num1, num2)
else Console.WriteLine("{0} is greater than {1}", num1, num2)
if...then...else ExpressionsWhen chaining multiple if...then...else expressions together, you can use the keyword elif instead of else if; they are equivalent.
let marks = 79;
if marks >= 90 then Console.WriteLine("A+")
elif marks >= 80 then Console.WriteLine("A")
elif marks >= 70 then Console.WriteLine("B")
elif marks >= 60 then Console.WriteLine("C")
elif marks >= 50 then Console.WriteLine("D")
else Console.WriteLine("F")
In the above example, a series of comparisons of a variable marks to check if it is one of the grades (such as A+, A, B, C, or D). Every following comparison is done only in the case that the previous comparison was not true. In the end, if none of the if conditions is not fulfilled, the last else clause is executed.
The result of the above example is shown below.
B