Printfn. In F# we often write values to the console. We use printfn with multiple arguments to print values—a format string is required.
Format codes. The first argument to printfn or printf must be a format string. The second argument (a value) is inserted in it and written. Printfn has a trailing newline. Printf does not.
First example. This program uses printfn with various format codes. We introduce a mutable id variable. We print it as an integer (numeric) value with the code "%d."
Next We use the "%A" code, which inserts any value. It does not require a specific type of argument.
Detail We can use some surrounding text in our printfn statement. The variable is inserted into this string.
let mutable id = 10
// Print the variable as an int.printfn"%d" id
// Print it as a value.
printfn "%A" id
// Use some surrounding text.
printfn "The ID is %A." id
// Modify the id variable and print it again.
id <- 20
printfn "%A" id10
10
The ID is 10.
20
List, dict. With the "%A" code we can print entire lists or dicts in a single statement. No for-in loop is needed to write all the elements. This makes programs easier to develop.
let codes = [10; 20; 30]
let data = ["cat", 10; "frog", 40; "rat", 100]
// Use printfn on list and dict types.// ... All included values are printed to the console.printfn"%A" codes
printfn "%A" data[10; 20; 30]
[("cat", 10); ("frog", 40); ("rat", 100)]
Printf, no newlines. This method does not add a trailing newline. So we can use it to write multiple values to a single line in a for-in loop. We print values as strings with the "%s" code.
let values = ["parrot"; "finch"; "bluebird"]
// Use printf on values in a list.// ... No newline is inserted after the values.for v in values do
printf "%s... " vparrot... finch... bluebird...
Sprintf. Sometimes we do not want to write to the console. With sprintf we can insert values into a string, and use that elsewhere in the program.
// Call sprintf to write values to a string.
let result = sprintf"Result: %A, %A" 10 20
// Now print our string.
printfn "%A" result"Result: 10, 20"
Parenthesis. When calling a method in a printfn statement, we need to surround it with parentheses. This treats the method as something to be evaluated (a value).
A review. With printfn and printf, we write values to the screen (the console). This helps us develop small and readable F# programs. Format codes help simplify the output.
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.
