printfn ExamplesWrite values to the screen with the printfn and printf methods. See format codes for strings and ints.
This page was last reviewed on Dec 10, 2022.
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" id
10 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... " v
parrot... 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.
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