F# - dict: Dictionary Examples

Dict. In F# programs we often need to map keys to values—a color name might have an index value. We could use a dict to make these links.

Type info. A dict is read-only: we create it once and cannot modify its contents. We use the dict keyword. A Dictionary, which can be changed, may also be used when mutability is needed.

Item example. Let us begin with a simple example. We create a "colors" dictionary with string keys and int values. This is a read-only sequence of key-value pairs.

Info Dict allows quick construction of a lookup table. For simple requirements, this is a good choice.

Also Item returns the value for a key in the dict. We use printfn with an "%A" to print the int value returned.

// Create a dictionary with 2 key-value pairs. let colors = dict["blue", 40; "red", 700] // Print value of "blue" key. printfn "%A" (colors.Item("blue")) // Print value of "red" key. printfn "%A" (colors.Item("red"))

40 700

Count, for-loop. A dict has many features—it supports all of the methods from the IDictionary interface. Often we must count, or iterate over, a dict's contents.

Info Count returns an int. Here we see our dict has two key-value pairs. We store this value in the constant "c."

Next We use a for-in loop to print all the values of the dict. We use a do keyword to indicate the start of the loop body.

// Create a dictionary with dict keyword. let sizes = dict["small", 0; "large", 10] // Get count of pairs and display it. let c = sizes.Count printfn "%A" c // Loop over pairs in the dictionary. // ... Display each KeyValuePair. for pair in sizes do printfn "%A" pair

2 [small, 0] [large, 10]

Keys. With this property we get a collection of the keys in the dict. With this collection (which is not a list) we can use a function like Seq.iter. Here we print all the keys.

Seq

fun

printfn

let places = dict["Canada", 10; "Germany", 20] // Get keys in a collection. let keys = places.Keys // Display all keys with labels. Seq.iter (fun key -> printfn "Key = %A" key) keys

Key = "Canada" Key = "Germany"

Error. A dict is read-only. It is immutable. This means we cannot set or add items (keys or values) to it. To add keys we must create a new dict.

// Create a lookup table. let lookup = dict["100", false; "200", true] // This causes an exception. lookup.Item("300") <- false

Unhandled Exception: System.NotSupportedException: This value cannot be mutated

Key-value pairs. We can use the dict keyword with existing key-value pairs. We can create these pairs with tuple syntax. Then we just pass them into the dict expression.

Tuple

EntryPoint

[<EntryPoint>] let main argv = let pair = ("carrot", "orange") let pair2 = ("apple", "red") // Use dict with Key-Value pairs already created. let fruit = dict[pair; pair2] // Write the Count. printfn "COUNT: %d" fruit.Count 0

COUNT: 2

A review. A dict is immutable in F#. This helps with functional programming, but sometimes we want a more versatile lookup table. We use a Dictionary for this purpose.

Final notes. Special F# constructs like dict can be used with other constructs like the tuple syntax. We can use a 2-item tuple to create an entry in a dict. Immutability is retained.