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.

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" pair2
[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.

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") <- falseUnhandled 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.

[<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 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.