F# dict: Dictionary Examples, Item and KeysCreate a dictionary with dict. Use Item, Keys and TryGetValue to access the dictionary.
Dict. A connection is made. A wolf howls in the dark of night. A cat has soft fur. We link things to values of things: in F# 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.Keywords
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.
Dict: This keyword allows quick construction of a lookup table. For simple requirements, this is a good choice.
Item: This returns the value for a key in the dict. We use printfn with an "%A" to print the int value returned.
F# program that creates dictionary, gets values
// Create a dictionary with two 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"))
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.
Count: This returns an int. Here we see our dict has two key-value pairs. We store this value in the constant "c."
For: 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.
F# program that uses count, for-loop on dict
// 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
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.SeqFunPrintfn
F# program that displays keys in dict
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.
F# program that causes dict exception
// 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
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
F# program that uses Key-Value pairs, dict
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
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.
In this language, 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.
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