27:T51a,# Introduction Cairo provides dictionaries as part of its core libraries represented by the `Felt252Dict` type. The dictionary key type in Cairo is restricted to `felt252`, which differs from other languages where it is possible to define the type of the key type. To create a dictionary, you can use the `default` method of the `Default` trait, like: ```rust let mut foo: Felt252Dict = Default::default(); ``` The basic operations that are most often used are: 1. `insert(felt252, T) -> ()` to write values to a dictionary instance and 2. `get(felt252) -> T` to read values from it. And here's how you would use these operations: ```rust let mut foo: Felt252Dict = Default::default(); // Add a value to the dictionariy foo.insert('bar', 42); // Here we update the element of 'bar' foo.insert('bar', 73); // To retrieve a dictionary value, you can use baz = foo.get('bar'); ``` If you try to retrieve the value for a key which does not exist in the dictionary, it will return the zero value of the value type. This can confuse you, especially if the default value of your value type (for example, 0 for an integer), is a valid value. This also means there is no way to delete data from a dictionary. Something to take into account when incorporating this structure into your code. 28:T485,# Hints ## General - [Dictionaries in The Cairo Book][book] - [Cairo cheatsheet][cheatsheet] ## 1. Store the unit of measurement in your program - To store the measurements in your program, you can insert them one by one into a dictionary ## 2. Create a new bill - To create a new bill, all you need to do is initialize a dictionary ## 3. Add item to the customer bill - To check whether the given unit of measurement is correct, you can test whether your measurement dictionary retrieves a non-zero value for a key ## 4. Remove item from the customer bill - To check whether the given item is in customer bill, you can test whether your bill dictionary retrieves a non-zero value for a key - To check whether the given unit of measurement is correct, you can test your measurement dictionary retrieves a non-zero value for a key ## 5. Return the number of specific item that is in the customer bill - To check whether the given item is in customer bill, you can simply retrieve it from the dictionary [book]: https://book.cairo-lang.org/ch03-02-dictionaries.html [cheatsheet]: https://starknet-by-example.voyager.online/cairo_cheatsheet/dict 29:T4be,use core::dict::Felt252Dict; pub fn units() -> Felt252Dict { let mut units: Felt252Dict = Default::default(); units.insert('quarter_of_a_dozen', 3); units.insert('half_of_a_dozen', 6); units.insert('dozen', 12); units.insert('small_gross', 120); units.insert('gross', 144); units.insert('great_gross', 1728); units } pub fn new_bill() -> Felt252Dict { Default::default() } pub fn add_item( ref bill: Felt252Dict, ref units: Felt252Dict, item: felt252, unit: felt252, ) -> bool { let measurement = units.get(unit); if measurement == 0 { return false; } let qty = bill.get(item); bill.insert(item, qty + measurement); true } pub fn remove_item( ref bill: Felt252Dict, ref units: Felt252Dict, item: felt252, unit: felt252, ) -> bool { let qty = bill.get(item); if qty == 0 { return false; } let measurement = units.get(unit); if measurement == 0 { return false; } if qty < measurement { return false; } bill.insert(item, qty - measurement); true } pub fn get_item(ref bill: Felt252Dict, item: felt252) -> u32 { bill.get(item) } 2a:T21f7,