STARKLINGS APP - SOLUTION TO EXERCISES

Exercises

• Intro 1

  For this one your just need to run the code!

• Intro 2

  Add the main function. A cairo code base need the main function to be able to compile.

  • Variable 1

    Add the 'let' keyword to declare a variable.

  • Variable 2

    Give the variale 'x' a value.

  • Variable 3

    Give the variale 'x' a value. It won't compile without a value.

  • Variable 4

    Add the 'mut' keyword to where you first declared x. You can't reassign without adding the 'mut' keyword.

  • Variable 5

    This is called shadowing, where you shadow a variable by redeclaring it. This will compile just fine.

  • Variable 6

    You can declare constant variables outside the main function. This will compile just fine.

  • Primitive types 1

    Change the value of is_evening to true to print out "Good evening!".

    • Primitive types 2

    Complete the declaration by adding mut, variable name and value to make the test pass.

    • Primitive types 3

    Destructure name and age to make the code compile.

    • Primitive types 4

    Make all the function parameter types the same with that of the test.

    • Primitive types 3

    Make all the function parameter types the same with that of the test.

    • Operations 1

    Declare res and return the solution.

    • Operations 2

    Declare res for each functions and return the arithemetic solution.

    • If 1

    Add an if comparison block to make the test pass.

    • If 2

    Add an if comparison block to make the test pass.

Hints

• Options 1

  Options can have a Some value, with an inner value, or a None value, without an inner value. There's multiple ways to get at the inner value, you can use unwrap, or pattern match. Unwrapping is the easiest, but how do you do it safely so that it doesn't panic in your face later? https://book.cairo-lang.org/ch06-01-enums.html#the-option-enum-and-its-advantages

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Options 2

  check out: https://github.com/starkware-libs/cairo/blob/main/corelib/src/option.cairo to see the implementation of the Option type and its methods. Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Options 3

  Reminder: You can use a match statement with an Option to handle both the Some and None cases. This syntax is more flexible than using unwrap, which only handles the Some case, and contributes to more robust code.

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Arrays 1

  You can declare an array in Cairo using the following syntax: let your_array = ArrayTrait::new(); You can append elements to an array using the following syntax: your_array.append(element); The pop_front method removes the first element from the array and returns an Option::Some(value) if the array is not empty, or Option::None() if the array is empty.

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Arrays 2

  How can you remove the first element from the array? Take a look at the previous exercise for a hint. Don't forget to call .unwrap() on the returned value. This will prevent the Variable not dropped error.

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Arrays 3

  The test fails because you are trying to access an element that is out of bounds! By using array.pop_front(), we remove the first element from the array, so the index of the last element is no longer 2. Without changing the index accessed, how can we make the test pass? Is there a method that returns an option that could help us?

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Structs 1

  Cairo has a single type of struct that are named collections of related data stored in fields. In this exercise you need to complete and implement a struct. Read more about structs in the Structs section of this article: https://book.cairo-lang.org/ch05-01-defining-and-instantiating-structs.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Structs 2

  Cairo requires you to initialize all fields when creating a struct and there is no update syntax available at the moment. You can have multiple data types in a struct, and even other structs.

  Read more about structs in the Structs section of this article: https://book.cairo-lang.org/ch05-01-defining-and-instantiating-structs.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Structs 3

  For is_international: What makes a package international? Seems related to the places it goes through right?

  For get_fees: This method takes an additional argument, is there a field in the Package struct that this relates to?

  Looking at the test functions will also help you understand more about the syntax. This section will help you understanding more about methods https://book.cairo-lang.org/ch05-03-method-syntax.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Move Semantics 1

  So you've got the "ref argument must be a mutable variable." error on line 10, right? The fix for this is going to be adding one keyword, and the addition is NOT on line 10 where the error is. Also: Try accessing arr0 after having called fill_arr(). See what happens! Read more about move semantics and ownership here: https://book.cairo-lang.org/ch04-01-what-is-ownership.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Move Semantics 2

  So, arr0 is passed into the fill_arr function as an argument. In Cairo, when an argument is passed to a function and it's not explicitly returned, you can't use the original variable anymore. We call this "moving" a variable. Variables that are moved into a function (or block scope) and aren't explicitly returned get "dropped" at the end of that function. This is also what happens here. There's a few ways to fix this, try them all if you want:

  1. Make another, separate version of the data that's in arr0 and pass that to fill_arr instead.
  2. Make fill_arr mutably borrow a reference to its argument (which will need to be mutable) with the ref keyword , modify it directly, then not return anything. Then you can get rid of arr1 entirely -- note that this will change what gets printed by the first print
  3. Make fill_arr borrow an immutable view of its argument instead of taking ownership by using the snapshot operator @, and then copy the data within the function in order to return an owned Array<felt>. This requires an explicit clone of the array and should generally be avoided in Cairo, as the memory is write-once and cloning can be expensive. To clone an object, you will need to import the trait clone::Clone and the implementation of the Clone trait for the array located in array::ArrayTCloneImpl

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Move Semantics 3

  The difference between this one and the previous ones is that the first line of fn fill_arr that had let mut arr = arr; is no longer there. You can, instead of adding that line back, add mut in one place that will change an existing binding to be a mutable binding instead of an immutable one :)

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Move Semantics 4

  Stop reading whenever you feel like you have enough direction :) Or try doing one step and then fixing the compiler errors that result! So the end goal is to:

  • get rid of the first line in main that creates the new array
  • so then arr0 doesn't exist, so we can't pass it to fill_arr
  • we don't want to pass anything to fill_arr, so its signature should reflect that it does not take any arguments
  • since we're not creating a new array in main anymore, we need to create a new array in fill_arr, similarly to the way we did in main

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Move Semantics 5

  Carefully reason about how each function takes ownership of the variable passed. It depends on the keyword used to pass the variable. What happens when a function takes ownership of a variable and then returns it? Can we still use it later on?

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Move Semantics 6

  The first problem is that get_value is taking ownership of the Number struct. So Number is moved and can't be used for set_value number is moved to get_value first, meaning that set_value cannot manipulate the data. What can we use to pass an immutable reference to get_value? What special operator do we use for that? What other operator do we use to "desnap" a snapshot? Hint: It involves the @ and * operators.

  Once you've fixed that, set_value's function signature will also need to be adjusted. Can you figure out how?

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

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• Traits 1

  If you want to implement a trait for a type, you have to implement all the methods in the trait. Based on the signature of the method, you can easily implement it.

  In the test, you need to instantiate two objects of type Animal. You can call the method of a trait by using the MyTrait::foo() syntax. How would you instantiate the two objects with AnimalTrait? Maybe you need to specify the type of the object? https://book.cairo-lang.org/ch08-02-traits-in-cairo.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Traits 2

  No hints for this one! It is very similar to the previous exercise.

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Traits 3

  You can implement multiple traits for a type. When a trait is destined to be implemented by a single type, you don't need to use generics. If you're having trouble updating the distance value in the Fish and Dog impls, remember that you need to first

  1. Destructure the object into mutable variables
  2. Update the distance variable
  3. Reconstruct self with the updated variables (self = MyStruct { ... })

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

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• Dict 1

  More info about the Felt252Dict type can be found in the following chapter : https://book.cairo-lang.org/ch03-02-dictionaries.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Dict 2

  More info about the Felt252Dict type can be found in the following chapter : https://book.cairo-lang.org/ch03-02-dictionaries.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Dict 3

  Example of custom data structures using dicts can be found in this chapter : https://book.cairo-lang.org/ch03-03-custom-data-structures.html

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

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• Modules 1

  You can bring a parent's modules items in the current module with super::item_name

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

• Modules 2

  While using functions/structs and other items from outside the module, you can refer to them with their full path or import them in the current context with the use keyword.

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.

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• Starknet 1

  No hints this time ;)

  Remember that you can always check the Cairo book at https://book.cairo-lang.org/ or the Cairo documentation at https://docs.cairo-lang.org/.