I would always recommend starting with the [official documentation](https://www.cairo-lang.org/docs/) (especially [hands on path](https://www.cairo-lang.org/docs/hello_cairo/index.html#hello-cairo)) and the [playground](https://www.cairo-lang.org/playground/). Learning more about how Cairo works is also beneficial for a deeper understanding of the StarkNet ecosystem, but I think it is not necessary for the beginning.

Once you learn about data representation, you may also want to compare two `felt`s or `Uint256`s, since there are no number comparison operators like in standard languages. You can check `cmp` utils functions built into the language: [`math_cmp`](https://github.com/starkware-libs/cairo-lang/blob/master/src/starkware/cairo/common/math_cmp.cairo) and [`uint256_cmp`](https://github.com/starkware-libs/cairo-lang/blob/master/src/starkware/cairo/common/uint256.cairo).

If you're coming from a Solidity background, you probably heard about OpenZeppelin. They have a [repo with StarkNet contracts](https://github.com/OpenZeppelin/cairo-contracts) implemented in Cairo. I find it useful, not just as a good library, but as an inspiration for code abstraction and organization, and Cairo best practices.

Other useful resources:
* [Cairo Mummies](https://triality.org/education/course/cairomummies) courses
* [Cairo resources Twitter thread](https://mobile.twitter.com/Starknet_edu/status/1529371701334581249)
* [What is Cairo? Resources and Tools – Nostra Devs 01](https://mirror.xyz/0x845605C411132BAA06024a521a85B653F3C802dF/oOISOf8vdxf-uiXrAGJVYxXdXuW81IauMIC7RzrznLQ) (also there's `felt` and `Uint256` post on the blog).
* [Practical StarkNet lessons learned](https://hackmd.io/@RoboTeddy/BJZFu56wF)
* Not strictly Cairo resources, but useful for better understanding StarkNet: [STARK math Twitter thread](https://twitter.com/EliBenSasson/status/1578380154476208131)

There are many people who are pretty new to Cairo Language, so this thread is about some good resources, to begin with. What projects, educational courses, Github Repos etc. about StarkNet can you recommend?

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Newton

What resources (Github repos, projects) do you follow about Cairo Language? Why?

# Hello starknet fam!

I heard with the last release of protostar it is now possible to make multiple call to different contracts functions in one protostar invoke CLI.

Can someone show me a good exemple on how to use it?

Like multicall with approve and swap.

Thank you very much!

How to use multicall with protostar 0.9.0?

I have a struct with : 

```struct Binary:
felt a:
felt* b:
```

How to return this in a view function?
I see this error:
```
Can't identify return variable
```


how to return struct

Hey this message looks very confusing.
`GatewayError: {'contract_address': ['Expected hex string, got: "0x01C438231cD70f47d1acB8404E51734c1Fb4d79AC55813b3E861fB1EF78Da0D6".']}`

Any hints on this?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

CairoLang error: `GatewayError: {'contract_address': ['Expected hex string, got: 

Hello! This is day 15 of the 17 days of the Cairo Challenge. And I have no idea how to solve the [playground](https://www.cairo-lang.org/playground/) exercise “SHARP”.  Perhaps you can help me?

```
// The program below lets you choose a number `x` and output its square `y = x*x`
// (proving that you know the square root of `y`).
//
// 1. Replace "YOUR_NUMBER" with a value of your choice, run the program and
//    make sure you get the expected output (the square of the value you chose).
//
// Take a look at the output line saying "Program hash: ...".
// The program hash is the (Pedersen) hash of the compiled program.
//
// 2. The compiled program does not include the [hints](https://cairo-lang.org/docs/how_cairo_works/hints.html),
//    so changing any code within a hint does not change the resulting program
//    hash.
//    Try changing `x` to a different number and note that the program hash remains
//    unchanged after rerunning.
//
// The Cairo SHARP collects several programs and creates a proof that they ran
// successfully. Such a batch of programs is called a "train".
// Note that it may take a while until a train is created, as the service waits
// to collect enough programs (after a certain amount of time, the train will
// be dispatched, even if it's not full).
//
// 3. Click on "Prove with SHARP" to prove the program using the shared proving service.
//
// After the proof is created, SHARP sends it to an [Ethereum goerli contract](https://goerli.etherscan.io/address/0xAB43bA48c9edF4C2C4bB01237348D1D7B28ef168).
// For each job in the train, a corresponding fact is registered on the contract,
// attesting to the validity of the run with its particular output.
//
// 4. You can monitor the status of your job, using the link in the output pane.
//
// For each job, the corresponding fact is computed as follows:
//   keccak(program_hash, keccak(program_output))
// where:
// * program_hash is the hash of the compiled program, which you can find in
//   the output pane (32 bytes).
// * program_output is the output of the program as a list of 32-byte
//   elements.
//
// For example, if you chose x=3, the output will be [9] and you can compute
// the fact in Python using:
//
//   from web3 import Web3
//
//   program_hash = 0x114952172aed91e59f870a314e75de0a437ff550e4618068cec2d832e48b0c7
//   program_output = [9]
//   Web3.solidityKeccak(['uint256', 'bytes32'], [
//     program_hash,
//     Web3.solidityKeccak(['uint256[]'], [program_output])
//   ])
//
// You can also find your job's fact in the SHARP tracking page you opened
// in step 4.
//
// 5. Once you see that the fact is registered, you can call the
//    [contract's isValid() method](https://goerli.etherscan.io/address/0xAB43bA48c9edF4C2C4bB01237348D1D7B28ef168#readContract) to make sure it's indeed registered.

%builtins output

from starkware.cairo.common.serialize import serialize_word

func main{output_ptr: felt*}() {
    alloc_locals;
    local x;
    %{ ids.x = YOUR_NUMBER %}

    serialize_word(x * x);
    return ();
}
```

---

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> 

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What is SHARP in Cairo Language?

Hey,
As each storage value is by default initialized to 0, does writing it to zero actually do something? Does it use storage in layer 1 with value 0? Or does it just do nothing?
Should I check that the value I want to write is different than 0 before writing it otherwise I will waste storage?

The same idea: does writing a storage value to 0 free storage on l1

Does writing it to zero actually do something?

Does AA work on Mobile?

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Does Account Abstraction work on Mobile?

In the medium article about 1.0.0 this is shown as the syntax for the for loop

```let sum = 0
for x in iter {
  sum = sum + x;
}```

That doesn't compile for me and loops are not mentioned on github release note.

Will recursion still be necessary if they work as they do in other languages? 

Will it be possible to invoke functions from within the loops?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

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