You can do the same thing as deploy just change one keyword to declare. This w


First compile you code.


`starknet-compile contract2.cairo \`


 `--output contract2_compiled.json \`


 `--abi contract2_abi.json`


Then declare it to get the contract class hash


`starknet declare --contract contract2_compiled.json`


ㅤ
_This answer was originally posted on [Triality](https://triality.org/question/554)_

When deploying contract2 from contract1 in Cairo, I need to get the CLASS_HASH. How do I achieve this?


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/554)_

Getting the CLASS_HASH of a contract in Cairo

Hello! This is day 13 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 “Pedersen Hash”.  Perhaps you can help me?

```
// The following code implements a Pedersen hash chain.
// Given the [Pedersen hash](https://docs.starkware.co/starkex-docs/crypto/pedersen-hash-function) function `H`, which
// takes two field elements and returns one representing their hash,
// a hash chain on inputs x_0, x_1, ..., x_n is the result of
// H(H(...H(H(x_0, x_1), x_2), ...,x_{n-1}), x_n)
// For example, the hash chain of (1, 2, 3, 4) is H(H(H(1, 2), 3), 4).
//
// Fix the function `hash_chain` to correctly implement the
// hash chain. Run your code to test it.
//
// Note: Don't use starkware.cairo.common.hash_chain (it computes the chain differently).

// Use the Pedersen builtin.
%builtins pedersen

from starkware.cairo.common.alloc import alloc
from starkware.cairo.common.cairo_builtins import HashBuiltin
from starkware.cairo.common.hash import hash2

// Computes the Pedersen hash chain on an array of size `length` starting from `data_ptr`.
func hash_chain{hash_ptr: HashBuiltin*}(data_ptr: felt*, length: felt) -> (result: felt) {
    if (length == 2) {
        let (result) = hash2(x=[data_ptr], y=[data_ptr + 1]);
        return (result=result);
    } else {
        // Fix here:
        return (result=123456);
    }
}

func main{pedersen_ptr: HashBuiltin*}() {
    alloc_locals;
    // Allocate a new array.
    let (local data_ptr: felt*) = alloc();
    // Fill the new array with field elements.
    assert [data_ptr] = 314;
    assert [data_ptr + 1] = 159;
    // Run the hash chain.
    let (result) = hash_chain{hash_ptr=pedersen_ptr}(data_ptr=data_ptr, length=2);
    // Check the result.
    assert result = 307958720726328212653290369969069617958360335228383070119367204176047090109;

    // Fill two more cells in the array.
    assert [data_ptr + 2] = 265;
    assert [data_ptr + 3] = 358;
    // Compute the hash chain on all four cells.
    let (result) = hash_chain{hash_ptr=pedersen_ptr}(data_ptr=data_ptr, length=4);
    assert result = 1828757374677754028678056220799392919487521050857166686061558043629802016816;
    return ();
}
```

---

> Answers to this question are a part of the ✨ 17 days of Cairo Lang with Playground & Newton. ✨
> 

*Vote for your favorite answer - the best answer will win a $10 award. A new day – a new reward! During the next 17 days, our goal is to attract more developers to the Cairo language and to systematize the knowledge of Cairo lang. [Read rules](https://www.notion.so/17-days-of-Cairo-Lang-with-Newton-550dfcda6a2d449ca468be293c776578)*

How to use Pedersen Hash in StarkNet / Cairo Language?

Hi everyone, I have a technical question:

We know that L2s are taking transaction load off of the Ethereum, but how is that process taking place? Is there a threshold of optimum transaction speed/cost? Are L2s only intervening when the L1 is congested or that threshold is passed?

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

 Is there a threshold of optimum transaction speed/cost in L2s?

# Hello world!

I try to implement the function `abs_value` of the package `starkware.cairo.common.math` but i have this error

```
AssertionError: Expected main to contain the following arguments (in this order): ['range_check_ptr_ptr']. Found: ['range_check_ptr'].
```

I dont understand the error. 

Can someone explain the error to me, why it happens ? 

> I am doing the challenge #17daysOfCairo of the day 3


Full code

```
%builtins range_check_ptr
from starkware.cairo.common.math import abs_value

func main{range_check_ptr}() {
    let x = abs_value(-1);
    return ();
}
```


AssertionError with range_check_ptr_ptr

Is there a way to call another function or contract and 'expect it to fail' without failing the whole transaction?


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/410)_

Expect function call to fail in cairo?

Hi, I'm using pytest for testing cairo contract. I found that a simple starknet setup and initial call for contract cost 34 seconds. Is there a way to accelerate test?

upd: yes, [this works](https://www.notion.so/Hi-I-m-using-pytest-for-testing-cairo-contract-I-found-that-a-simple-starknet-setup-and-initial-ca-db2282fc02f2463aadaf63eac655df6f). but I found that compiling contract cost 7s and deploying contract cost 25s. These two operations are the most time-consuming

 Is there a way to accelerate pytest for cairo contract?

How do I access the address 0, ie 0x0 in Cairo? Like address(0) in solidity


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/656)_

Cairo: Getting the zero address

Some questions that could be answered separately.

1. The only way to invoke a contract is using migrations (this applies for both devnet and testnet?)
2. When doing a migration file I receive the “Hint is not whitelisted. This may indicate that this library function cannot be used in StarkNet contracts.” error. For example for this code:

```
%lang starknet

@external
func up() {
    %{
        contract_address = deploy_contract(
             "./build/main.json",
             config={"wait_for_acceptance": True}
        ).contract_address

        invoke(
            contract_address,
            "initialize",
            {"new_authority": 123},
            config={
                "auto_estimate_fee": True,
                "wait_for_acceptance": True,
            }
        )
    %}

    return ();
}
```

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

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