If you execute multiple transactions that change one particular storage value, and all the transactions are included in one block, Starknet can bundle them together so you only need to pay once :)


ㅤ
_This answer was originally posted on [Triality](https://triality.org/question/788)_

I am working through [ex03](https://github.com/starknet-edu/starknet-cairo-101/blob/main/contracts/ex03.cairo) of starknet-cairo-101 and I noticed that I was not being charged a gas fee for calling `increment_counter` multiple times back to back, it only charged me once. Does anyone know why this is happening?


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/788)_

Did not have to pay gas fee when calling same function multiple times in Starknet?

Hey folks, do you know how I could read a felt* from ap?

I want to do something like this:

```rs
    let (hello : felt*) = alloc();
    tempvar world : felt* = hello;
    let world_like : felt* = [ap-1];
```

when I do tempvar world = hello, I ensure I add hello (the pointer) to [ap] and then I increase ap so world_like = [ap-1] should be the same thing than world

But I get this error:

```
Cannot assign an expression of type 'felt' to a reference of type 'felt*'.
    let world_like : felt* = [ap-1];
```

When doing a similar code:

```rust
        let (func_pc: felt*) = get_label_location(storage_var);
        _prepare_call(pedersen_ptr, range_check_ptr, len_inputs, inputs + len_inputs);
        call abs ap[11 - 6 * 2];
```

I get ```
Cannot apply subscript-operator to non-pointer, non-tuple type 'felt'.
        call abs ap[11 - 6 * 2];  // removing alloc_locals by calling ap[11-6*2] ?
```

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

how I could read a felt* from ap in Cairo lang?

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)_

The only way to invoke a contract is using migrations (this applies for both devnet and testnet?)

When running my python tests I get:


 NotImplementedError: Invoking through an account contract is not supported yet, please use execute() instead.


 Has anyone seen this problem and know of a solution?


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/1179)_

Cairo Testing Error

In the following function I am presented with the following error, even though my code does not make it to this function. What am I doing wrong?


`func fill_in_row{output_ptr: felt*}(row_idx : felt, row_size : felt, curr_row : felt*) {` 


 `if (row_idx == row_size-2) {`


 `return ();`


 `}`


 `let val : felt = curr_row[row_idx-1] + curr_row[row_idx];`


 `serialize_word(val);`


 `curr_row[row_idx+1] = val;`


 `return fill_in_row(row_idx+1, row_size, curr_row);`


`}`


Error: code:80:5: While expanding the reference 'curr_row' in:


 curr_row[row_idx+1] = val;


 ^******^


code:72:73: Expected a register. Found: [fp + (-4)].


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/1156)_

Cairo Error: Getting Error while setting value of item in array

How can I prepare my Cairo lang project for an audit?

Hey there, is there an existing Math64x61_pow implementation to compute power of a floating point elevated to an integer power?

edit: looks like there is 🙂
https://github.com/influenceth/cairo-math-64x61/blob/master/contracts/cairo_math_64x61/math64x61.cairo#L278

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

Is there a Cairo Lang implementation to compute power of a floating point?

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 ();
}
```

---

> 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)*

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Did not have to pay gas fee when calling same function multiple times in Starknet?

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