`gas_usage` is `amount * gas_price`. `gas_price` is by default set to 100 Gwei, but you can update the price on devnet with the following command: `starknet-devnet --gas-price 999`, where '999' would be the new gas price.


ㅤ
_This answer was originally posted on [Triality](https://triality.org/question/967)_

yup , i am listen


ㅤ
_This answer was originally posted on [Triality](https://triality.org/question/967)_

When calling `estimateFee`, I get a struct with `amount`, `unit`, `gas_price` and `gas_usage`. Wondering what is the difference between `amount` and `gas_usage` ?


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/967)_

Cairo: Difference between amount and gas_fee

does anybody have a working local setup ? I find myself having to run tests on the testnet...

I tried both nile and protostar. the problem is that I can set it up but I don't know how to get access to the accounts that are pre-loaded with ETH. new accounts that I create don't have ETH and I don't have a way to transfer ETH to them..

if anybody has a go-to setup that they can share that'd be much appreciated 🙏

How to setup local Starknet node with Account contracts?


I can often make a terminal recursive version of my functions with a little less elegant code. Should I do it because it could reduce the fees or should I keep the unoptimised version?


For example, here is an "unoptimised" function which sums elements of an array:



```
@view
func get_convoy_strength{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr}(
    convoy_id : felt
) -> (strength : felt):
    alloc_locals
    let (convoyables_len : felt, convoyables : felt*) = get_convoyables(convoy_id)
    return _get_convoyables_strength(convoyables_len, convoyables)
end

```

and here is the tail call optimization:



```
func _get_convoyables_strength_tc{
    syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr
}(convoyables_len : felt, convoyables : felt*, sum : felt) -> (strength : felt):
    if convoyables_len == 0:
        return (sum)
    else:
        let convoyable_id = [convoyables]
        alloc_locals
        let (convoyable_strength) = _get_strength(convoyable_id)
        return _get_convoyables_strength_tc(
            convoyables_len - 1, convoyables + 1, sum + convoyable_strength
        )
    end
end

```

As you can see it's a little less friendly because it requires an additional argument (which will always be 0). On a normal computer this could be optimized to not fill the call stack but as FeedTheFed pointed out, the memory is immutable here so it doesn't seem to be useful. He did say, however, that it could be interesting for "not wasting memory cells for the intermediate return values". It would be very helpful to me to have a more detailed explanation as I'm not sure to understand.


Here is the cairo doc related to this: <https://www.cairo-lang.org/docs/how_cairo_works/functions.html?highlight=tail#tail-recursion>



ㅤ
_This question was originally posted on [Stack Overflow](https://stackoverflow.com/questions/72321871/when-to-use-tail-call-optimization-in-a-cairo-smartcontract)_

when to use tail call optimization in a cairo smartcontract

Hey there! 

I am using the Oz Nile framework for smart contract development on Starknet. More specifically, I am testing the proxy upgrade feature, but my tests fail with the error message TypeError: int() can't convert non-string with explicit base. This happens when nile.signer tries to sign a transaction. Here's the snippet in the stack trace below. 

```
def sign_transaction(self, sender, calls, nonce, max_fee):
        """Sign a transaction for an Account."""
        (call_array, calldata) = from_call_to_call_array(calls)
        message_hash = get_transaction_hash(
>           int(sender, 16), call_array, calldata, nonce, int(max_fee)
        )
```

Some relevant info:
cairo-lang==0.10.0
cairo-nile==0.8.2
openzeppelin-cairo-contracts==0.4.0b
pytest==7.1.2
python=3.9 

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

Cairo/Nile: Testing the proxy upgrade feature, but my tests fail with the error message TypeError: int() can't convert non-string with explicit base.

How to convert tokens on starknet? Any DEXs AMMs?

Something that isn't on testnet.

Am going through all these projects on the ecosystem page but am coming up with nothing.

Would be nice if there was a DEX/AMM section ... instead of making people go through all those projects one by one to see what they actually do.

I'm up to Magnety ...

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

How to convert tokens on starknet? Any DEXs AMMs?


Are you able to nest mappings in Cairo like you can in Solidity?

e.g. in Solidity you can do
```solidity
mapping(address => mapping(uint256 => bool));
```
is this doable in Cairo? I want to do something similar to

```cairo

func mappingA(value: felt) -> func mappingB(value: felt)  -> (value: felt){
}
```

mapping(x, y) -> (z) is not gonna work, here is some context:

I'm working through the sample ERC721_Metadata.cairo and ERC721_Metadata_Base contract shown here https://github.com/starknet-edu/starknet-erc721/tree/main/contracts/token/ERC721

It looks like from what I can tell the tokenUri is set for each domain, and even though calling the @view function does take in a tokenId, calling to set the tokenUri does not and it's not clear to me how that mapping could exist otherwise. So I wanted to modify the contract to have a token_id -> token_uri mapping. Because of the way those mappings exist though, you need to provide both the token_uri_len: felt and token_uri: felt*  when writing them. This means it can only be done once, otherwise you'd need a specific @storage_var for every token that's minted, just to hold it's tokenuri. I was hoping to do something like 

```cairo
// Represents a tokenUri
struct TokenUri {
    // The length of the token bytes
    token_uri_len: felt,
    // The pointer to the beginning of the token uri bytes
    token_uri: felt*,
}

@storage_var
func token_uris(token_id: felt) -> (token_uri: TokenUri) {
}
```


But because of the token_url: felt*  pointer in that struct, it can't be used as a storage variable return type. It gives an error when I try to compile showing
```cairo
starknet_contracts/ERC721_Metadata.cairo:29:36: The return type of storage variables must be a felts-only type (cannot contain pointers).
func token_uris(token_id: felt) -> (token_uri: TokenUri) {
```

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

Are you able to nest mappings in Cairo like you can in Solidity?

Hello guys, I would like to know if CAIRO work in python3.10 venv? Because I have a problem to install python 3.9.


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

Does Cairo work in python3.10 venv? I can’t install python 3.9

I am trying to index into an array in cairo, and getting the following error:


`Unknown value for memory cell at address 13:2`


The line of code throwing the error is:


`let item = arr[index]`


ㅤ
_This question was originally posted on [Triality](https://triality.org/question/606)_

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