## Receive data from a Cairo contract :

When you perform a call, you have the result in an object :

- With a contract.call : `const result=contract.call("read_val", myParameters)`.
- With a contract.meta-class : `const result=contract.read_val(...myParameters)`.

| Type in Cairo | Cairo code | Type expected in JS/TS | JS/TS function to recover data |
| --- | --- | --- | --- |
| felt (251 bits max) | func getV()->(total:felt) | BN | const total:BN = result.total |
|  |  | number (64 bits max) | const total:number = parseInt(result.total) |
|  |  | string representing an hex number | const address:string = number.toHex(result.address) |
| Uint256 (256 bits max) | func getV()->(balance:Uint256) | BN | const balance:BN = uint256.uint256toBN(result.balance) |
| array of felt | func getV()->(list_len:felt, list:felt*) | BN[] | const list:BN[]= result.list |
| shortString (31 ASCII characters max) | func getV()->(title:felt) | string | const title:string = shortString.decodeShortString(result.title) |

Newton

What do I have as the result of a Cairo contract in JavaScript?

Hello! This is day 4 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 “Variables”. Can you launch this code with the instructions and write down how many steps the program has in the output?

[BONUS]: try to make it faster

```
// The following code outputs the 9th power of 2.
// 1. Click on "Debug" to run the code, and then click on
//    the arrows in the memory panel to trace the program
//    step by step.
//
// Observe that the computation of `z` take 8 steps and
// creates 8 temporary variables. This is because `let`
// simply replaces each instance of `y` with `x * x * x`
// and each instance of `z` with `y * y * y`, which is
// expanded to nine multiplications.
//
// 2. Change the two `let`s to `tempvar`s and run the program.
//
// "tempvar" stands for "temporary variable", which means that
// a variable will be allocated for the intermediate values
// (`x * x * x` and `y * y * y`). This reduces the same
// computation to use only 4 steps and 2 new variables.
//
// 3. Replace all three instance of the keyword `tempvar`
//    with `local`, uncomment the line containing `alloc_locals`
//    and run the code again.
//
// The behavior of the code doesn't change. This is because
// the keyword `local` allocates a new variables for the
// intermediate values as well. The main difference between
// `local` and `tempvar` is explained in the next challenge.

// Use the output builtin.
%builtins output

// Import the serialize_word() function.
from starkware.cairo.common.serialize import serialize_word

// Outputs the value 2^9.
func main{output_ptr: felt*}() {
    // alloc_locals;
    tempvar x = 2;
    let y = x * x * x;
    let z = y * y * y;
    // Because of the `let`s, the following line is equivalent to
    // serialize_word(x * x * x * x * x * x * x * x * x).
    serialize_word(z);
    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)*

What is the difference between tempvar/let in Cairo Lang? How to use allow_locals and local?

Hello, Do I understand this correctly: Pathfinder JSON-RPC V0.1.0 does not support InvokeTXNV1 yet, right? At least, I do not `nonce` in `add_invoke_transaction`.

Do you plan to add it in JSON-RPC V0.1.0 like devnet (not being compliant to the spec) or will you introduce it with JSON-RPC V0.2.0?

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

Can anyone explain Pathfinder JSON-RPC InvokeTXNV1?

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

Is anyone experiencing slowness on goerli? my transactions aren't being processed - stuck in `RECEIVED`

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

Is anyone experiencing slowness on goerli StarkNet?

Hello! This is day 8 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 “Field Elements”.  Perhaps you can help me?

```
// Unlike most systems where the primitive type is a 32 or 64-bit integer,
// the primitive type in Cairo is "felt", which stands for "field element".
// When we say "a field element" we mean an integer in the range -P / 2 < x < P / 2,
// where P is a very large (prime) number.
// Currently we use a 252-bit number (76 decimal digits).
// When we add, subtract or multiply and the result is outside the range above,
// there is an overflow.
//
// The most important difference between integers and field elements is division:
// Division of field elements (and therefore division in Cairo) is not the
// integer division you may be used to.
// In the next challenge, you'll see how to use unsigned_div_rem()
// for truncated (i.e., rounding down) integer division.
//
// Make the code below output the results of the following expressions:
// 1. 1 + 100, 1 - 100, 9 * 9.
// 2. pow(10, 74), pow(10, 75), pow(10, 76)
// 3. 12 / 3, (-12) / 3
// 4. 13 / 3 (This is an example of division in the field)

%builtins output

from starkware.cairo.common.serialize import serialize_word

// Computes base^exp.
func pow(base: felt, exp: felt) -> (res: felt) {
    if (exp == 0) {
        return (res=1);
    }

    let (res) = pow(base=base, exp=exp - 1);
    return (res=res * base);
}

func main{output_ptr: felt*}() {
    serialize_word(1 + 100);
    // Add your code here.

    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 make math operation with Field Elements (felts) in Cairo lang?

Hello! This is day 10 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 “Bitwise operations”.  Perhaps you can help me?

```
// The bitwise builtin allows you to perform bitwise operations
// (and, or, xor) on integers up to 251 bits.
//
// The following code implements the function xor_last_hex_digits,
// which takes the two last hexadecimal digits and "xor"s them.
// For example, in 0x1234 the last two digits are 3 and 4,
// and their xor is 3^4=7.
//
// Fix the missing code and make sure it works.

%builtins bitwise

from starkware.cairo.common.bitwise import bitwise_and, bitwise_xor
from starkware.cairo.common.cairo_builtins import BitwiseBuiltin

// Returns the xor of the last two hexadecimal digits.
func xor_last_hex_digits{bitwise_ptr: BitwiseBuiltin*}(x: felt) -> (res: felt) {
    // Fix and uncomment the line below.
    // let (digit0) = ...

    let (shifted_digit1) = bitwise_and(x, 0xF0);
    tempvar digit1 = shifted_digit1 / 0x10;

    return bitwise_xor(digit0, digit1);
}

func main{bitwise_ptr: BitwiseBuiltin*}() {
    let (res) = xor_last_hex_digits(0x1234);
    assert res = 7;

    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 make Bitwise operations in Cairo Lang using %builtins bitwise, bitwise_xor?

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)_

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What do I have as the result of a Cairo contract in JavaScript?

1 answers

Receive data from a Cairo contract :

Your answer

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You may find useful

Type in Cairo	Cairo code	Type expected in JS/TS	JS/TS function to recover data
felt (251 bits max)	func getV()->(total:felt)	BN	const total:BN = result.total
		number (64 bits max)	const total:number = parseInt(result.total)
		string representing an hex number	const address:string = number.toHex(result.address)
Uint256 (256 bits max)	func getV()->(balance:Uint256)	BN	const balance:BN = uint256.uint256toBN(result.balance)
array of felt	func getV()->(list_len:felt, list:felt*)	BN[]	const list:BN[]= result.list
shortString (31 ASCII characters max)	func getV()->(title:felt)	string	const title:string = shortString.decodeShortString(result.title)