Revoke reference means compiler has lost the reference to the location we want to access.
In this example, reference to the location where x has been stored is lost.


When we use alloc_locals, then reference to local variables are fixed and are in relation to frame pointer fp. First variables location is [fp + 0], next local variable reference is [fp + 1] and so on, and compiler does not lose the reference to local variables.


The solution Ishita Rastogi has mentioned all result in compiler not losing the reference to the variables in the program.


I don’t recommend using low level programing constructs (ap, fp and such) to beginners. I am also a beginner. I hope Cairo compiler will mature over time and will to better optimization. 


The program below does not give revoked reference issue, since reference to x is not lost (function foo is not called again).

In the original  program, since function foo is called again, and “alloc_locals” was not used, compiler lost the location of x.

======================================================================


%builtins output

from starkware.cairo.common.serialize import serialize_word

// Returns a^3 for a != 0 and 1 otherwise.
func foo(a) -> (res: felt) {
    if (a == 0) {
        return (res=1);
    } else {
        return (res=a * a * a);
    }
}

// Outputs the value 10^3 + 5^3.
func main{output_ptr: felt*}() {
    let (x) = foo(10);
    let y = 5;
    serialize_word(x + y);
    return ();
}

============================================================

Below are my  notes when I was dealing with issue of “Reference 'syscall_ptr' was revoked” while solving one of the exercise from ZKP Boot Camp

If..else statement in Cairo.

•	Seems like we need to put return statements (example below) inside if…else statements, else we get revoked references issue  Reference 'syscall_ptr' was revoked

@external
func bombard{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr, bitwise_ptr: BitwiseBuiltin*}(game_idx : felt, x : felt, y : felt, square_reveal : felt):

    alloc_locals

    #get address of the caller
    let (caller) = get_caller_address()
    ### some logic
    ### some logic
    ### some logic

    ### now some complex if...else logic
    if current_game.next_player == 0:
        ### logic
        ### logic
        ### logic
        return () ### --> pay attention
    else:
    ### logic
        ### some logic
        ### some logic
        ### some logic
        ### some logic
        if hit_by_prev_player == 1:
            if caller == current_game.player2.address:
                ### some logic
                ### some logic
                ### some logic
                ### some logic
                return ()   ### --> pay attention
            else:
                ### some logic
                ### some logic
                ### some logic
                return ()   ### --> pay attention
            end
        end
    end

    return()
end



>Hello! This is day 5 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 “Revoked references”. Can you run these 6 steps and write down what you see?

```
// The scope of some types of references in Cairo (e.g., return values of
// functions and temporary variables) is somewhat restricted.
// For example, a call to another function may revoke those references.
//
// 1. Try to run the following code. You should get an error that `x` was revoked
//    (this is because of the second call to `foo`).
// 2. In simple cases, like this one, the compiler can solve this automatically
//    if the "alloc_locals" keyword is used within the function's scope.
//    Add "alloc_locals;" to the beginning of main(), and make sure the code works.
// 3. To understand what happens behind the scenes, replace "alloc_locals;" with
//    "ap += SIZEOF_LOCALS;" - this command allocates memory cells for local variables
//    (you'll need them below) but doesn't enable the mechanism that
//    automatically resolves revoked references.
//    Trying to run the code now should result in the original error.
// 4. Solve the problem manually by copying the value of `x` to a local variable
//    (e.g., "local x2 = x;"). Local variables are not revoked when functions
//    are called.
//    You can read more about local variables [here](https://www.cairo-lang.org/docs/how_cairo_works/consts.html#local-vars).
// 5. You can even use the same name: "local x = x;".
//    This is called "reference rebinding", where the meaning of `x` changes:
//      Before this line, `x` refers to the return value.
//      After that, `x` refers to the local variable.
// 6. A shorter syntax is "let (local x) = foo(10);" which achieves the same goal.
//    Try it!

%builtins output

from starkware.cairo.common.serialize import serialize_word

// Returns a^3 for a != 0 and 1 otherwise.
func foo(a) -> (res: felt) {
    if (a == 0) {
        return (res=1);
    } else {
        return (res=a * a * a);
    }
}

// Outputs the value 10^3 + 5^3.
func main{output_ptr: felt*}() {
    let (x) = foo(10);
    let (y) = foo(5);
    serialize_word(x + y);
    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)*



Cairo Lang / StarkNet: What are Revoked references? What is alloc_locals?

Here is my solution
``


```
// Use the output builtin.
%builtins output

from starkware.cairo.common.alloc import alloc
from starkware.cairo.common.serialize import serialize_word


// Fills `new_array` with the squares of the first `length` elements in `array`.
func _inner_sqr_array(array: felt*, new_array: felt*, length: felt) {
    if (length == 0) {
        return ();
    }

    assert [new_array] = [array] * [array];

    _inner_sqr_array(array=array + 1, new_array=new_array + 1, length=length - 1);
    return ();
}

func sqr_array(array: felt*, length: felt) -> (new_array: felt*) {
    // Write your code here.
    alloc_locals;
    let (local new_array) = alloc();
     _inner_sqr_array(array=array, new_array=new_array, length=length);
     return(new_array=new_array);
}

func main{output_ptr: felt*}() {
    alloc_locals;
    // Allocate a new array.
    let (local array) = alloc();
    // Fill the new array with field elements.
    assert [array] = 1;
    assert [array + 1] = 2;
    assert [array + 2] = 3;
    assert [array + 3] = 4;

    let (new_array) = sqr_array(array=array, length=4);
    serialize_word([new_array]);
    serialize_word([new_array + 1]);
    serialize_word([new_array + 2]);
    serialize_word([new_array + 3]);

    return ();
}
```


Hello! This is day 6 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 “Dynamic allocation”.  Perhaps you can help me?

```
// Dynamic allocation in Cairo is done using the `alloc` function,
// which itself is implemented in Cairo using the
// [segments](https://www.cairo-lang.org/docs/how_cairo_works/segments.html) mechanism.
// Thanks to this mechanism, `alloc` allocates an array of an arbitrary size,
// which does not need to be specified in the call.
//
// The function `sqr_array` should compute and return an array
// of the square values of a given array.
// Write the body of `sqr_array` using the given helper function
// `_inner_sqr_array` and check that the program output
// is 1, 4, 9, 16.
// `sqr_array` should allocate the new array it returns.

// Use the output builtin.
%builtins output

from starkware.cairo.common.alloc import alloc
from starkware.cairo.common.serialize import serialize_word

// Fills `new_array` with the squares of the first `length` elements in `array`.
func _inner_sqr_array(array: felt*, new_array: felt*, length: felt) {
    if (length == 0) {
        return ();
    }

    assert [new_array] = [array] * [array];

    _inner_sqr_array(array=array + 1, new_array=new_array + 1, length=length - 1);
    return ();
}

func sqr_array(array: felt*, length: felt) -> (new_array: felt*) {
    // Write your code here.
}

func main{output_ptr: felt*}() {
    alloc_locals;
    // Allocate a new array.
    let (local array) = alloc();
    // Fill the new array with field elements.
    assert [array] = 1;
    assert [array + 1] = 2;
    assert [array + 2] = 3;
    assert [array + 3] = 4;

    let (new_array) = sqr_array(array=array, length=4);
    serialize_word([new_array]);
    serialize_word([new_array + 1]);
    serialize_word([new_array + 2]);
    serialize_word([new_array + 3]);

    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 can I use dynamic allocation in Cairo? 

My answer:

``

```
// The following code prints the sum of the numbers from 1 to 10.
// Modify the function `compute_sum` to print all the intermediate sums:
// 1, 1 + 2, 1 + 2 + 3, ..., 1 + 2 + ... + 10.
// Note: you'll have to add the implicit argument output_ptr to the
// declaration of compute_sum (in order to use the serialize_word function):
//   func compute_sum{output_ptr: felt*}(n: felt) -> (sum: felt)

// Use the output builtin.
%builtins output

from starkware.cairo.common.serialize import serialize_word

func compute_sum{output_ptr: felt*}(n: felt) -> (sum: felt) {
    if (n == 0) {
        // When 0 is reached, return 0.
        return (sum=0);
    }

    // Otherwise, call `compute_sum` recursively to compute 1 + 2 + ... + (n-1).
    let (sum) = compute_sum(n=n - 1);
    // Add the new value `n` to the sum.
    let new_sum = sum + n;
    serialize_word(new_sum);
    return (sum=new_sum);
}

func main{output_ptr: felt*}() {
    let (res) = compute_sum(n=10);

    // Output the result.
    //serialize_word(res);
    return ();
}
```


Hello! This is day 7 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 “Recursion”.  Perhaps you can help me?

```
// The following code prints the sum of the numbers from 1 to 10.
// Modify the function `compute_sum` to print all the intermediate sums:
// 1, 1 + 2, 1 + 2 + 3, ..., 1 + 2 + ... + 10.
// Note: you'll have to add the implicit argument output_ptr to the
// declaration of compute_sum (in order to use the serialize_word function):
//   func compute_sum{output_ptr: felt*}(n: felt) -> (sum: felt)

// Use the output builtin.
%builtins output

from starkware.cairo.common.serialize import serialize_word

func compute_sum(n: felt) -> (sum: felt) {
    if (n == 0) {
        // When 0 is reached, return 0.
        return (sum=0);
    }

    // Otherwise, call `compute_sum` recursively to compute 1 + 2 + ... + (n-1).
    let (sum) = compute_sum(n=n - 1);
    // Add the new value `n` to the sum.
    let new_sum = sum + n;
    return (sum=new_sum);
}

func main{output_ptr: felt*}() {
    let (res) = compute_sum(n=10);

    // Output the result.
    serialize_word(res);
    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 recursive function in Cairo Lang?

My solution

```


// 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) {
  alloc_locals;

  if (length == 2) {
        let (result) = hash2(x=[data_ptr], y=[data_ptr + 1]);
        return (result=result);
    } else {
        // Fix here:
        let (result_int) = hash2(x=[data_ptr], y=[data_ptr+1]);
        // data_ptr = data_ptr+2, new_length = length-2
        let (result) = cal_hash(result_int=result_int, data_ptr=data_ptr+2, length=length-2);
        return (result=result);
    }
}

func cal_hash{hash_ptr: HashBuiltin*}(result_int : felt, data_ptr : felt*, length: felt) -> (result : felt) {
    if(length == 0) {
        return (result=result_int);
    } else {
        let (result_int2) = hash2(x=result_int, y=[data_ptr]);
        let (result) = cal_hash(result_int=result_int2, data_ptr=data_ptr+1, length=length-1);
        return (result=result);
    }
    
}
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 ();
}
```


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?

1. assert is essential as this constitutes verification part. If we remove assert, then we are omitting verification and the program won't be able to verify correctness of output.

2. My solution for fourth root:


```
%builtins output

from starkware.cairo.common.serialize import serialize_word

// Computes the square root (over the integers) of `n`.
// Prover assumption: The square root exists.
func sqrt(n) -> (res: felt) {
    alloc_locals;
    local res;

    // Set the value of res using a Python hint.
    %{
        import math

        # Use the ids variable to access the value of a Cairo variable.
        #ids.res = int(math.sqrt(ids.n))
        ids.res = int(math.sqrt(int(math.sqrt(ids.n))))
        print(f"res = {ids.res}")
    %}

    // From the verifier's point of view, the hint is completely transparent.
    // The following line guarantees that `res` is the square root
    // (either the positive or negative) of `n`.
    assert n = res * res * res * res;
    return (res=res);
}

func main{output_ptr: felt*}() {
    let (res) = sqrt(256);
    serialize_word(res);
    return ();
}
```


Hello! This is day 14 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 “Hints”.  Perhaps you can help me?

```
// The following function uses hints to efficiently compute the square root `res`
// of the argument `n`.
// The idea is that the verifier only needs to be convinced that res * res = n,
// it doesn't really care how res was computed.
// In such cases, we don't have to compute res in pure Cairo - we can write a
// piece of Python code inside the Cairo program, which is called a "hint".
// A hint is a piece of code that the prover runs to initialize
// some memory cells. Note that it is completely transparent from the verifier's
// point of view, so the result of the hint *must* be verified using pure Cairo
// instructions (e.g., the "assert n = res * res" instruction below).
//
// 1. Comment out the line "assert n = res * res;" and run the code. Does it still output
//    the expected value? Can you explain why the assert is nonetheless essential?
//    Hint: Recall that from the verifier's point of view, the hint does not exist.
// 2. Uncomment the assert line and change the function to compute the fourth
//    root.

%builtins output

from starkware.cairo.common.serialize import serialize_word

// Computes the square root (over the integers) of `n`.
// Prover assumption: The square root exists.
func sqrt(n) -> (res: felt) {
    alloc_locals;
    local res;

    // Set the value of res using a Python hint.
    %{
        import math

        # Use the ids variable to access the value of a Cairo variable.
        ids.res = int(math.sqrt(ids.n))
    %}

    // From the verifier's point of view, the hint is completely transparent.
    // The following line guarantees that `res` is the square root
    // (either the positive or negative) of `n`.
    assert n = res * res;
    return (res=res);
}

func main{output_ptr: felt*}() {
    let (res) = sqrt(256);
    serialize_word(res);
    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 Hints in Cairo Lang?

Hey guys,

I am having issue with generating _abi.json file when I use namespace,
Please see example below.

Does someone know solution of how to generate _abi.json file when namespace is used? I really need help 🙏 

This code generates starknet_abi.json

%lang starknet
from starkware.cairo.common.cairo_builtins import HashBuiltin
from starkware.starknet.common.syscalls import get_caller_address

@External
   func store_name{syscall_ptr: felt, pedersen_ptr: HashBuiltin, range_check_ptr}(_count: felt) -> (even : felt) {
      alloc_locals;
      local even;

      let (caller) = get_caller_address();
      if (_count == 2) {
         even = 1;
      }
      return (even=even);
   }
=============================================================
This code does not generates starknet_abi.json (please see use of  namespace)

%lang starknet
from starkware.cairo.common.cairo_builtins import HashBuiltin
from starkware.starknet.common.syscalls import get_caller_address

namespace ToMoon {
@External
   func store_name{syscall_ptr: felt, pedersen_ptr: HashBuiltin, range_check_ptr}(_count: felt) -> (even : felt) {
      alloc_locals;
      local even;

      let (caller) = get_caller_address();
      if (_count == 2) {
         even = 1;
      }
      return (even=even);
   }
}



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