Arrays

Can be fixed or dynamic sized at compile-time

  • T[k] -> fixed size of type T, with k elements

  • T[] -> dynamic size of type T

  • T[][5] -> 5 dynamic-sized arrays of T (read in reversed)

Storage arrays

Can be fixed or dynamic.

For fixed:

  • size must be specified during declaration.

  • Once declared, the array size cannot be changed.

  • Any attempt to reference beyond the last array position will result in an error.

For dynamic:

  • size of the array is not mentioned during the declaration.

  • size of the dynamic array can be changed during the runtime as elements are added/removed.

Methods

  • length

  • delete

  • push/pop (only for dynamic)

In a fixed-size array, we can not insert a new element as it increases the length of an array. To insert a new element in the dynamic size array we use the push method; this increases the length.

push(): append a zero-initialized element

push(X): append X

Init arrays

Fixed arrays:

  • all elements initialize to 0 -> fixedArr1[1] = 0

  • can partially assign, as seen in initializeFixedArr2(). fixedArr2 = [6,0,0]

  • length is 3, as elements are initialized to default values.

contract FixedArray {

    // Fixed sized array, all elements initialize to 0
    uint[3] public fixedArr1;
    uint[3] public fixedArr2;
    uint[3] public x = [8, 15, 32];

    function updateFixedArr1() public {
        fixedArr1 = [21, 232, 43];
    }

    function updateFixedArr2() public {
        fixedArr2[0] = 6;
    }

    function getFixedLength() public view returns (uint) {
        return fixedArr1.length;
    }
}

Dynamic arrays:

  • have not defined number of elements.

  • therefore no elements or default values exist.

  • cannot call dynArr[0]. will revert.

  • default length of 0.

  • must init w/ values first.

contract DynamicArray {

    // Several ways to initialize an array
    uint[] public dynArr;
    uint[] public dynArr2 = [1, 2, 3];    
    
    // will fail if dynArr has not been assigned any elements.
    // also, not needed, since dynArr is public.
    function get(uint i) public view returns (uint) {
        return dynArr[i];
    }

    function getLength() public view returns (uint) {
        return dynArr.length;
    }
}

  • When calling get on dynArr it will revert, as no assignment has been done.

  • Can call length on an unassigned array, it will return 0.

contract Array {

    // Several ways to initialize an array
    uint256[] public dynArr; 
    
    // will fail is dynArr has not been assigned any elements.
    // also, not needed, since dynArr is public.
    function get(uint i) public view returns (uint) {
        return dynArr[i];
    }

    // Append to array: increases length by 1.
    function push() public {
        dynArr.push(11);
        dynArr.push(12);
        dynArr.push(13);
    }
    
    // Remove last element from array: decreases length by 1
    function pop() public {
        dynArr.pop();
    }

    function getLength() public view returns (uint) {
        return dynArr.length;
    }

    // Delete does not change the array length.
    // It resets the value at index to it's default value,
    function remove(uint index) public {
        delete dynArr[index];
    }
}

Push and Pop

  • dynArr being initialized but unassigned.

  • call push(): this assigns 3 elements to it. its length is now 3.

  • dynArr[2]: returns 13

  • call pop(): removes last element, 13. length is now 2.

    • dynArr[2]: will revert. there is no element there anymore.

    • dynArr[1]: returns 12

Delete

  • call push() -> dynArr = [11, 12, 13]

  • delete dynArr[1] -> dynArr = [11, 0, 13]

  • length remains 3

  • push/pop modify length

  • delete does not modify length

Removing a specific element

  1. Remove array element by shifting elements from right to left

Copy all the elements forward, starting from the specified index. Pop the last element as its repeated.

contract ArrayRemoveByShifting {

    // [1, 2, 3] -- remove(1) --> [1, 3, 3] --> [1, 3]
    // [1, 2, 3, 4, 5, 6] -- remove(2) --> [1, 2, 4, 5, 6, 6] --> [1, 2, 4, 5, 6]
    // [1, 2, 3, 4, 5, 6] -- remove(0) --> [2, 3, 4, 5, 6, 6] --> [2, 3, 4, 5, 6]
    // [1] -- remove(0) --> [1] --> []

    uint[] public arr;

    function remove(uint _index) public {
        require(_index < arr.length, "index out of bound");

        // copy all the elements forward, starting from the specified index.
        // pop the last element as its repeated.
        // [1, 2, 3] -- remove(1) --> [1, 3, 3] --> [1, 3]
        for (uint i = _index; i < arr.length - 1; i++) {
            arr[i] = arr[i + 1];
        }
        arr.pop();
    }

    function test() external {
        arr = [1, 2, 3, 4, 5];
        remove(2);
        // [1, 2, 4, 5]
        assert(arr[0] == 1);
        assert(arr[1] == 2);
        assert(arr[2] == 4);
        assert(arr[3] == 5);
        assert(arr.length == 4);

        arr = [1];
        remove(0);
        // []
        assert(arr.length == 0);
    }
}

  1. Remove array element by copying last element into to the place to remove

  • Deleting an element creates a gap in the array.

  • One trick to keep the array compact is to move the last element into the place to delete.

contract ArrayReplaceFromEnd { 

uint[] public arr;

    function remove(uint index) public {
        // Move the last element into the place to delete
        arr[index] = arr[arr.length - 1];
        // Remove the last element
        arr.pop();
    }

    function test() public {
        arr = [1, 2, 3, 4];
    
        remove(1);
        // [1, 4, 3]
        assert(arr.length == 3);
        assert(arr[0] == 1);
        assert(arr[1] == 4);
        assert(arr[2] == 3);
    
        remove(2);
        // [1, 4]
        assert(arr.length == 2);
        assert(arr[0] == 1);
        assert(arr[1] == 4);
    }
}

Arrays of structs

Memory Arrays

Only fixed-size memory array. Dynamic array cannot be created in memory. No push/pop available

  • new keyword is used to initialise arrays in memory.

contract Demo {
    
    // init, then assign later
    function method1() public pure returns(uint) {
        //fixed size of 2
        uint[] memory val = new uint[](2);
        
        val[0] = 100;        
        val[1] = 99;
        return (val[0] + val[1]);       
    }
    
    //init and assign together
    function method2() public() {
        string[3] memory myString = ["one", "two", "three"];
    }

    // no assignment done
    function test1() public pure returns (uint256) {
        uint256[5] memory myArr;
    
        return myArr.length; //length = 5
    }
}
  • uint256[] memory val = new uint[](2);
// or
uint256[] memory val = [1,2,3];
// or
uint256[5] memory myArr;

Bad example

    function test() public pure {
        
        uint256[] memory tree;
        // reverts on assignment 
        tree[0] = 1;
    }

  • you can compile this, but calling test() will revert.

  • you could remove the assignment and the function will no longer revert - but obviously this achieves nothing.

Assignments from memory to memory only create references.

  • y, z and zz are all arrays in memory

  • Changes to one memory variable affect all others referencing the same data.

contract Demo {

    function f(uint256[] memory y) public {
        // both z and zz reference the same memory location as y
        uint256[] memory z = y;
        uint256[] memory zz = y;
        
        z[1] = 10;
        assert(y[1] == 10); // true
        assert(zz[1] == 10); // true
        
        zz[0] = 6;
        assert(y[0] == 6); // true
        assert(z[0] == 6); // true
    }
}

Layout in storage

Dynamic arrays

  • Storage slot of a dynamic array stores the length of the array; and updates it accordingly.

  • Dynamic array elements are stored elsewhere.

  • Why: compiler has no way of knowing how many elements there would be; cos dynamic, so it cannot store the elements consecutively starting from the storage slot at which the dynamic array is declared. Danger of clashing into the next storage slot which could hold some other state variable.

Location of array elements

hash(slot) = location of the first array element, and the rest follow

// hash(slot = dynamicArraySlot)
uint256(keccak256(abi.encode(dynamicArraySlot)));

Nested dynamic arrays

uint256[][]s;    //storage location 0

s.push();       // push an empty dynArr, s[0]
s[0].push(4);
s[0].push();    //empty uint = 0
s[0].push(6);

  • s points to another array, s[0]

  • s[0] points to the series of elements, s[0] = [4,0,6]

  • s[1] pointer would be stored beside s[0], and so forth

storage layout of dynamic nested arrays: https://www.youtube.com/watch?v=Zi4BANKFNP8

Storage pointers and Data Location Effects

  • myArray is not a separate unique array that is created from x.

  • its a pointer to the location in storage where coders resides.

  • modifying values through the storage pointer will affect both myArray and x.

contract StorageDemo {
  
    uint256[] x; // storage

    constructor() {
        x.push(2);
        x.push(25);
        x.push(12);
    }
    
    function fuckAround() public returns(uint256){

        //Creates storage pointer to x. DOES NOT COPY ARRAY
        uint[] storage myArray = x;
        
        // this creates a copy in memory
        uint[] memory memArray = x;

        // Overwrites x[0]: 2 -> 66
        myArray[0] = 66;
        assert(x[0] == 66); // true

        // returns 2: mem copy unchanged
        return memArray[0]; 
    } 
}

  • Assignments between storage and memory (or from calldata) always create an independent copy

  • see: uint[] memory memArray = x;

  • Assignments from storage to a local storage variable also only assign a reference

  • see: uint[] storage myArray = x;

Assignments to storage always copy, even if sourcing from reference variables

contract Storage {

    uint256[] public x; // storage by default

    function f(uint256[] memory y) public {

        // x is a copy of y, not a reference
        x = y; 
        
        // only alters x, NOT y
        x[0] = 100; 
        
        assert(x[0] != y[0]); // true
    }

    function g(uint256[] calldata z) public {
        // x is a copy of z, not a reference
        x = z; 
        
        // only alters x, NOT z
        x[0] = 200; 
        
        assert(x[0] != z[0]); // true
    }
}

Dangling references to storage array elements

  1. add(): Create a dynamic array with a single nested dynamic array uint256[][] s, such that s[0] = [4,0,6]

  2. fuckAround(): create a storage pointer to the last element in s, which is a pointer to s[0]; since there is only 1 element.

  3. pop s[0]

  4. push some hex value into the same storage location 

contract Storage {
    uint256[][] s;

    function add() public {
        s.push();
        s[0].push(4);
        s[0].push();
        s[0].push(6);
    }

    function pop() public {
        s.pop();
    }

    function fuckAround() public {
        // ptr := s[0], reference to the storage location
        uint[] storage ptr = s[s.length - 1];
        // remove s[0]
        s.pop();
        // push a value into the str location of s[0][0]
        ptr.push(0x42);
        // re-assign s[0]. references same location
        s.push();
        // s[0][0] == 0x42, cos of clashing storage location
        assert(s[s.length - 1][0] == 0x42);
    }

    // HELPER TO READ FROM STORAGE SLOTS
    function readStorageSlot(uint256 i) public view returns (bytes32 content) {
        assembly {
            content := sload(i)
        }
    }

    function getLocationOfDynamicArray(uint dynamicArraySlot) public pure returns (uint) {
        // dynamicArraySlot: the slot that the dynamic array itself sits in
        return uint256(keccak256(abi.encode(dynamicArraySlot)));
    }
}
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