A data structure is a way to store and organise data for effective usage and optimisation in any programming language. The primitive data structures in solidity are:
- uint
- int
- address See My Web3 Journey
Other complex data structures in Solidity includes:
- Arrays
- Mapping
- Struct
- Enum
Arrays
Arrays are used for storing a list of data. I liken arrays to a shopping list that contain the list of items we want to buy at the mall.
Arrays in Solidity can be of dynamic length where we can add infinite amount of items in our shopping list and we also have arrays of fixed length.
Arrays must contain the same type. Array in solidity has a length property that gives the number of items in the array. Array index is zero based. (starts counting from 0)
//SPDX-License-Identifier:MIT
pragma solidity ^0.8.4;
contract ArrayDataStructure {
uint[] public myNumber; //dynamic array
//initialize an array
uint[] public arr = [1,3,4,9];
//arrays with a fixed length
string[2] public names = [ "Jamie", "Jason"];
}
`
Arrays Methods
push and pop
We can add items to an array by using the push method of the array. The push method only works for dynamic length arrays. To add an item to a fixed length array, we have to specific the index we are adding the item.
`
//SPDX-License-Identifier:MIT
pragma solidity ^0.8.4;
contract ArrayDataStructure {
//initialize an array
uint[] public arr = [1,3,4,9];
//arrays with a fixed length
string[3] public names = [ "Jamie", "Jason"];
//push is used to add an element to the back of the array
function addToArray(uint _number) public {
//adding the value of _number into the array named arr in
//storage
arr.push(_number);
//we can not push items into a fixed element array. Fixed
//element array don't have the push method
//names.push("Nnena"); this won't work
names[2] = "Nnena";
}
//pop removes the last element from an array. This reduces //the length of the array by 1.
function popItem() public {
arr.pop();
}
}
`
Remove item at an index
You can use the index of an array to remove the item at that index. Doing this though only initialises the item at that index to the default value of the array. It does not reduce the length of the array.
To remove an item and also reduced the length of the array, we could:
move the items from the right to the left stopping at the index we want to remove. The last item is then pop out from the array thereby reducing the array length.
we can move the last item of the array to the index of the item we want removed from the array. We can then pop out the last item. (if the order of the items don't matter. This is the right way to remove an item ).
`
//SPDX-License-Identifier:MIT
pragma solidity ^0.8.4;
contract ArrayDataStructure {
//initialize an array
uint[] public arr = [1,3,4,9, 67,78 ];
//push is used to add an element to the back of the array
function removeItemByMovingRightToLeft(uint _index) public {
//check if the array is not out of bound before continuing
//require in solidity is used to check for true or false //conditions.more like a concise if statement.
require(_index < arr.length - 1, "Array index out of bound");
//loop over the array
//for loops in solidity are almost the same in Javascript
for(uint i=_index; i < arr.length; i++){
//moving from the right to the left
arr[i] = arr[i + 1];
}
//pop out the last item
arr.pop();
}
function removeBySwappingLastWithIndexToRemove(uint _index) public {
require(_index < arr.length, "Array index out of bound");
//replace the index with the last item
arr[_index] = arr[arr.length-1];
//remove the last item
arr.pop();
}
function getArray() public view returns (uint[] memory) {
return arr;
}
}
`
Try it on the Remix Online Editor
Mapping
A map is Solidity is a data structure with a key:value pair store of data.
Maps are created with the syntax mapping(keyType => keyValue)
KeyType could be value type of uint, address, bytes. (the keys of mapping ).
ValueType can be any type including another mapping. Mapping are not iterable meaning you cannot loop over them as you would an array.
`
//SPDX-License-Identifier:MIT
pragma solidity ^0.8.4;
contract MappingDataStructure {
mapping(address => uint) public myMap;
//this creates a mapping on the blockchain called with a
//key of address and value of uint
function get(address _address) public view returns (uint) {
return myMap[_address];
//it returns the value at that address but if the value has //not been set, it returns the default value which in this //case is 0;
}
function set(address _address, uint _value ) public {
myMap[_address] = _value;
}
}
`
Nested Mapping
`
//SPDX-License-Identifier:MIT
pragma solidity ^0.8.4;
contract NestedMappingDataStructure {
mapping(uint => mapping(address => uint )) public nestedMap;
function get(uint _index, address _address ) public view returns (uint) {
return nestedMap[_index][_address];
}
function set(uint _index, address _address, uint _value ) public {
nestedMap[_index][_address] = _value;
}
}
`
Structs
Structs are your own defined types. A structs can be defined outside a contract and then imported into the contract. They can contain any value type.
`
pragma solidity ^0.8.4;
contract Todos {
struct Todo {
string text;
bool completed;
}
// An array of 'Todo' structs
Todo[] public todos;
function create(string memory _text) public {
// 3 ways to initialize a struct
// - calling it like a function
todos.push(Todo(_text, false));
// key value mapping
todos.push(Todo({text: _text, completed: false}));
// initialize an empty struct and then update it
Todo memory todo;
todo.text = _text;
// todo.completed initialized to false
todos.push(todo);
}
// update text
function update(uint _index, string memory _text) public {
Todo storage todo = todos[_index];
todo.text = _text;
}
// update completed
function toggleCompleted(uint _index) public {
Todo storage todo = todos[_index];
todo.completed = !todo.completed;
}
}
//Example copied from https://solidity-by-example.org/structs/
`
One optimisation technique to save gas when using the struct data structure is to save the same type close together.
`
pragma solidity ^0.8.4;
struct GiftItems {
string giftItems;
string ownerName;
uint price;
uint number;
uint total;
bool paid;
}
`
Saving this struct like this where the same value type are close to each other will help to reduce the gas spent, though minimal. ("at all at all na bad pass" --- Nigerian proverb)
In this journey we have been able to explain some complex data structure in Solidity. I left out the Enum data structure because it is similar to how Enum (enumeration) is implemented in other languages.
Remember that arrays with fixed length don't have a push method. Don't spend hours like me trying to figure out why I can't push into this array of fixed length of string.
Until next time,
Happy coding......
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