> For the complete documentation index, see [llms.txt](https://calnix.gitbook.io/eth-dev/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://calnix.gitbook.io/eth-dev/learning-solidity/module-1/mapping.md).

# Mapping

Mapping is a data structure in Solidity that allows a programmer to store, retrieve and modify data in a key-value pair format. It is similar to a hash table or dictionary in other languages.

**Example:**

Let's assume we want to store a list of people and their ages. We can do this with a mapping like this:

```solidity
mapping (address => uint) public peopleAges;
```

## Initialization & Assignment

* On Initialization, all possible key-value pairs are already initialized. 
* Accessing unassigned key-value pairs will return the default value for that value type.
* This is because the hash that points to the location in storage has not been generated & we have not written a value thus far.

#### Assignment of values must be done inside a function

* cannot simply do `myMapping[1] = "test"`  (like python)

**Example**

```solidity
//SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

contract SimpleMappingExample {

    mapping(uint => bool) public myMapping;
    mapping(address => bool) public myAddressMapping;

    function setValue(uint _index) public {
        myMapping[_index] = true;
    }

    function setMyAddressToTrue() public {
        myAddressMapping[msg.sender] = true;
    }
}
```

![](https://1628544884-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTgomzlmn9NrxUY0OQ3cD%2Fuploads%2F3JBODZ404fNxFBrKCfUu%2Fimage.png?alt=media\&token=b9254e35-02f0-4c78-9939-cb124a5ba6ea)         ![](https://1628544884-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTgomzlmn9NrxUY0OQ3cD%2Fuploads%2F1D2RElmpnxMWZQ1ns1zA%2Fimage.png?alt=media\&token=e3bb9de3-1e91-4637-84ae-ae5c6d9dba97)

1. On initialization, `myMapping[1]` will return the default bool value of false. 
2. Then we call `setValue` setting -> `myMapping[1]` = true
3. Subsequently, when we call `myMapping[1]`, we are returned true as set earlier.

### Nested Mapping

```solidity
mapping(address => mapping(uint256 => string)) public userData;

// accessing nested mapping
userData[address][uint256] = 'John'
```

* This creates a mapping from an address to a **mapping of uints to strings.** 
* This allows us to **store a string value associated with a given address and a given uint**. 
* This could be used to store user data associated with a particular address and a particular event or transaction.