# Solidity Minimalist Tutorial: 7. Mapping
Recently, I have been relearning Solidity, consolidating the finer details, and also writing a "Solidity Minimalist Tutorial" for newbies to learn. Lectures are updated 1~3 times weekly.
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In this section, we will introduce the hash table in Solidity: mapping
type.
# Mapping
With mapping
type, people can query the corresponding Value
by using a Key
. For example, a person's wallet address can be queried by their id
.
The format of declaring the mapping
is mapping(_KeyType => _ValueType)
, where _KeyType
and _ValueType
are the variable types of Key
and Value
respectively. For example:
mapping(uint => address) public idToAddress; // id maps to address
mapping(address => address) public swapPair; // mapping of token pairs, from address to address
# Rules of mapping
- Rule 1: The
_KeyType
should be selected among default types insolidity
such asuint
,address
, etc. No custom struct can be used. However,_ValueType
can be any custom types. The following example will throw an error, because_KeyType
uses a custom struct:
// define a struct
struct Student{
uint256 id;
uint256 score;
}
mapping(Student => uint) public testVar;
Rule 2: The storage location of the mapping must be
storage
: it can serve as the state variable or thestorage
variable inside function. But it can't be used in arguments or return results ofpublic
function.Rule 3: If the mapping is declared as
public
then Solidity will automatically create agetter
function for you to query for theValue
by theKey
.Rule 4:The syntax of adding a key-value pair to a mapping is
_Var[_Key] = _Value
, where '_Var' is the name of the mapping variable, and '_Key' and '_Value' correspond to the new key-value pair. For example:
function writeMap (uint _Key, address _Value) public {
idToAddress[_Key] = _Value;
}
# Principle of mapping
Principle 1: The mapping does not store any
key
information or length information.Principle 2: Mapping use
keccak256(key)
as offset to access value.Principle 3: Since Ethereum defines all unused space as 0, all
key
that are not assigned aValue
will have an initial Value of 0.
# Verify on Remix (use Mapping.sol
as example)
Deploy
Mapping.sol
Check initial value of map
idToAddress
.Write new key-value pair
# Summary
In this section,we introduced the mapping
type in Solidity. So far, we've learned all kinds of common variables, and then we'll learn control flow such as if-else
and while
in the coming tutorials.