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Merge the documentation about mappings

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Alex Beregszaszi 2016-10-15 22:50:46 +01:00
parent 1b3713742f
commit fcba4d927c
2 changed files with 27 additions and 46 deletions
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46
docs/frequently-asked-questions.rst
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@ -104,52 +104,6 @@ This example demonstrates how to send funds from a contract to an address.
See `endowment_retriever <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/30_endowment_retriever.sol>`_.
What is a ``mapping`` and how do we use them?
=============================================
A mapping is very similar to a K->V hashmap.
If you have a state variable of type ``mapping (string -> uint) x;``, then you can
access the value by ``x["somekeystring"]``.
How can I get the length of a ``mapping``?
==========================================
Mappings are a rather low-level data structure. It does not store the keys
and it is not possible to know which or how many values are "set". Actually,
all values to all possible keys are set by default, they are just
initialised with the zero value.
In this sense, the attribute ``length`` for a mapping does not really apply.
If you want to have a "sized mapping", you can use the iterable mapping
(see below) or just a dynamically-sized array of structs.
Are ``mapping``'s iterable?
===========================
Mappings themselves are not iterable, but you can use a higher-level
datastructure on top of it, for example the `iterable mapping <https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol>`_.
Can I put arrays inside of a ``mapping``? How do I make a ``mapping`` of a ``mapping``?
=======================================================================================
Mappings are already syntactically similar to arrays as they are, therefore it doesn't make much sense to store an array in them. Rather what you should do is create a mapping of a mapping.
An example of this would be::
contract C {
struct myStruct {
uint someNumber;
string someString;
}
mapping(uint => mapping(string => myStruct)) myDynamicMapping;
function storeInMapping() {
myDynamicMapping[1]["Foo"] = myStruct(2, "Bar");
}
}
Can you return an array or a ``string`` from a solidity function call?
======================================================================

27
docs/types.rst
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@ -617,6 +617,33 @@ Because of this, mappings do not have a length or a concept of a key or value be
Mappings are only allowed for state variables (or as storage reference types
in internal functions).
It is possible to mark mappings ``public`` and have Solidity create an accessor.
The ``_KeyType`` will become a required parameter for the accessor and it will
return ``_ValueType``.
::
pragma solidity ^0.4.0;
contract MappingExample {
mapping(address => uint) public balances;
function update(uint newBalance) {
balances[msg.sender] = newBalance;
}
}
contract MappingUser {
function f() returns (uint) {
return MappingExample(<address>).balances(this);
}
}
.. note::
Mappings are not iterable, but it is possible to implement a data structure on top of them.
For an example, see `iterable mapping <https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol>`_.
.. index:: assignment, ! delete, lvalue
Operators Involving LValues