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Add updated iterable mapping example
Remove unused var
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@ -53,7 +53,100 @@ each ``_KeyType``, recursively. For example with a mapping:
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}
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}
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.. index:: !iterable mappings
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.. _iterable-mappings:
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.. note::
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Mappings are not iterable, but it is possible to implement a data structure
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on top of them. For an example, see `iterable mapping <https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol>`_.
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Iterable Mappings
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-----------------
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Mappings are not iterable, but it is possible to implement a data structure on
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top of them and iterate over that. For example, the code below implements an
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``IterableMapping`` library that the ``User`` contract then adds data too, and
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the ``sum`` function iterates over to sum all the values.
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::
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pragma solidity >=0.4.0 <0.7.0;
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library IterableMapping {
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struct itmap {
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mapping(uint => IndexValue) data;
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KeyFlag[] keys;
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uint size;
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}
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struct IndexValue { uint keyIndex; uint value; }
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struct KeyFlag { uint key; bool deleted; }
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function insert(itmap storage self, uint key, uint value) internal returns (bool replaced) {
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uint keyIndex = self.data[key].keyIndex;
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self.data[key].value = value;
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if (keyIndex > 0)
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return true;
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else {
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keyIndex = self.keys.length++;
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self.data[key].keyIndex = keyIndex + 1;
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self.keys[keyIndex].key = key;
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self.size++;
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return false;
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}
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}
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function remove(itmap storage self, uint key) internal returns (bool success) {
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uint keyIndex = self.data[key].keyIndex;
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if (keyIndex == 0)
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return false;
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delete self.data[key];
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self.keys[keyIndex - 1].deleted = true;
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self.size --;
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}
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function contains(itmap storage self, uint key) internal view returns (bool) {
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return self.data[key].keyIndex > 0;
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}
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function iterate_start(itmap storage self) internal view returns (uint keyIndex) {
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return iterate_next(self, uint(-1));
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}
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function iterate_valid(itmap storage self, uint keyIndex) internal view returns (bool) {
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return keyIndex < self.keys.length;
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}
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function iterate_next(itmap storage self, uint keyIndex) internal view returns (uint r_keyIndex) {
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keyIndex++;
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while (keyIndex < self.keys.length && self.keys[keyIndex].deleted)
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keyIndex++;
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return keyIndex;
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}
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function iterate_get(itmap storage self, uint keyIndex) internal view returns (uint key, uint value) {
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key = self.keys[keyIndex].key;
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value = self.data[key].value;
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}
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}
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// How to use it
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contract User {
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// Just a struct holding our data.
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IterableMapping.itmap data;
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// Insert something
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function insert(uint k, uint v) public returns (uint size) {
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// Actually calls itmap_impl.insert, auto-supplying the first parameter for us.
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IterableMapping.insert(data, k, v);
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// We can still access members of the struct - but we should take care not to mess with them.
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return data.size;
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}
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// Computes the sum of all stored data.
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function sum() public view returns (uint s) {
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for (uint i = IterableMapping.iterate_start(data);
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IterableMapping.iterate_valid(data, i);
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i = IterableMapping.iterate_next(data, i)) {
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(, uint value) = IterableMapping.iterate_get(data, i);
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s += value;
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}
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}
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}
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