I Use this sample for search in Map but not work :|:
var xmenList = ['4','xmen','4xmen','test'];
var xmenObj = {
'first': '4',
'second': 'xmen',
'fifth': '4xmen',
'author': 'test'
};
print(xmenList.indexOf('4xmen')); // 2
print(xmenObj.indexOf('4xmen')); // ?
but I have error TypeError: xmenObj.indexOf$1 is not a function on last code line.
Pelease help me to search in map object simple way same as indexOf.
I found the answer:
print(xmenObj.values.toList().indexOf('4xmen')); // 2
or this:
var ind = xmenObj.values.toList().indexOf('4xmen') ;
print(xmenObj.keys.toList()[ind]); // fifth
Maps are not indexable by integers, so there is no operation corresponding to indexOf. If you see lists as specialized maps where the keys are always consecutive integers, then the corresponding operation should find the key for a given value.
Maps are not built for that, so iterating through all the keys and values is the only way to get that result.
I'd do that as:
K keyForValue<K, V>(Map<K, V> map, V value) {
for (var entry in map.entries) {
if (entry.value == value) return key;
}
return null;
}
The entries getter is introduced in Dart 2. If you don't have that, then using the map.values.toList().indexOf(value) to get the iteration position, and then map.keys.elementAt(thatIndex) to get the corresponding key.
If you really only want the numerical index, then you can skip that last step.
It's not amazingly efficient (you allocate a new list and copy all the values). Another approach is:
int indexOfValue<V>(Map<Object, V> map, V value) {
int i = 0;
for (var mapValue in map.values) {
if (mapValue == value) return i;
i++;
}
return -1;
}
You can search using .where(...) if you want to find all that match or firstWhere if you assume there can only be one or you only want the first
var found = xmenObj.keys.firstWhere(
(k) => xmenObj[k] == '4xmen', orElse: () => null);
print(xmenObj[found]);
Related
I have a list of elements and I need to get a list containing the first element followed by every nth element afterwards. For example: given n = 3 and the list [banana, cherry, apple, pear, kiwi], I need to get the list [banana, pear]. I need this regardless of specific content, since the list depends on user input.
How do I do this using Dart?
You may access list in dart by providing an index like for example:
List<String> fruits = ["banana","cherry","apple","pear","kiwi"];
print(fruits[0]); // Will print to the console "banana";
On your case, you are trying to access index 0 and index 3 which is "banana" and "pear".
You may create a function that accepts an index like:
String getFruit(int index, List<String> fruits) => fruits[index];
print(getFruit[0]); // Will print "banana";
or if you need to actually get the specific ranges you may use:
List<String> fruits =["banana","cherry","apple","pear","kiwi"].getRange(0,4);
// Will give you "banana","cherry","apple","pear
You may check : https://api.dart.dev/be/180791/dart-core/List-class.html for more information.
Edited answer based off the comment:
List<String> getElements(List userInput, nIndex){
List elements = [];
for(int x = 0; x<userInput.length;x++){
if(x % nIndex == 0){
elements.add(userInput[x]);
}
}
return elements;
}
List fruits = ["banana","cherry","apple","pear","kiwi"];
print(getElements(fruits,2));
or you may try to look and use List.retainWhere() depending on your use case.
Dart has a great set of collection operators that make this type of problem pretty straightforward to solve. For example, we could do something like:
extension X<T> on List<T> {
List<T> everyNth(int n) => [for (var i = 0; i < this.length; i += n) this[i]];
}
main() {
final fruit = ["banana", "cherry", "apple", "pear", "kiwi"];
print(fruit.everyNth(3));
}
Output:
[banana, pear]
You can use this extension method, which will work on lists of any type:
extension GetEveryN<T> on List<T> {
List<T> elementsEveryN(int n) {
List<T> result = [];
for(int index = 0; index < length; index +=1) {
if(index % n == 0) {
result.add(this[index]);
}
}
return result;
}
}
Trying it in an example:
List<String> list = ["banana", "cherry","apple", "pear","kiwi"];
print(list.elementsEveryN(2)); // [banana, pear]
I am trying to code a smart contract and I am using a mapping (address => bool) ; i make it true when a certain conditions meet. Now for another condition I want my mapping to reset and loose all of its data , i want it to be where it was in the beginning a empty mapping.
CODE IMAGE
Problem statement : I am setting all true for whoever voted using in my Dapp using a mapping ( address => bool ) now after voting is ended i want all my mapping values to be false , is there any efficient way to solve this
Thank you in advance..
Now for another condition I want my mapping to reset and loose all of its data , i want it to be where it was in the beginning a empty mapping.
The only way to reset mapping is to iterate through all the keys and set values to zero. This is not practical in Solidity.
Instead, you probably want to use mapping with the third layer and having data tuple of (version, key, value). Then just increment version when you want to do a full reset.
Alternatively you can do (key, version, value) and compare the version every time the key is read. This approach could be more gas efficient, as you are using the same storage lot when rewriting values.
I have found this as most feasible so far;
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.0 <0.9.0;
contract Map {
mapping (uint => uint)[] array;
function allocate(uint newMaps) public {
for (uint i = 0; i < newMaps; i++)
array.push();
}
function writeMap(uint map, uint key, uint value) public {
array[map][key] = value;
}
function readMap(uint map, uint key) public view returns (uint) {
return array[map][key];
}
function eraseMaps() public {
delete array;
}
}
I have a situation where I have a list that can be at most 4 elements.
However, if I have only 1-3 elements to put in that list, how can I fill the remainder with null values?
For example, a List<int?> of length 4 with 2 given elements, should result in:
[1,3] -> [1,3,null,null]
Here's what I'm doing, but maybe there is a better way
List.generate(4, (index) {
try {
final id = given.elementAt(index);
return id;
} catch (error) {
return null;
}
});
The simplest version would probably be:
[for (var i = 0; i < 4; i++) i < given.length ? given[i] : null]
You can use List.generate, but in this case, the function is simple enough that a list literal can do the same thing more efficiently.
(Or, as #jamesdlin says, if you want a fixed-length list, use List.generate).
A more indirect variant could be:
List<GivenType?>.filled(4, null)..setAll(0, given)
where you create a list of four nulls first, then write the given list into it. It's probably just more complicated and less efficient.
I am trying to create a custom collector in order to count valid elements of a list. I have done it using one of the already provide collectors:
arr.stream()
.filter(e -> e.matches("[:;][~-]?[)D]"))
.map(e -> 1)
.reduce(0, Integer::sum);
but as a challenge for myself, I wanted to create my own custom collector in order to understand it better. And this is where I got stuck.
It is probably something trivial but I am learning this and can't figure a supplier, an accumulator, and a combiner. I guess I still don't understand something about them. For instance, I have a similar stream:
arr1.stream()
.filter(e -> e.matches("[:;][~-]?[)D]"))
.map(e -> 1)
.collect(temporary array, adding to array, reduce);
AFAIK supplier is a function without arguments, which returns something. I studied standard examples and it is usually a method reference for a new collection, e.g. ArrayList::new. I tried to use constant 0, e -> 0 because I want to return a scalar. I think it is wrong because it makes the stream returning 0. If using method reference for a temporary collection, Java complains about a mismatch of types of a supplier and returning object. I am also confused about using an accumulator if the final result is a number as a combiner would reduce all elements to a number, e.g. (a,b) -> a + b`.
I'm completely stumped.
Probably part of your problem is that you cannot obviously create an accumulator for an Integer type since it is immutable.
You start with this:
System.out.println(IntStream.of(1,2,3).reduce(0, Integer::sum));
You can extend to this:
System.out.println(IntStream.of(1,2,3).boxed()
.collect(Collectors.reducing(0, (i1,i2)->i1+i2)));
Or even this, which has an intermediate mapping function
System.out.println(IntStream.of(1,2,3).boxed()
.collect(Collectors.reducing(0, i->i*2, (i1,i2)->i1+i2)));
You can get this far with your own Collector
Collector<Integer, Integer, Integer> myctry = Collector.of(
()->0,
(i1,i2)->{
// what to do here?
},
(i1,i2)->{
return i1+i2;
}
);
The accumulator is A function that folds a value into a mutable result container with mutable being the keyword here.
So, make a mutable integer
public class MutableInteger {
private int value;
public MutableInteger(int value) {
this.value = value;
}
public void set(int value) {
this.value = value;
}
public int intValue() {
return value;
}
}
And now:
Collector<MutableInteger, MutableInteger, MutableInteger> myc = Collector.of(
()->new MutableInteger(0),
(i1,i2)->{
i1.set(i1.intValue()+i2.intValue());
},
(i1,i2)->{
i1.set(i1.intValue()+i2.intValue());
return i1;
}
);
And then:
System.out.println(IntStream.of(1,2,3)
.mapToObj(MutableInteger::new)
.collect(myc).intValue());
Reference:
Example of stream reduction with distinct combiner and accumulator
EDIT: The finisher just does whatever with the final result. If you don't set it on purpose then it is set by default to IDENTITY_FINISH which is Function.identity() which says just to return the final result as is.
EDIT: If you're really desperate:
Collector<int[], int[], int[]> mycai = Collector.of(
()->new int[1],
(i1,i2)->i1[0] += i2[0],
(i1,i2)->{i1[0] += i2[0]; return i1;}
);
System.out.println(IntStream.of(1,2,3)
.mapToObj(v->{
int[] i = new int[1];
i[0] = v;
return i;
})
.collect(mycai)[0]);
I would like to obtain an object from a List based on a specific search criteria of its member variable
this is the code I am using
class foo
{
foo(this._a);
int _a;
}
List<foo> lst = new List<foo>();
main()
{
foo f = new foo(12);
lst.add(f);
List<foo> result = lst.where( (foo m) {
return m._a == 12;
});
print(result[0]._a);
}
I am getting the error and not sure how to resolve this
Uncaught exception:
TypeError: Instance of 'WhereIterable<foo>': type 'WhereIterable<foo>' is not a subtype of type 'List<foo>'
I am trying to search for an object whose member variable a == 12. Any suggestions on what I might be doing wrong ?
The Iterable.where method returns an iterable of all the members which satisfy your test, not just one, and it's a lazily computed iterable, not a list. You can use lst.where(test).toList() to create a list, but that's overkill if you only need the first element.
You can use lst.firstWhere(test) instead to only return the first element, or you can use lst.where(test).first to do effectively the same thing.
In either case, the code will throw if there is no element matched by the test.
To avoid throwing, you can use var result = lst.firstWhere(test, orElse: () => null) so you get null if there is no such element.
Another alternative is
foo result;
int index = lst.indexWhere(test);
if (index >= 0) result = lst[index];
The answer is simple. Iterable.where returns an Iterable, not a List. AFAIK this is because _WhereIterable does its computations lazily.
If you really need to return a List, call lst.where(...).toList().
Otherwise, you can set result to be an Iterable<foo>, instead of a List<foo>.
or you can go crazy and do this:
bool checkIfProductNotFound(Map<String, Object> trendingProduct) {
bool isNotFound = this
._MyProductList
.where((element) => element["id"] == trendingProduct["id"])
.toList()
.isEmpty;
return isNotFound ;
}