I want to have a map with duplicate keys. Is there such a map in Dart or a utility library that has this functionality?
I'm using the following get a count of items:
myList.forEach(
(element) {
if (!myMap.containsKey(element)) {
myMap[element] = 1;
} else {
myMap[element] += 1;
}
},
);
then convert keys/values to lists: Need to switch key/values...
final keys = myMap.keys.toList();
final itemSpit = keys.map((e) => e.toString().split('§º')).toList();
final values = myMap.values.toList();
put it in a map
final map = Map();
for (var i = 0; i < values.length; i++) {
map[values[i]] = itemSpit[i];
}
Obviously the keys are overridden in the for loop.
then
iterate over map (Put keys/values in flutter widgets)
final cells = map.entries
.map((e) => ........
THe first method increases the value count if there's a duplicate value. So I have this. ... {breadwhitelarge: 3, cornyellowsmall:5 ..etc..}
I then have to split the strings and have output like this
5 bread white large
3 corn yellow small
Instead of defining a map which allows duplicated keys you can instead create a Map<K,List<V>> like this example:
void main() {
final map = <String, List<int>>{};
addValueToMap(map, 'Test 1', 1);
addValueToMap(map, 'Test 1', 2);
addValueToMap(map, 'Test 2', 3);
addValueToMap(map, 'Test 1', 4);
addValueToMap(map, 'Test 2', 5);
addValueToMap(map, 'Test 3', 6);
print(map); // {Test 1: [1, 2, 4], Test 2: [3, 5], Test 3: [6]}
}
void addValueToMap<K, V>(Map<K, List<V>> map, K key, V value) =>
map.update(key, (list) => list..add(value), ifAbsent: () => [value]);
You can then ask for a given key and get a list of all values connected to this key.
package:quiver provides a MultiMap class with List-based and Set-based implementations.
Could you just create the Flutter widgets directly from the first map?
var widgets = [for (var e in myMap.entries) MyWidget(
count: e.value,
strings: [... e.key.split("§º")])];
Building the intermediate map seems to be what is causing the trouble.
this is an example map has duplicate keys
withDuplicateKey() {
List<dynamic> demoList = [
{1},
{2},
{3},
{1}
];
var toRemove = {};
demoList.forEach((e) {
toRemove.putIfAbsent(e, () => e);
});
print(toRemove.keys.toList());
}
output is ( printed list of key )
[{1}, {2}, {3}, {1}]
Related
Is there a method we use to reach the desired number in an array given in dart language.. I can do this for binary ones, but I can't do it for a code that finds the sum of 3 or more elements
For example
Input: candidates = [10,1,2,7,6,1,5], target = 8
Output:
[
[1,1,6],
[1,2,5],
[1,7],
[2,6]
]
this is the my code i have done until now
void main() {
var candidates = [10, 1, 2, 7, 6, 1, 5], target = 8;
var answer = [];
for (int i = 0; i < candidates.length; i++) {
for (int j = 0; j < candidates.length; j++) {
if (candidates[i] + candidates[j] == target && i != j && i < j) {
answer.add([candidates[i], candidates[j]]);
}
}
}
}
I am sure this can be done more efficient but since the solution is for some Leetcode assignment, I don't really want to spend too much time on optimizations.
I have tried added some comments in the code which explains my way of doing it:
void main() {
getSumLists([10, 1, 2, 7, 6, 1, 5], 8).forEach(print);
// [5, 1, 2]
// [1, 6, 1]
// [1, 7]
// [6, 2]
getSumLists([2, 5, 2, 1, 2], 5).forEach(print);
// [2, 1, 2]
// [5]
}
Iterable<List<int>> getSumLists(
List<int> candidates,
int target, {
List<int>? tempAnswer,
int sum = 0,
}) sync* {
// We cannot use default value in parameter since that makes list const
final tempAnswerNullChecked = tempAnswer ?? [];
if (sum == target) {
// We got a result we can return.
// OPTIMIZATION: If you know the returned list from each found result is not
// being used between found results, you can remove the `.toList()` part.
yield tempAnswerNullChecked.toList();
} else if (sum > target) {
// No need to search further in this branch since we are over the target
return;
}
// Make a copy so we don't destroy the input list but also so it works even
// if provided list as input is non-growing / non-modifiable
final newCandidates = candidates.toList();
while (newCandidates.isNotEmpty) {
// We take numbers from the end of the list since that is more efficient.
final number = newCandidates.removeLast();
// Recursive call where we return all results we are going to find given
// the new parameters
yield* getSumLists(
newCandidates,
target,
tempAnswer: tempAnswerNullChecked..add(number),
sum: sum + number,
);
// Instead of creating a new tempAnswerNullChecked, we just reuse it and
// make sure we remove any value we are temporary adding
tempAnswerNullChecked.removeLast();
// Ensure we don't get duplicate combinations. So if we have checked the
// number `1` we remove all `1` so we don't try the second `1`.
newCandidates.removeWhere((element) => element == number);
}
}
I'm trying to create a custom world map where countries are merged into regions instead of having individual countries. Unfortunately for some reason something seems to get messed up with the winding order along the process.
As base data I'm using the natural earth 10m_admin_0_countries shape files available here. As criteria for merging countries I have a lookup map that looks like this:
const countryGroups = {
"EUR": ["ALA", "AUT", "BEL"...],
"AFR": ["AGO", "BDI", "BEN"...],
...
}
To merge the shapes I'm using topojson-client. Since I want to have a higher level of control than the CLI commands offer, I wrote a script. It goes through the lookup map and picks out all the topojson features that belong to a group and merges them into one shape and places the resulting merged features into a geojson frame:
const topojsonClient = require("topojson-client");
const topojsonServer = require("topojson-server");
const worldTopo = topojsonServer.topology({
countries: JSON.parse(fs.readFileSync("./world.geojson", "utf-8")),
});
const geoJson = {
type: "FeatureCollection",
features: Object.entries(countryGroups).map(([region, ids]) => {
const relevantCountries = worldTopo.objects.countries.geometries.filter(
(country, i) =>
ids.indexOf(country.properties.ISO_A3) >= 0
);
return {
type: "Feature",
properties: { region, countries: ids },
geometry: topojsonClient.merge(worldTopo, relevantCountries),
};
}),
};
So far everything works well (allegedly). When I try to visualise the map using github gist (or any other visualisation tool like vega lite) the shapes seem to be all messed up. I'm suspecting that I'm doing something wrong during the merging of the features but I can't figure out what it is.
When I try to do the same using the CLI it seems to work fine. But since I need more control over the merging, using just the CLI is not really an option.
The last feature, called "World", should contain all remaining countries, but instead, it contains all countries, period. You can see this in the following showcase.
var w = 900,
h = 300;
var projection = d3.geoMercator().translate([w / 2, h / 2]).scale(100);
var path = d3.geoPath().projection(projection);
var color = d3.scaleOrdinal(d3.schemeCategory10);
var svg = d3.select('svg')
.attr('width', w)
.attr('height', h);
var url = "https://gist.githubusercontent.com/Flave/832ebba5726aeca3518b1356d9d726cb/raw/5957dca433cbf50fe4dea0c3fa94bb4f91c754b7/world-regions-wrong.topojson";
d3.json(url)
.then(data => {
var geojson = topojson.feature(data, data.objects.regions);
geojson.features.forEach(f => {
console.log(f.properties.region, f.properties.countries);
});
svg.selectAll('path')
// Reverse because it's the last feature that is the problem
.data(geojson.features.reverse())
.enter()
.append('path')
.attr('d', path)
.attr('fill', d => color(d.properties.region))
.attr('stroke', d => color(d.properties.region))
.on('mouseenter', function() {
d3.select(this).style('fill-opacity', 1);
})
.on('mouseleave', function() {
d3.select(this).style('fill-opacity', null);
});
});
path {
fill-opacity: 0.3;
stroke-width: 2px;
stroke-opacity: 0.4;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/5.7.0/d3.js"></script>
<script src="https://d3js.org/topojson.v3.js"></script>
<svg></svg>
To fix this, I'd make sure to always remove all assigned countries from the list. From your data, I can't see where "World" is defined, and if it contains all countries on earth, or if it's a wildcard assignment.
In any case, you should be able to fix it by removing all matches from worldTopo:
const topojsonClient = require("topojson-client");
const topojsonServer = require("topojson-server");
const worldTopo = topojsonServer.topology({
countries: JSON.parse(fs.readFileSync("./world.geojson", "utf-8")),
});
const geoJson = {
type: "FeatureCollection",
features: Object.entries(countryGroups).map(([region, ids]) => {
const relevantCountries = worldTopo.objects.countries.geometries.filter(
(country, i) =>
ids.indexOf(country.properties.ISO_A3) >= 0
);
relevantCountries.forEach(c => {
const index = worldTopo.indexOf(c);
if (index === -1) throw Error(`Expected to find country ${c.properties.ISO_A3} in worldTopo`);
worldTopo.splice(index, 1);
});
return {
type: "Feature",
properties: { region, countries: ids },
geometry: topojsonClient.merge(worldTopo, relevantCountries),
};
}),
};
Suppose I have a Map:
Map<String, int> source = {'a':1, 'b':2, 'c':3};
I want to get this:
Map<String, int> expected = {'a': 1, 'b':4, 'c':9 };
I want to achieve the result using map function:
Map<String,int> actual = source.map((key,value)=> {key: value * value});
However, I got this error:
The return type 'Map<String, int>' isn't a 'MapEntry<String, int>', as required by the closure's context
Can't we use the map function of map to get another map like this?
The mapping method should return a MapEntry instance since you can change both the key and value. So your code should instead be something like:
void main() {
final source = {'a': 1, 'b': 2, 'c': 3};
final actual = source.map((key, value) => MapEntry(key, value * value));
print(actual); // {a: 1, b: 4, c: 9}
}
Using collection-for might be more straightforward than using Map.map:
final source = {'a': 1, 'b': 2, 'c': 3};
final actual = {
for (var entry in source.entries)
entry.key: entry.value * entry.value,
};
The Map interface doesn't seem to provide access to the entries as an iterable, nor does it expose a where method to filter entries. Am I missing something? Is there a simple workaround?
e.g.
Map map;
final filteredMap = map.where((k, v) => k.startsWith("foo"));
Update: with control flow collection statements you can also do this:
final filteredMap = {
for (final key in map.keys)
if (!key.startsWith('foo')) key: map[key]
};
Original answer: Dart 2.0.0 added removeWhere which can be used to filter Map entities. Given your example, you could apply this as:
Map map;
final filteredMap = Map.from(map)..removeWhere((k, v) => !k.startsWith("foo"));
It's not the where method you asked for, but filtering Map entities is certainly doable this way.
Since Dart 2.0 Maps have an entries getter that returns an Iterable<MapEntry<K, V>> so you can do:
MapEntry theOne = map.entries.firstWhere((entry) {
return entry.key.startsWith('foo');
}, orElse: () => MapEntry(null, null));
You can use
library x;
void main(List<String> args) {
Map map = {'key1': 'aölsjfd', 'key2': 'oiweuwrow', 'key11': 'oipoip', 'key13': 'werwr'};
final filteredMap = new Map.fromIterable(
map.keys.where((k) => k.startsWith('key1')), key: (k) => k, value: (k) => map[k]);
filteredMap.forEach((k, v) => print('key: $k, value: $v'));
}
I use dartx and it's filter method
var myMap = {
"a": [1, 2, 3],
"b": [4, 5, 6],
"c": [7, 8, 9],
};
var result = myMap.filter((entry) => entry.key != "a");
You can just create an extension function and then use it anywhere in your code.
Put this in any file (I called mine MapUtils.dart)
extension MapUtils<K, V> on Map<K, V> {
Map<K, V> where(bool Function(K, V) condition) {
Map<K, V> result = {};
this.entries.forEach((element) {
if (condition(element.key, element.value)) {
result[element.key] = element.value;
}
});
return result;
}
}
and then use it like so:
Map<String, int> peopleHeight = {"Bob":170, "Alice":130};
Map<String, int> shortPeople = peopleHeight.where((name, height) => height < 140);
How do you get the min and max values of a List in Dart.
[1, 2, 3, 4, 5].min //returns 1
[1, 2, 3, 4, 5].max //returns 5
I'm sure I could a) write a short function or b) copy then sort the list and select the last value,
but I'm looking to see if there is a more native solution if there is any.
Assuming the list is not empty you can use Iterable.reduce :
import 'dart:math';
main(){
print([1,2,8,6].reduce(max)); // 8
print([1,2,8,6].reduce(min)); // 1
}
If you don't want to import dart: math and still wants to use reduce:
main() {
List list = [2,8,1,6]; // List should not be empty.
print(list.reduce((curr, next) => curr > next? curr: next)); // 8 --> Max
print(list.reduce((curr, next) => curr < next? curr: next)); // 1 --> Min
}
You can now achieve this with an extension as of Dart 2.6:
import 'dart:math';
void main() {
[1, 2, 3, 4, 5].min; // returns 1
[1, 2, 3, 4, 5].max; // returns 5
}
extension FancyIterable on Iterable<int> {
int get max => reduce(math.max);
int get min => reduce(math.min);
}
An example to get Min/Max value using reduce based on condition for a list of Map objects
Map studentA = {
'Name': 'John',
'Marks': 85
};
Map studentB = {
'Name': 'Peter',
'Marks': 70
};
List<Map> students = [studentA, studentB];
// Get student having maximum mark from the list
Map studentWithMaxMarks = students.reduce((a, b) {
if (a["Marks"] > b["Marks"])
return a;
else
return b;
});
// Get student having minimum mark from the list (one liner)
Map studentWithMinMarks = students.reduce((a, b) => a["Marks"] < b["Marks"] ? a : b);
Another example to get Min/Max value using reduce based on condition for a list of class objects
class Student {
final String Name;
final int Marks;
Student(this.Name, this.Marks);
}
final studentA = Student('John', 85);
final studentB = Student('Peter', 70);
List<Student> students = [studentA, studentB];
// Get student having minimum marks from the list
Student studentWithMinMarks = students.reduce((a, b) => a.Marks < b.Marks ? a : b);
If your list is empty, reduce will throw an error.
You can use fold instead of reduce.
// nan compare to any number will return false
final initialValue = number.nan;
// max
values.fold(initialValue, (previousValue, element) => element.value > previousValue ? element.value : previousValue);
// min
values.fold(initialValue, (previousValue, element) => element.value < previousValue ? element.value : previousValue);
It can also use to calculate sum.
final initialValue = 0;
values.fold(initialValue, (previousValue, element) => element.value + previousValue);
Although fold is not cleaner than reduce for getting min/max, it is still a powerful method to do more flexible actions.
For empty lists: This will return 0 if list is empty, the max value otherwise.
List<int> x = [ ];
print(x.isEmpty ? 0 : x.reduce(max)); //prints 0
List<int> x = [1,32,5];
print(x.isEmpty ? 0 : x.reduce(max)); //prints 32
int minF() {
final mass = [1, 2, 0, 3, 5];
mass.sort();
return mass[0];
}
void main() {
firstNonConsecutive([1,2,3,4,6,7,8]);
}
int? firstNonConsecutive(List<int> arr) {
var max = arr.reduce((curr, next) => curr > next? curr: next);
print(max); // 8 --> Max
var min = arr.reduce((curr, next) => curr < next? curr: next);
print(min); // 1 --> Min
return null;
}
If you need a more sophisticated min/max, such as finding an object with a min/max of a field, or use of a comparison predicate, use minBy() and maxBy() from the collection package:
import 'package:collection/collection.dart';
class Person {
final String name;
final int age;
Person(this.name, this.age);
#override
String toString() => '$name (age $age)';
}
main() {
final alice = Person('Alice', 30);
final bob = Person('Bob', 40);
final chris = Person('Chris', 25);
final dan = Person('Dan', 35);
final people = [alice, bob, chris, dan];
print('Youngest is ${minBy(people, (e) => e.age)}');
print('Oldest is ${maxBy(people, (e) => e.age)}');
print('First alphabetically is ${minBy(people, (e) => e.name)}');
print('Last alphabetically is ${maxBy(people, (e) => e.name)}');
print('Largest name length times age is ${maxBy(people, (e) => e, compare: (a, b) => (a.name.length * a.age).compareTo(b.name.length * b.age))}');
}
Output:
Youngest is Chris (age 25)
Oldest is Bob (age 40)
First alphabetically is Alice (age 30)
Last alphabetically is Dan (age 35)
Largest name length times age is Alice (age 30)```