I have a List that contains Maps.
I would like to know what the list index of a specific Map is by providing a Map value.
In my case, the unique value is for the key "Item ID".
In the example below, how would I find the list index of the map where key 'itemID' == '222'?
List<Map<String, dynamic>> itemList = [
{
'itemID': '111',
'itemDescription': 'Lolipop',
},
{
'itemID': '222',
'itemDescription': 'Haribo',
},
{
'itemID': '333',
'itemDescription': 'KitKat',
},
];
Updated to add code example. Sorry, really new to this and Dart.
Try:
int indexWhereValue<K, V>(List<Map<K, V>> maps, K key, V value) =>
maps.indexWhere((map) => map[key] == value);
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]
Hello I am new to dart and trying to find an item by property name in a list of list.
class Product{
String id;
String title;
Product(this.id,this.title);
}
void main(){
List<List<Product>> allProdcuts=[
//New Prodcuts
[
Product("1","Hammer"),
Product("3","Nails"),
Product("2","Screws"),
],
futureItems,
//Old Prodcuts
[
Product("4","Rock"),
Product("5","Paper"),
Product("6","Scissor"),
],
//Rare Items
[
Product("7","Plank"),
Product("8","Wires"),
Product("9","Box"),
],
];
print(allProdcuts.where((itemsList)=>itemsList.contains((product)=>product.title='Wires')));
//Returns ()
}
I have tried using for a single List:
List<Product> futureItems= [
Product("101","Galactic Hammer"),
Product("301","Galactic Nails"),
Product("201","Galactic Screws"),
];
print(newProduct.firstWhere((p)=>p.title=='Hammer'));
//Instance of 'Product'
Also tried this:
print(allProdcuts.map((itemList)=>itemList.firstWhere((p)=>p.title=='Nails')));
// Bad state: No elementError: Bad state: No element.
But there is an element with the title='Nails'.I don't understand what I am doing wrong.
You are calling itemList.firstWhere((p)=>p.title=='Nails') on each list, also the ones with no element with title "Nails". Since firstWhere throws if there is no matching value, it does that for two of your three lists. Also, in the example, itemsList.contains(...) does not take a callback, so you are just checking whether a function is in the list, which it isn't. You might want to use any for that, but it won't solve the problem here.
To do this efficiently, I'd probably create helper function:
Product findByTitle(List<List<Product>> allProducts, String title) {
for (var products in allProducts) {
for (var product in products) {
if (product.title == title) return product;
}
}
// Or return `null`.
throw ArgumentError.value(title, "title", "No element with that title");
}
The return in the middle allows you to skip out of the double iteration the moment you have a match, something which is harder to do with firstWhere/map/forEach etc.
One alternative solutions would be:
var product = allProducts.expand((l) => l.where((p) => p.title == title)).first;
which finds all the products with the given title and flattens them into a single iterable, then picks the first one (if there are any). Because iterables are lazy, it will actually stop at the first match.
There are many ways to solve this.
One example is to use the forEach() method:
allProdcuts.forEach(
(List<Product> l)=>l.forEach(
(Product p){
if (p.title=="Nails")
print(p.id);
}
)
);
The for each method receives a function and applies this function to every element on the list. If you have a lists of lists, you can do this twice to get a function applied to each element of the sub lists.
The above code prints 3, which is the desired result.
Another solution would be to flatten the list first, so you can have an easier search later.
print(allProdcuts.any((innerListOfProducts) =>
innerListOfProducts.any((product) => product.title == 'Wires')));
This code will return true if 'Wires' is in the inner list, and false otherwise.
I'm trying to sort snapshot by timestamp but returns original order.
data structure looks like this
I have two snapshot and timestamps are -1536025466539 and -1536025893015.
So, I expect -1536025893015 to come first after sorted.
Does anyone know how to sort correctly?
Code:
Map<dynamic, dynamic> map = snapshot.data?.snapshot?.value;
map.values.toList().sort((a, b) {
return a['timestamp'].compareTo(b['timestamp']);
// also not work return b['timestamp'].compareTo(a['timestamp']);
});
From the above code, it looks like you are not having a variable to hold the list.
Map<dynamic, dynamic> map = {
"one": {"timestamp": 1},
"two": {"timestamp": 2}
};
List list = map.values.toList(); //variable which holds new list created from Map.
//As it new list, any change in list will not have no impact on map.
list.sort((a, b) {
return b["timestamp"].compareTo(a["timestamp"]);
}); // inplace sort
print(list); // [{timestamp: 2}, {timestamp: 1}]
If you want key also in the result,
var list = map.entries.toList();
list.sort((a, b) => b.value["timestamp"].compareTo(a.value["timestamp"]));
var list2 = list.map((a) => {a.key: a.value}).toList();
print(list2); // [{two: {timestamp: 2}}, {one: {timestamp: 1}}]
Unfortunately when you call snapshot.data?.snapshot?.value, the resulting Map doesn't have space for the order of the items anymore.
In most Firebase SDKs you can solve this by looping over the children of the snapshot in the value event listener. But in Flutter you will need to listen to onChildAdded events to maintain the value of the items in Flutter, at least until this issue is addressed: https://github.com/flutter/flutter/issues/20745.
From one of my own projects:
ref.orderByKey().equalTo("child1").onChildAdded.listen((event) {
print(event.snapshot.key);
});
ref.child("child1").once().then((snapshot) {
print("Done loading all data for child1");
});
This will first print the keys of all child nodes (in the requested order), and then "Done loading all data for child1".
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]);
I am trying to create multiple nodes in Neo4j using Cypher by passing properties as parameters as part of an UNWIND function, but I keep receiving the error Type mismatch: expected Collection<T> but was Map.
This happens even when using the following example from the Neo4j documentation (link):
UNWIND {
props : [ {
name : "Andres",
position : "Developer"
}, {
name : "Michael",
position : "Developer"
} ]
} AS map
CREATE (n)
SET n = map
Can anyone point out what I am doing wrong here?
Note, the example above is not exactly as in the Neo4j documentation. Their example wraps the property names in double quotes, but this causes my instance of Neo4j to throw the errorInvalid input '"': expected whitespace...)
UNWIND is expecting a collection, not a map as you're currently passing in, try this instead (just remove the wrapping curly braces and prop top level field):
UNWIND [ {
name : "Andres",
position : "Developer"
}, {
name : "Michael",
position : "Developer"
} ] AS map
CREATE (n)
SET n = map
Chris's answer is of course the correct one, but here's why your solution doesn't work when you're following the documentation: you're not copying the documentation.
The documentation shows the use of a named parameter:
UNWIND { props } AS map
CREATE (n)
SET n = map
with props passed in the map of parameters, which would look like:
{
"props" : [ {
"name" : "Andres",
"position" : "Developer"
}, {
"name" : "Michael",
"position" : "Developer"
} ]
}
if you displayed the map as JSON. It means the {props} placeholder will be replaced by the value for the props key. Which is exactly what Chris did.
Here's what the Java code would look like:
GraphDatabaseService db = /* init */;
Map<String, Object> andres = new HashMap<>();
andres.put("name", "Andres");
andres.put("position", "Developer");
Map<String, Object> michael = new HashMap<>();
michael.put("name", "Michael");
michael.put("position", "Developer");
Map<String, Object> params = new HashMap<>();
params.put("props", Arrays.asList(andres, michael));
try (Transaction tx = db.beginTx()) {
db.execute("UNWIND {props} AS map CREATE (n) SET n = map", params);
tx.success();
}