I have a Stream, that yields Strings like this:
'apple'
'orange'
'banana'
When I listen to the Stream, I want to assign numbers to the results from 1 to n (Stream lenght) in the order they were yielded like this:
1: 'apple'
2: 'orange'
3: 'banana'
What is the best way to do that?
As there is no enumerate() function like in Python, my attempts so far are pretty bad:
inside Stream definition
int n = 0
await for (var nextString in originStream) {
++n;
yield [n, nextString];
}
when stream is called
myStream.transform(enumerateFunc)
List enumerateFunc(String myStr){
return [n, myStr];
}
I got a suggestion to create a class for this. But I don't want to create a class for simply assing numbers to Stream events in the order they happenned.
Is there a better way to do this?
As #julemand101 suggested, a pretty viable option is to create a class for this purpose and yield the class instead of List:
Stream<String> fruits() async* {
int n = 1;
await for (final String fruit in fruitChannel) {
yield Enumerated(n, fruit);
n++;
}
}
Related
I need to loop through list items and I need index, not value.
I know three options:
Classic:
for (final i = 0; i < list.length; i++) {
With asMap:
for (final i in list.asMap().keys)
With Iterable:
for (final i in Iterable.generate(list.length)) {
Non-classic options look to be easier to read, write and less error prone.
How about performance? It seems both options produce iterable, so they should not create performance overhead. Is it correct assessment or I am missing something?
Micro benchmark here (usual caveats apply) https://gist.github.com/jakemac53/16c782ed92f6bbceb98ad83cd257c760.
If your code is perf sensitive, use the "classic" for loop.
While existing alternatives for the classic way are not great from performance perspective, there is work in progress to introduce a better way to loop lists: https://github.com/dart-lang/collection/pull/259#discussion_r1090563595
For now you the extension will enable nice looping:
extension Indexes<T> on List<T> {
Iterable<int> get indexes sync* {
for (var i = 0; i < length; i++) yield i;
}
}
Then we can write:
for (final i in list.indexes)
Using package:collection, you can get a .mapIndexed on an Iterable to give you both the item and the index of the item.
final result = someList.mapIndexed((n, e) => "item $n is $e");
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 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 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 have one line data like this:
a\tb1,b2,..,bn\tc1,c2,..,cn
in which n is uncertain. And now, I want transform it to some lines like this:
a\tb1\tc1
a\tb2\tc2
...
a\tbn\tcn
Is it possible by pig latin, or has to use UDF?
If using the script:
A = LOAD 'file' AS (a, b, c);
B = FOREACH A GENERATE a, FLATTEN(TOKENIZE(b)), FLATTEN(TOKENIZE(c));
dump B;
I will get the resulr as following:
a\tb1\tc1
a\tb1\tc2
..
a\tb1\tcn
a\tb2\tc1
a\tb2\tc2
..
a\tb2\tcn
..
It isn't the data I wanted. Does anyone have ideas?
IMO too many people who use Pig are resistant to write UDFs. In your case, the UDF you'd need to do this is fairly simple. Here's sample code (untested)
public class InSequenceJoin extends EvalFunc<DataBag>
{
public DataBag exec(Tuple input) throws IOException {
String b = (String) input.get(0);
String c = (String) input.get(1);
String[] bArray = b.split(",");
String[] cArray = c.split(",");
DataBag bag = BagFactory.getInstance().newDefaultBag();
for (int i = 0; i < bArray.length && i < cArray.length; i++) {
Tuple tuple = TupleFactory.getInstance.newTuple(2);
tuple.set(0, bArray[i]);
tuple.set(1, cArray[i]);
bag.add(tuple);
}
return bag;
}
}
define InSequenceJoin mysourcepath.InSequenceJoin();
A = LOAD 'file' AS (a, b, c);
B = FOREACH A GENERATE a, FLATTEN(InSequenceJoin(b,c));
dump B;
You could add validation on if the sizes of the arrays match if you need to in the UDF. You could replace the String split I used in example with whatever you truly require.
I'd try to use datafu's bag UDFs.
Load the data as you've done, then use Enumerate to enumerate the bag elements, then flatten (which gives you the cross join between the bag elements as you've seen) and then you can filter on the indexes added to the bag elements.
See here: https://github.com/linkedin/datafu