Java 8- forEach method iterator behaviour - foreach

I recently started checking new Java 8 features.
I've come across this forEach iterator-which iterates over the Collection.
Let's take I've one ArrayList of type <Integer> having values= {1,2,3,4,5}
list.forEach(i -> System.out.println(i));
This statement iteates over a list and prints the values inside it.
I'd like to know How am I going to specify that I want it to iterate over some specific values only.
Like, I want it to start from 2nd value and iterate it till 2nd last value. or something like that- or on alternate elements.
How am I going to do that?

To iterate on a section of the original list, use the subList method:
list.subList(1, list.length()-1)
.stream() // This line is optional since List already has a foreach method taking a Consumer as parameter
.forEach(...);

This is the concept of streams. After one operation, the results of that operation become the input for the next.
So for your specific example, you can follow #Joni's command. But if you're asking in general, then you can create a filter to only get the values you want to loop over.
For example, if you only wanted to print the even numbers, you could create a filter on the streams before you forEached them. Like this:
List<Integer> intList = Arrays.asList(1,2,3,4,5);
intList.stream()
.filter(e -> (e & 1) == 0)
.forEach(System.out::println);
You can similarly pick out the stuff you want to loop over before reaching your terminal operation (in your case the forEach) on the stream. I suggest you read this stream tutorial to get a better idea of how they work: http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/

Related

Dart: how to apply function to each inner element of list of lists

Obviously playing with dart and still new to it and implementing tic tac toe. So I have a list of lists of winning positions and want to check if any of the lists is sublist of the inputed moves list. Sounds pretty easy right, so I apply for each element of inner list I wanna check my condition. What are the dart ways to achieve this. This isn't working because map expect function of type void and I return some calls to print for each element of every inner list obviously. Why it isn't showing me any mistake at all.
I tried with .forEach and .takeWhile examples but coudn't get it working.
List <List<int>> winMoves = new List.generate(8, (i) => new List(3));
winMoves[0]=[1,2,3];
winMoves[1]=[4,5,6];
winMoves[2]=[7,8,9];
winMoves[3]=[1,4,7];
winMoves[4]=[2,5,8];
winMoves[5]=[3,6,9];
winMoves[6]=[1,5,9];
winMoves[7]=[3,5,7];
winMoves.map((list) => (list.forEach((el) => print(el))));
import 'package:collection/collection.dart' show ListEquality;
...
const eq = const ListEquality<int>();
print(winMoves.indexWhere((list) => eq.equals(list, [1, 5, 9])));
Depending on what result you want you can also use where, firstWhere, any, ...
There are many ways to solve this problem, depending on what the exact desired result is.
You have a list of elements, and you want to check something about these elements (in this case the elements are lists too, but that's not important to begin with).
If you want to know whether any of the elements satisfies your check, you can use:
bool hasWin = winMoves.any((moves) => ...check...);
If you want to find an element which satisfies your check, you can use:
var win = winMoves.firstWhere((moves) => ...check..., orElse: null);
If you want to find all the winners:
var wins = winMoves.where((moves) => ...check...).toList(); // `where` is lazy.
How to write the check is a separate issue. In this case you seem to want, as Günther Zöchbauer has already written, something like
(moves) => moves.every(expectedMoves.contains)
Your example is not giving you any warnings or errors because it's correct, and it doesn't actually do anything. The map function is lazy, it creates an Iterable which won't do anything until you start iterating. If you call .toList() on it, it'll create a List<void>. Not very useful, but valid in Dart 2.
Also, your initial winMoves initialization is overly complex. You can just do:
List<List<int>> winMoves = []..length = 8; // for growable list, or
List<List<int>> winMoves = List(8); // for fixed-length list.
You actively fill the list with lists, then immediately overwrite those values with new lists in the following lines.

Reorder elements in Erlang

I want to redefine the order of a tuple looking for specific words
Example, I have a list of tuples like this:
[{"a",["r001"]},
{"bi",["bidder"]},
{"bo",["an"]}]
But sometimes the order of the tuples can change for example:
[{"bi",["bidder"]},
{"a",["r001"]},
{"bo",["an"]}]
or
[{"bo",["an"]},
{"a",["r001"]},
{"bi",["bidder"]}]
The first string/list of the tuple is my unique key ("bo","a","bi")
But I want to be able to reorder the list of tuples, always like:
[{"a",["r001"]},
{"bi",["bidder"]},
{"bo",["an"]}]
How can I achieve this?
This will do it:
lists:sort(fun({A,_},{B,_}) -> A =< B end, List).
Or this, which will sort by the tuples second element after the first:
lists:sort(List).
I offer the second version, because without the custom sort function, it is faster for data like this.
If you need to sort by specified element, you just sort by specified element
lists:keysort(1, List).

Dart: DoubleLinkedQueue length after entry prepend

I'm manipulating entries inside a DoubleLinkedQueue via the DoubleLinkedQueueElement.append/prepend methods. This results in the new elements being inserted into the queue, but fails to update the length, and the toList() method results in an error being thrown.
I understand queues are only supposed to have elements added at the start/end, but it looks like the interface should allow for adding in the middle via the entries. I find it hard to believe such a common/well understood data structure would have a bug at this point - so am I using DoubleLinkedQueues incorrectly? Is there another data structure that I should be using? I'm looking to merge values from another iterable into my own sorted iterable - a SplayTreeSet might get me there in n log n time, but a simple merge should get me there in linear time...
Example of code that acts unexpectedly:
main() {
var q = new DoubleLinkedQueue<int>.from([1]);
q.firstEntry().prepend(0);
print('length: ${q.length}');
int i = 0;
for (var qi in q){
print('${i++}: $qi');
}
}
Output:
length: 1
0: 0
1: 1
It looks like the length getter is only pointing to an internal counter. This is done because counting the elements everytime might take very long for long lists.
The internal counter is only updated if you use the methods that directly operate on the list instead of using the prepend method of an element. In your example you should use q.addFirst(0); which results in the length being updates. The .prepend() method just inserts a new element and changes the pointer. Which results in correct traversation of the elements, but the counter is wrong anyway.
Unfortunately it looks like you cannot insert elements in the middle of the list, nor can you make the list recount the elements. You should consider creating a bug over at www.dartbug.com.
// Update:
toList() throws an error because there are more elements than length.

detect if a combination of string objects from an array matches against any commands

Please be patient and read my current scenario. My question is below.
My application takes in speech input and is successfully able to group words that match together to form either one word or a group of words - called phrases; be it a name, an action, a pet, or a time frame.
I have a master list of the phrases that are allowed and are stored in their respective arrays. So I have the following arrays validNamesArray, validActionsArray, validPetsArray, and a validTimeFramesArray.
A new array of phrases is returned each and every time the user stops speaking.
NSArray *phrasesBeingFedIn = #[#"CHARLIE", #"EAT", #"AT TEN O CLOCK",
#"CAT",
#"DOG", "URINATE",
#"CHILDREN", #"ITS TIME TO", #"PLAY"];
Knowing that its ok to have the following combination to create a command:
COMMAND 1: NAME + ACTION + TIME FRAME
COMMAND 2: PET + ACTION
COMMAND n: n + n, .. + n
//In the example above, only the groups of phrases 'Charlie eat at ten o clock' and 'dog urinate'
//would be valid commands, the phrase 'cat' would not qualify any of the commands
//and will therefor be ignored
Question
What is the best way for me to parse through the phrases being fed in and determine which combination phrases will satisfy my list of commands?
POSSIBLE solution I've come up with
One way is to step through the array and have if and else statements that check the phrases ahead and see if they satisfy any valid command patterns from the list, however my solution is not dynamic, I would have to add a new set of if and else statements for every single new command permutation I create.
My solution is not efficient. Any ideas on how I could go about creating something like this that will work and is dynamic no matter if I add a new command sequence of phrase combination?
I think what I would do is make an array for each category of speech (pet, command, etc). Those arrays would obviously have strings as elements. You could then test each word against each simple array using
[simpleWordListOfPets containsObject:word]
Which would return a BOOL result. You could do that in a case statement. The logic after that is up to you, but I would keep scanning the sentence using NSScanner until you have finished evaluating each section.
I've used some similar concepts to analyze a paragraph... it starts off like this:
while ([scanner scanUpToString:#"," intoString:&word]) {
processedWordCount++;
NSLog(#"%i total words processed", processedWordCount);
// Does word exist in the simple list?
if ([simpleWordList containsObject:word]) {
//NSLog(#"Word already exists: %#", word);
You would continue it with whatever logic you wanted (and you would search for a space rather than a ",".

how to sort documents using the erlang map reduce in riak

i'm using riak to store json documents right now, and i want to sort them based on some attribute, let's say there's a key, i.e
{
"someAttribute": "whatever",
"order": 1
}
so i want to sort the documents based on the "order".
I am currently retrieving the documents in riak with the erlang interface. i can retrieve the document back as a string, but i dont' really know what to do after that. i'm thinking the map function just reduces the json document itself, and in the reduce function, i'd make a check to see whether the item i'm looking at has a higher "order" than the head of the rest of the list, and if so append to beginning, and then return a lists:reverse.
despite my ideas above i've had zero results after almost an entire day, i'm so confused with the erlang interface in riak. can someone provide insight on how to write this map/reduce function, or just how to parse the json document?
As far as I know, You do not have access to Input list in Map. You emit from Map a document as 1 element list.
Inputs (all the docs to handle as {Bucket, Key}) -> Map (handle single doc) -> Reduce (whole list emitted from Map).
Maps are executed per each doc on many nodes whereas Reduce is done once on so called coordinator node (the one where query was called).
Solution:
Define Inputs (as a list or bucket)
Retrieve Value in Map and emit whole doc or {Id, Val_to_sort_by)
Sort in Reduce (using regular list:keysort)
This is not a map reduce solution but you should check out Riak Search.
so i "solved" the problem using javascript, still can't do it using erlang.
here is my query
{"inputs":"test",
"query":[{"map":{"language":"javascript",
"source":"function(value, keyData, arg){ var data = Riak.mapValuesJson(value)[0]; var obj = {}; obj[data.order] = data; return [ obj ];}"}},
{"reduce":{"language":"javascript",
"source":"function(values, arg){ return [ values.reduce(function(acc, item){ for(var order in item){ acc[order] = item[order]; } return acc; }) ];}",
"keep":true}}
]
}
so in the map phase, all i do is create a new array, obj, with the key as the order, and the value as the data itself. so visually, the obj is like this
{"1":{"firstName":"John","order":1}
in the reduce phase, i'm just putting it in the accumulator, so basically that's the sort if you think about it, because when you're done, everything will be put in order for you. so i put 2 json documents for testing, one is above, the ohter is just firstName: Billie, order 2. and here is my result for the query above
[{"1":{"firstName":"John","order":1},"2":{"firstName":"Billie","order":2}}]
so it works! . but i still need to do this in ERLANG, any insights?

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