How to apply a math function to each object of an ArrayList

Question

I'm having trouble figuring out how to apply a math operation to each item of an ArrayList. The ArrayList will be user inputted so there's no telling how many items would be within it. Is there a method that might aid in doing this?

Foreach loop? http://stackoverflow.com/questions/85190/how-does-the-java-for-each-loop-work — Neil Locketz, Sep 28 '16 at 20:45
You can use stream().map to apply a function to each element of a List — puhlen, Sep 28 '16 at 20:46
This gives an example of using stream.map() : https://www.mkyong.com/java8/java-8-filter-a-map-examples/ — StvnBrkdll, Sep 28 '16 at 20:49
What _exactly_ do you want to do? Are the types going to be the same after calculation? Can you give an example? — Tunaki, Sep 28 '16 at 21:00

score 1 · Answer 1 · answered Sep 28 '16 at 20:53

Use ListIterator -> https://docs.oracle.com/javase/7/docs/api/java /util/ListIterator.html

Unlike plain Iterator, ListIterator will allow you to store newly computed value back to the list

ArrayList<Integer> source = ...

ListIterator<Integer> iter = source.listIterator();

while( iter.hasNext() )
{
  Integer value = iter.next();

  Integer newValue = Integer.valueOf( value.intValue() * 2 );

  iter.set(newValue);
}

Tokazio · Answer 2 · 2016-09-28T21:23:25.293

0

as @puhlen says, in java 8, use stream and lambda expression

List liste = new ArrayList<>();
liste.stream().forEach(o->{
    //apply your math on o
});

stream provides you many other functionnalities to filter, order, collect... use it if you're on java8

or as @neil-locketz says in java before 8 use a foreach loop

 List<type> liste = new ArrayList<>();
for(type o : liste){
    //math on object o here
}

edited Sep 28 '16 at 21:23

answered Sep 28 '16 at 20:49

Tokazio

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Actually, that code won't do anything whatsoever. – Bohemian Sep 28 '16 at 20:50
1

Your stream code does nothing because there is no terminating method. That is, `map()` isn't called until the terminating method (eg `collect()`, `reduce()` etc) requests the next element. With no terminating method, literally no code will fire. – Bohemian Sep 28 '16 at 20:56
sorry, i think forEach and i write map... edited – Tokazio Sep 28 '16 at 21:24

How to apply a math function to each object of an ArrayList

2 Answers2