Iterate over consecutive object pairs in an ArrayList

问题

I want to get pairs of objects from an ArrayList so I can perform calculations between the elements of each object. Ideally it should iterate over pairs of objects. For example in a List with {obj1, obj2, obj3, obj4} it should go over {obj1,obj2}, {obj2,obj3} and {obj3,obj4}.

What I have tried so far is as follows.

public class Sum {
    public ArrayList<Double> calculateSum(ArrayList<Iter> iter) {
        ListIterator<Iter> it = iter.listIterator();
        ArrayList<Double> sums = new ArrayList<>();

        while (it.hasNext()) {
            Iter it1 = it.next();
            Iter it2;
            if(it.hasNext()){
                it2 = it.next();
            } else {    break;  }

            double sum = it1.getValue() + it2.getValue();
            sums.add(sum);
        }
        return sums;
    }
}

Here, it just iterates as {obj1,obj2} and {obj3,obj4}. How can I fix this?

All help is greatly appreciated. Thanks!

回答1:

A very normal loop, except that you need to loop up to list.size() - 1, the before last element of the array.

public ArrayList<Double> calculateSum(ArrayList<Iter> list) {
    ArrayList<Double> sums = new ArrayList<>();

    for (int i = 0; i < list.size() - 1; i++) {
        double sum = list.get(i).getValue() + list.get(i + 1).getValue();
        sums.add(sum);
    }
    return sums;
}

EDIT

Using an iterator in this case will not be faster than doing a normal loop and just makes the logic unnecessarily complicated and can easily introduce bugs.

回答2:

A little modification to Davide's answer

for (int i = 0; i < list.size() - 1; i ++) {
    sums.add(list.get(i) + list.get(i+1));
}

Because the OP wanted {obj1, obj2} {obj2, obj3} ...etc

Using a iterator

itr = list.iterator();
while(itr.hasNext()) {
   Double x = itr.next();
   if(itr.hasNext()){
      x+= itr.next();
      sum.add(x);}
   itr.previous();
  }

This is not recommended.

回答3:

Simply use a for loop and stop at element before last one.

for (int i = 0; i < iter.size() - 1; i++) {
    Iter first = iter.get(i);
    Iter second = iter.get(i + 1);
    // Your code here
}

回答4:

 public static ArrayList<Double> calculateSum(ArrayList<Iter> iter) {
     ListIterator<Iter> it = iter.listIterator();
     ArrayList<Double> sums = new ArrayList<>();
     if (it.hasNext()) {
         double prev = it.next().getValue();
         while (it.hasNext()) {
             double current = it.next().getValue();
             double sum = prev + current;
             sums.add(sum);
             prev = current;
         }
     }
     return sums;
 }

回答5:

Try this :-

public ArrayList<Double> calculateSum(ArrayList<Iter> inputList) {
    ArrayList<Double> sums = new ArrayList<Double>();
    int inputSize =  inputList.size(); // size should be saved as an int , instead of calling size() over the list again and again in for loop 
    int incVar = 1; // 1 incremented value
    double sum = 0.0d;
    for(int i = 0; incVar < inputSize; i++,incVar++){
          sum = inputList.get(i).getValue() + inputList.get(incVar).getValue();
          sums.add(sum);
    }
    return sums;
}

来源：https://stackoverflow.com/questions/31222023/iterate-over-consecutive-object-pairs-in-an-arraylist

标签

java

arraylist