CODEWITHSUNDEEP
javaarrayslistfor-looparraylist
robert hue
robert hue

Reputation: 9

how to iterate over consecutive pairs in arrayList

I want to create a loop that iterates through an ArrayList of objects.

for (int i = 0,  k=i+1; k < arr.size(); i++) { 
     int tprice;
     tprice=0;
     if (arr.get(i).getID()=arr.get(i+1).getID()) {
        tprice = arr.get(i).getPrice() + arr.get(i+1).getPrice();
     }
     else {
           if (tprice!=0) {
              System.out.println(tprice);
              }
           else {
              System.out.println(arr.get(i).getDuration());
              }
       }
    }

I want to compare element i to element i+1 of the ArrayList, without creating a nested for loop as I am not interested in all the permutations. (e.g, 1->2 then 2->3, not 1->2 then 1->3)

However, my line of code always causes an error because index k at the end of the loop is referencing an out-of-bounds index (or the equivalent for ArrayList).

Say i have three arrays : i=0 compares to i=1, i=1 compares to i=2, i=2 compares to i=3 but throws back an error since it doesn't exist.

Is there any way to overcome this? It seems silly, but I couldn't find anything about it across the forums.

exemple hardcode: 

         arr.add(new object(444,24))  // 444 is ID, 24 is Price
         arr.add(new object(444,29))  // 444 is ID, 29 is Price
         arr.add(new object(121,33))  // ...

wanted outcome : 

       53
       53
       33

side-question : if i am dealing with the same ID, i would ideally wish to print only once. Is there anyway to do so?

Upvotes: 1

Views: 1972

Answers (2)

Hearen
Hearen

Reputation: 7828

If all you want to check is the adjacent elements in the array, just start from the index 1 till the end and compare the adjacent: current and previous. 

for (int i = 1; i < arr.size(); ++i) {
    if (arr.get(i) > arr.get(i-1)) ....
}

Upvotes: -1

Nikolas
Nikolas

Reputation: 44398

If you touch the two consequent elements of the array, you have to stop one element before you read the end:

for (int i = 0; i < arrayList.size() - 1; i++) {
   // do stuff with arrayList.get(i) and arrayList(i+1)
}

Of course, the arrayList must contain at least two elements otherwise the loop will never be entered since the arrayList.size() - 1 would be equal to 0.

Moreover, you confuse arrays with ArrayList<T>.

  • List<T> of size list.size() has methods list.add(T t) and list.get(int index).
  • An array int[] array of size (length) array.length access to items with index array[0].

Upvotes: 1

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