I dont want to enter the size of List but I also want to dynamically add number in List

Question

I am using List for dynamically storing the values, but I have to specify the size of List before taking input in the Binary search program. I want help in that I don't want to take size as an input.

I have tried using length but that doesn't work, someone suggested me to use size(), but I don't know how to use it.

import java.util.*;

class Binary
{
    public static void main(String args[])
    {
        Scanner s=new Scanner(System.in);
        System.out.println("Enter size of array");
        int n=s.nextInt();
        System.out.println("Enter array elements in ascending order");
        List<Integer> L=new ArrayList<>();
        for(int i=0;i<n;i++)
        {
            int e=s.nextInt();
            L.add(e);
        }
        System.out.println("Enter the element you want to search");
        int h=s.nextInt();
        int left=L.get(0);
        int right=L.get(n-1);
        while(left<=right)
        {
            if(h<=right){
                int m=(left+right)/2;

                if(m==h)
                {
                    System.out.println("Element found at index:"+L.indexOf(m)+" starting from 0");
                    return;
                }
                if(m>h)
                {
                    right=m-1;
                }
                if(m<h)
                {
                    left=m+1;
                }
            }
            else{
                System.out.println("Element not present");
            }   
        }
    }
}

Now I am expecting the user to enter elements till he desires and then to find index of element using Binary Search

If you don't want the user to specify the number of elements he wants to enter, how do you want him to indicate that he is done with entering all elements? — Turamarth, Oct 01 '19 at 14:09
You can read numbers until user enters an escape number. Then use `L.get(L.size() - 1);` instead of `L.get(n-1);` — SomeFire, Oct 01 '19 at 14:18
If we declare 0 as an escape element then the user will no be able to enter 0 in between, so we cannot use any number as an escape element.@SomeFire — Rohit Mittal, Oct 02 '19 at 05:16
how about a string, `end`? You'll need to either multiline your number input, or you can treat a single line of input as the array of numbers and split it by whitespace or some other delimiter (thus eliminating a need for the end of input, it'll be the enter key) — Rogue, Oct 02 '19 at 16:27

fedup · Answer 1 · 2019-10-02T16:45:49.270

Your question is really a duplicate of How to terminate Scanner when input is complete?.

But here are a few pointers to get your program working.

This part of the code is incorrect.

    int left=L.get(0);
    int right=L.get(n-1);

You need to be setting the left and right to be the bounds of the search - not the values the user entered into those positions.

    int left=0;
    int right=L.size() - 1;

Then when you need to compare the values the user entered.

    if (L.get(m) < h)

L.indexOf(m) doesn't really make sense. It would work if you had L.indexOf(L.get(m)) but that is the same as just m.

    System.out.println("Element found at index:" + m + " starting from 0");

I dont want to enter the size of List but I also want to dynamically add number in List

1 Answers1