I'm trying to do a recursive binary search on an ArrayList but not sure what the issue with my code is. I have a text file with a bunch of integer numbers that look like this:
217 320 550 212 12 17 3560 2999 211
The problem is that when I input a number to search the ArrayList, it always says the element does not exist, but I know it does!
public static int binarySearch(List<Integer> arr, Integer l, Integer r, Integer x)
{
if (r>=l)
{
Integer mid = l + (r - l)/2;
// If the element is present at the middle itself
if ((arr.size()/2) == x)
return mid;
// If element is smaller than mid, then it can only
// be present in left subarray
if ((arr.size()/2) > x)
return binarySearch(arr, l, mid-1, x);
// Else the element can only be present in right
// subarray
return binarySearch(arr, mid+1, r, x);
}
// We reach here when element is not present in array
return -1;
}
}
Any help would be greatly appreciated, new programmer here!
The part where you check if the element is present is wrong, don't do this:
arr.size()/2 == x
arr.size()/2 > x
Instead, do this:
arr.get(mid) == x
arr.get(mid) > x
Because you're supposed to access the element at the mid
position, not checking if half the list's size equals x
.
Shouldn't you replace arr.size()
with arr[mid]
or something like that? Looks like you don't compare the value of an item, but rather the array size itself, which is constant and totally irrelevant.
You calculate mid
, and then use arr.size() / 2
, instead...
Replace them with arr.get(mid)
, something like this:
public static int binarySearch(final List<Integer> arr, final int l, final int r, final Integer x)
{
if (r >= l)
{
final int mid = l + (r - l) / 2;
// If the element is present at the middle itself
if (x.equals(arr.get(mid)))
return mid;
// If element is smaller than mid, then it can only
// be present in left subarray
if (arr.get(mid) > x)
return binarySearch(arr, l, mid - 1, x);
// Else the element can only be present in right
// subarray
return binarySearch(arr, mid + 1, r, x);
}
// We reach here when element is not present in array
return -1;
}
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