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javabinary-search

Logic Error: Binary search fails with more than two elements in a string array


The objective is to return the index of an element in a string array if present. The method uses a basic binary search using compareTo statements. With more than two elements in a tested array, the method will not detect the present element and return -1. The Java code this question is referring to is below. How do I make the binary search method work as intended?

   public static int binarySearch(String[] array, String x) {
        int high = array.length - 1;
        int low = 0;

        while (low <= high) {

            int mid = low + (high - low) / 2;

            if (x.compareTo(array[mid]) == 0) {
                return mid;
            }
            if (x.compareTo(array[mid]) > 0) {
                low = mid + 1;
            }
            else {
                high = mid - 1;
            }
        }
        return -1;
    }

Solution

  • add an extra variable and set it to -1;

    int loc=-1;
    

    change the code

    int mid=low+(high-low)/2;
    

    to

     int mid=(low+high)/2;
     if(x.compareTo(array[mid]==0)
     {
      loc=mid;
      break;
     }
     else if(x<array[mid])
     {
      last=mid-1;
     }
     else
     {
       low=mid+1;
     }
    

    then

    if(loc>=0)
    {
     System.out.println(loc+1);
    } 
    else
    {
      System.out.println("no element");
    }