Search in an array of strings where non-empty strings are sorted

Given an array of strings. The array has both empty and non-empty strings. All non-empty strings are in sorted order. Empty strings can be present anywhere between non-empty strings.

Examples:

Input :  arr[] =  {"for", "", "", "", "geeks", 
                   "ide", "", "practice", "" , 
                   "", "quiz", "", ""};
          str = "quiz"
Output :   10
The string "quiz" is present at index 10 in 
given array.

A simple solution is to linearly search given str in array of strings.

A better solution is to do modified Binary Search. Like normal binary search, we compare given str with middle string. If middle string is empty, we find the closest non-empty string x (by linearly searching on both sides). Once we find x, we do standard binary search, i.e., we compare given str with x. If str is same as x, we return index of x. if str is greater, we recur for right half, else we recur for left half.

Below is the implementation of the idea:

C++

// C++ program to find location of a str in
// an array of strings which is sorted and
// has empty strings between strings.
#include <bits/stdc++.h>
using namespace std;

// Compare two string equals are not
int compareStrings(string str1, string str2)
{
    int i = 0;
    while (str1[i] == str2[i] && str1[i] != '\0')
        i++;
    if (str1[i] > str2[i])
        return -1;

    return (str1[i] < str2[i]);
}

// Main function to find string location
int searchStr(string arr[], string str, int first,
                                        int last)
{
    if (first > last)
        return -1;

    // Move mid to the middle
    int mid = (last+first)/2;

    // If mid is empty , find closest non-empty string
    if (arr[mid].empty())
    {
        // If mid is empty, search in both sides of mid
        // and find the closest non-empty string, and
        // set mid accordingly.
        int left  = mid - 1;
        int right = mid + 1;
        while (true)
        {
            if (left < first && right > last)
                return -1;
            if (right<=last && !arr[right].empty())
            {
                mid = right;
                break;
            }
            if (left>=first && !arr[left].empty())
            {
                mid = left;
                break;
            }
            right++;
            left--;
        }
    }

    // If str is found at mid
    if (compareStrings(str, arr[mid]) == 0)
        return mid;

    // If str is greater than mid
    if (compareStrings(str, arr[mid]) < 0)
        return searchStr(arr, str, mid+1, last);

    // If str is smaller than mid
    return searchStr(arr, str, first, mid-1);
}

// Driver Code
int main()
{
    // Input arr of Strings.
    string arr[] = {"for", "", "", "", "geeks", "ide", "",
                     "practice", "" , "", "quiz", "", ""};

    // input Search String
    string str = "quiz";
    int n = sizeof(arr)/sizeof(arr[0]);

    cout << searchStr(arr, str, 0, n-1);
    return 0;
}

Java

// Java program to find location of a str in
// an array of strings which is sorted and
// has empty strings between strings.
import java.util.*;

class GFG
{

    // Compare two string equals are not
    static int compareStrings(String str1,
                              String str2)
    {
        int i = 0;
        while (i < str1.length() - 1 &&
                   str1.charAt(i) == str2.charAt(i))
            i++;

        if (str1.charAt(i) > str2.charAt(i))
            return -1;

        if (str1.charAt(i) < str2.charAt(i))
            return 1;
        else
            return 0;
    }

    // Main function to find string location
    static int searchStr(String[] arr, String str,
                            int first, int last)
    {
        if (first > last)
            return -1;

        // Move mid to the middle
        int mid = (last + first) / 2;

        // If mid is empty,
        // find closest non-empty string
        if (arr[mid].isEmpty())
        {

            // If mid is empty, search in both sides of mid
            // and find the closest non-empty string, and
            // set mid accordingly.
            int left = mid - 1;
            int right = mid + 1;
            while (true)
            {
                if (left < first && right > last)
                    return -1;
                if (right <= last && !arr[right].isEmpty())
                {
                    mid = right;
                    break;
                }
                if (left >= first && !arr[left].isEmpty())
                {
                    mid = left;
                    break;
                }
                right++;
                left--;
            }
        }

        // If str is found at mid
        if (compareStrings(str, arr[mid]) == 0)
            return mid;

        // If str is greater than mid
        if (compareStrings(str, arr[mid]) < 0)
            return searchStr(arr, str, mid + 1, last);

        // If str is smaller than mid
        return searchStr(arr, str, first, mid - 1);
    }

    // Driver Code
    public static void main(String[] args)
    {

        // Input arr of Strings.
        String[] arr = { "for", "", "", "", "geeks",
                         "ide", "", "practice", "",
                         "", "quiz", "", "" };

        // input Search String
        String str = "quiz";
        int n = arr.length;

        System.out.println(searchStr(arr, str, 0, n - 1));
    }
}

// This code is contributed by
// sanjeev2552

Python3

# Python3 program to find the location of
# an str in an array of strings which is sorted
# and has empty strings between strings.

# Compare two string equals are not
def compareStrings(str1, str2):

    i = 0
    while i < len(str1) - 1 and str1[i] == str2[i]:
        i += 1
    if str1[i] > str2[i]:
        return -1

    return str1[i] < str2[i]

# Main function to find string location
def searchStr(arr, string, first, last):

    if first > last:
        return -1

    # Move mid to the middle
    mid = (last + first) // 2

    # If mid is empty , find closest non-empty string
    if len(arr[mid]) == 0:

        # If mid is empty, search in both sides of mid
        # and find the closest non-empty string, and
        # set mid accordingly.
        left, right = mid - 1, mid + 1
        while True:

            if left < first and right > last:
                return -1

            if right <= last and len(arr[right]) != 0:
                mid = right
                break

            if left >= first and len(arr[left]) != 0:
                mid = left
                break

            right += 1
            left -= 1

    # If str is found at mid
    if compareStrings(string, arr[mid]) == 0:
        return mid

    # If str is greater than mid
    if compareStrings(string, arr[mid]) < 0:
        return searchStr(arr, string, mid+1, last)

    # If str is smaller than mid
    return searchStr(arr, string, first, mid-1)

# Driver Code
if __name__ == "__main__":

    # Input arr of Strings.
    arr = ["for", "", "", "", "geeks", "ide", "",
                    "practice", "" , "", "quiz", "", ""]

    # input Search String
    string = "quiz"
    n = len(arr)

    print(searchStr(arr, string, 0, n-1))

# This code is contributed by Rituraj Jain

C#

// C# program to find location of a str
// in an array of strings which is sorted
// and has empty strings between strings.
using System;
public class GFG
{
  // Compare two string equals are not
  static int compareStrings(String str1, String str2)
  {
    int i = 0;
    while (i < str1.Length - 1 && str1[i] == str2[i])
      i++;

    if (str1[i] > str2[i])
      return -1;

    if (str1[i] < str2[i])
      return 1;
    else
      return 0;
  }

  // Main function to find string location
  static int searchStr(String[] arr, String str, int first, int last)
  {
    if (first > last)
      return -1;

    // Move mid to the middle
    int mid = (last + first) / 2;

    // If mid is empty,
    // find closest non-empty string
    if (arr[mid] == "")
    {

      // If mid is empty (""), search in both sides of mid
      // and find the closest non-empty string, and
      // set mid accordingly.
      int left = mid - 1;
      int right = mid + 1;
      while (true)
      {
        if (left < first && right > last)
          return -1;
        if (right <= last && !(arr[right] == ""))
        {
          mid = right;
          break;
        }
        if (left >= first && !(arr[left] == ""))
        {
          mid = left;
          break;
        }
        right++;
        left--;
      }
    }

    // If str is found at mid
    if (compareStrings(str, arr[mid]) == 0)
      return mid;

    // If str is greater than mid
    if (compareStrings(str, arr[mid]) < 0)
      return searchStr(arr, str, mid + 1, last);

    // If str is smaller than mid
    return searchStr(arr, str, first, mid - 1);
  }

  // Driver Code
  public static void Main(string[] args)
  {

    // Input arr of Strings.
    String[] arr = { "for", "", "", "", "geeks", "ide", "", "practice", "", "", "quiz", "", "" };

    // input Search String
    String str = "quiz";
    int n = arr.Length;

    Console.WriteLine (searchStr(arr, str, 0, n - 1));
  }
}

// This code is contributed by ajaymakvana.

Javascript

<script>
    // Javascript program to find location of a str in
    // an array of strings which is sorted and
    // has empty strings between strings.

    // Compare two string equals are not
    function compareStrings(str1, str2)
    {
        let i = 0;
        while (i < str1.length - 1 &&
                   str1[i] == str2[i])
            i++;

        if (str1[i] > str2[i])
            return -1;

        if (str1[i] < str2[i])
            return 1;
        else
            return 0;
    }

    // Main function to find string location
    function searchStr(arr, str, first, last)
    {
        if (first > last)
            return -1;

        // Move mid to the middle
        let mid = parseInt((last + first) / 2, 10);

        // If mid is empty,
        // find closest non-empty string
        if (arr[mid] == "")
        {

            // If mid is empty, search in both sides of mid
            // and find the closest non-empty string, and
            // set mid accordingly.
            let left = mid - 1;
            let right = mid + 1;
            while (true)
            {
                if (left < right && right > last)
                    return -1;
                if (right <= last && arr[right] != "")
                {
                    mid = right;
                    break;
                }
                if (left >= right && !arr[left] == "")
                {
                    mid = left;
                    break;
                }
                right++;
                left--;
            }
        }

        // If str is found at mid
        if (compareStrings(str, arr[mid]) == 0)
            return mid;

        // If str is greater than mid
        if (compareStrings(str, arr[mid]) < 0)
            return searchStr(arr, str, mid + 1, last);

        // If str is smaller than mid
        return searchStr(arr, str, first, mid - 1);
    }

    // Input arr of Strings.
    let arr = [ "for", "", "", "", "geeks",
                    "ide", "", "practice", "",
                    "", "quiz", "", "" ];

    // input Search String
    let str = "quiz";
    let n = arr.length;

    document.write(searchStr(arr, str, 0, n - 1));

// This code is contributed by vaibhavrabadiya3.
</script>

Output:

  10

Using linear search would take us O(L*N), where L is the string comparison. To optimize the runtime, we use binary search making the time complexity O(L(logN)). 

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