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Index Mapping (or Trivial Hashing) with negatives allowed

Given a limited range array contains both positive and non-positive numbers, i.e., elements are in the range from -MAX to +MAX. Our task is to search if some number is present in the array or not in O(1) time.

Since range is limited, we can use index mapping (or trivial hashing). We use values as index in a big array. Therefore we can search and insert elements in O(1) time.

hmap

How to handle negative numbers?
The idea is to use a 2D array of size hash[MAX+1][2]

Algorithm:



Assign all the values of the hash matrix as 0.
Traverse the given array:
    If the element ele is non negative assign 
        hash[ele][0] as 1.
    Else take the absolute value of ele and 
         assign hash[ele][1] as 1.

To search any element x in the array.

  • If X is non-negative check if hash[X][0] is 1 or not. If hash[X][0] is one then the number is present else not present.
  • If X is negative take absolute vale of X and then check if hash[X][1] is 1 or not. If hash[X][1] is one then the number is present

Below is the implementation of the above idea.

C++

// CPP program to implement direct index mapping
// with negative values allowed.
#include <bits/stdc++.h>
using namespace std;
#define MAX 1000
  
// Since array is global, it is initialized as 0.
bool has[MAX + 1][2];
  
// searching if X is Present in the given array 
// or not.
bool search(int X)
{
    if (X >= 0) {
        if (has[X][0] == 1)
            return true;
        else
            return false;
    }
  
    // if X is negative take the absolute 
    // value of X.
    X = abs(X);
    if (has[X][1] == 1)
        return true;
  
    return false;
}
  
void insert(int a[], int n)
{
    for (int i = 0; i < n; i++) {
        if (a[i] >= 0) 
            has[a[i]][0] = 1;
       else
            has[abs(a[i])][1] = 1;
    }
}
  
// Driver code
int main()
{
    int a[] = { -1, 9, -5, -8, -5, -2 };
    int n = sizeof(a)/sizeof(a[0]);
    insert(a, n);
    int X = -5;
    if (search(X) == true)
       cout << "Present"
    else
       cout << "Not Present";
    return 0;
}

Java

// Java program to implement direct index 
// mapping with negative values allowed. 
class GFG
{
  
final static int MAX = 1000;
  
// Since array is global, it 
// is initialized as 0. 
static boolean[][] has = new boolean[MAX + 1][2];
  
// searching if X is Present in 
// the given array or not. 
static boolean search(int X) 
{
    if (X >= 0
    {
        if (has[X][0] == true
        {
            return true;
        
        else 
        {
            return false;
        }
    }
  
    // if X is negative take the 
    // absolute value of X. 
    X = Math.abs(X);
    if (has[X][1] == true
    {
        return true;
    }
  
    return false;
}
  
static void insert(int a[], int n) 
{
    for (int i = 0; i < n; i++) 
    {
        if (a[i] >= 0
        {
            has[a[i]][0] = true;
        
        else 
        {
            has[Math.abs(a[i])][1] = true;
        }
    }
}
  
// Driver code 
public static void main(String args[]) 
{
    int a[] = {-1, 9, -5, -8, -5, -2};
    int n = a.length;
    insert(a, n);
    int X = -5;
    if (search(X) == true)
    {
        System.out.println("Present");
    
    else 
    {
        System.out.println("Not Present");
    }
}
}
  
// This code is contributed
// by 29AjayKumar

Python3

# Python3 program to implement direct index
# mapping with negative values allowed.

# Searching if X is Present in the
# given array or not.
def search(X):

if X >= 0:
if has[X][0] == 1:
return True
else:
return False

# if X is negative take the absolute
# value of X.
X = abs(X)
if has[X][1] == 1:
return True

return False

def insert(a, n):

for i in range(0, n):
if a[i] >= 0:
has[a[i]][0] = 1
else:
has[abs(a[i])][1] = 1

# Driver code
if __name__ == “__main__”:



a = [-1, 9, -5, -8, -5, -2]
n = len(a)

MAX = 1000

# Since array is global, it is
# initialized as 0.
has = [[0 for i in range(2)]
for j in range(MAX + 1)]
insert(a, n)

X = -5
if search(X) == True:
print(“Present”)
else:
print(“Not Present”)

# This code is contributed by Rituraj Jain

C#

// C# program to implement direct index 
// mapping with negative values allowed. 
using System;
  
class GFG
{
  
static int MAX = 1000;
  
// Since array is global, it 
// is initialized as 0. 
static bool[,] has = new bool[MAX + 1, 2];
  
// searching if X is Present in 
// the given array or not. 
static bool search(int X) 
{
    if (X >= 0) 
    {
        if (has[X, 0] == true
        {
            return true;
        
        else
        {
            return false;
        }
    }
  
    // if X is negative take the 
    // absolute value of X. 
    X = Math.Abs(X);
    if (has[X, 1] == true
    {
        return true;
    }
  
    return false;
}
  
static void insert(int[] a, int n) 
{
    for (int i = 0; i < n; i++) 
    {
        if (a[i] >= 0) 
        {
            has[a[i], 0] = true;
        
        else
        {
            has[Math.Abs(a[i]), 1] = true;
        }
    }
}
  
// Driver code 
public static void Main() 
{
    int[] a = {-1, 9, -5, -8, -5, -2};
    int n = a.Length;
    insert(a, n);
    int X = -5;
    if (search(X) == true)
    {
        Console.WriteLine("Present");
    
    else
    {
        Console.WriteLine("Not Present");
    }
}
}
  
// This code is contributed
// by Akanksha Rai


Output:

Present

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.



This article is attributed to GeeksforGeeks.org

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