Data Modelling & AI Data Structure & Algorithm

Efficient method to store a Lower Triangular Matrix using row-major mapping

23 June 2025

3

Given a lower triangular matrix Mat[][], the task is to store the matrix using row-major mapping.

Lower Triangular Matrix: A Lower Triangular Matrix is a square matrix in which the lower triangular part of a matrix consists of non-zero elements and the upper triangular part consists of 0s. The Lower Triangular Matrix for a 2D matrix Mat[][] is mathematically defined as:

If i < j, set Mat[i][j] = 0.
If i >= j, set Mat[i][j] > 0.

Illustration: Below is a 5×5 lower triangular matrix. In general, such matrices can be stored in a 2D array, but when it comes to matrices of large size, it is not a good choice because of its high memory consumption due to the storage of unwanted 0s.
Such a matrix can be implemented in an optimized manner.

The efficient way to store the lower triangular matrix of size N:

Count of non-zero elements = 1 + 2 + 3 + … + N = N * (N + 1) /2.
Count of 0s = N²– (N * (N + 1) /2 = (N * (N – 1)/2.

Now let us see how to represent lower triangular matrices in our program. Notice that storing 0s must be avoided to reduce memory consumption. As calculated, for storing non-zero elements, N*(N + 1)/2 space is needed. Taking the above example, N = 5. Array of size 5 * (5 + 1)/2 = 15 is required to store the non-zero elements.

Now, elements of the 2D matrix can be stored in a 1D array, row by row, as shown below:

Apart from storing the elements in an array, a procedure for extracting the element corresponding to the row and column number is also required.
Using Row-Major Mapping for storing lower triangular matrix, the element at index Mat[i][j] can be represented as:

Index of Mat[i][j] matrix in the array A[] = [i*(i – 1)/2 + j – 1]

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Dimensions of a matrix
static int N = 5;
 
// Structure of the efficient matrix
class Matrix {
  public:
  int* A;
  int size;
};
 
// Function to set the
// values in the Matrix
void Set(Matrix mat, int i, int j, int x)
{
  if (i >= j)
    mat.A[i * (i - 1) / 2 + j - 1] = x;
}
 
// Function to store the
// values in the Matrix
int Get(Matrix mat, int i, int j)
{
  if (i >= j)
    return mat.A[i * (i - 1) / 2 + j - 1];
  return 0;
}
 
// Function to display the
// elements of the matrix
void Display(Matrix mat)
{
  int i, j;
  // Traverse the matrix
  for (i = 1; i <= mat.size; i++) {
    for (j = 1; j <= mat.size; j++) {
      if (i >= j)
        cout << mat.A[i * (i - 1) / 2 + j - 1]
        << " ";
      else
        cout << 0 << " ";
    }
    cout << endl;
  }
}
 
// Function to generate an efficient matrix
Matrix createMat(vector<vector<int> >& Mat)
{
  // Declare efficient Matrix
  Matrix mat;
  // Initialize the Matrix
  mat.size = N;
  mat.A = new int[(mat.size * (mat.size + 1)) / 2];
  int i, j;
  // Set the values in matrix
  for (i = 1; i <= mat.size; i++)
    for (j = 1; j <= mat.size; j++)
      Set(mat, i, j, Mat[i - 1][j - 1]);
  // Return the matrix
  return mat;
}
 
// Driver Code
int main()
{
  vector<vector<int> > Mat = { { 1, 0, 0, 0, 0 },
                              { 1, 2, 0, 0, 0 },
                              { 1, 2, 3, 0, 0 },
                              { 1, 2, 3, 4, 0 },
                              { 1, 2, 3, 4, 5 } };
 
  // Stores the efficient matrix
  Matrix mat = createMat(Mat);
   
  // Print the Matrix
  Display(mat);
  return 0;
}
 
// This code is contributed by Tapesh (tapeshdua420)

C

// C program for the above approach
#include <stdio.h>
#include <stdlib.h>
 
// Dimensions of a matrix
const int N = 5;
 
// Structure of the efficient matrix
struct Matrix {
    int* A;
    int size;
};
 
// Function to set the
// values in the Matrix
void Set(struct Matrix* mat,
         int i, int j, int x)
{
    if (i >= j)
        mat->A[i * (i - 1) / 2 + j - 1] = x;
}
 
// Function to store the
// values in the Matrix
int Get(struct Matrix mat, int i, int j)
{
    if (i >= j) {
        return mat.A[i * (i - 1) / 2 + j - 1];
    }
    else {
        return 0;
    }
}
 
// Function to display the
// elements of the matrix
void Display(struct Matrix mat)
{
    int i, j;
 
    // Traverse the matrix
    for (i = 1; i <= mat.size; i++) {
        for (j = 1; j <= mat.size; j++) {
 
            if (i >= j) {
                printf("%d ",
                       mat.A[i * (i - 1) / 2 + j - 1]);
            }
            else {
                printf("0 ");
            }
        }
        printf("\n");
    }
}
 
// Function to generate an efficient matrix
struct Matrix createMat(int Mat[N][N])
{
    // Declare efficient Matrix
    struct Matrix mat;
 
    // Initialize the Matrix
    mat.size = N;
    mat.A = (int*)malloc(
        mat.size * (mat.size + 1) / 2
        * sizeof(int));
 
    int i, j;
 
    // Set the values in matrix
    for (i = 1; i <= mat.size; i++) {
 
        for (j = 1; j <= mat.size; j++) {
 
            Set(&mat, i, j, Mat[i - 1][j - 1]);
        }
    }
 
    // Return the matrix
    return mat;
}
 
// Driver Code
int main()
{
    int Mat[5][5] = { { 1, 0, 0, 0, 0 },
                      { 1, 2, 0, 0, 0 },
                      { 1, 2, 3, 0, 0 },
                      { 1, 2, 3, 4, 0 },
                      { 1, 2, 3, 4, 5 } };
 
    // Stores the efficient matrix
    struct Matrix mat = createMat(Mat);
 
    // Print the Matrix
    Display(mat);
 
    return 0;
}

Java

// Java program for the above approach
class GFG
{
   
// Dimensions of a matrix
static int N = 5;
 
// Structure of the efficient matrix
static class Matrix {
    int[] A;
    int size;
};
 
// Function to set the
// values in the Matrix
static void Set(Matrix mat,
         int i, int j, int x)
{
    if (i >= j)
        mat.A[i * (i - 1) / 2 + j - 1] = x;
}
 
// Function to store the
// values in the Matrix
static int Get(Matrix mat, int i, int j)
{
    if (i >= j) {
        return mat.A[i * (i - 1) / 2 + j - 1];
    }
    else {
        return 0;
    }
}
 
// Function to display the
// elements of the matrix
static void Display(Matrix mat)
{
    int i, j;
 
    // Traverse the matrix
    for (i = 1; i <= mat.size; i++) {
        for (j = 1; j <= mat.size; j++) {
 
            if (i >= j) {
                System.out.printf("%d ",
                       mat.A[i * (i - 1) / 2 + j - 1]);
            }
            else {
                System.out.printf("0 ");
            }
        }
        System.out.printf("\n");
    }
}
 
// Function to generate an efficient matrix
static Matrix createMat(int Mat[][])
{
   
    // Declare efficient Matrix
    Matrix mat = new Matrix();
 
    // Initialize the Matrix
    mat.size = N;
    mat.A = new int[(mat.size*(mat.size + 1)) / 2];
    int i, j;
 
    // Set the values in matrix
    for (i = 1; i <= mat.size; i++)
    {
        for (j = 1; j <= mat.size; j++)
        {
            Set(mat, i, j, Mat[i - 1][j - 1]);
        }
    }
 
    // Return the matrix
    return mat;
}
 
// Driver Code
public static void main(String[] args)
{
    int Mat[][] = { { 1, 0, 0, 0, 0 },
                      { 1, 2, 0, 0, 0 },
                      { 1, 2, 3, 0, 0 },
                      { 1, 2, 3, 4, 0 },
                      { 1, 2, 3, 4, 5 } };
 
    // Stores the efficient matrix
    Matrix mat = createMat(Mat);
 
    // Print the Matrix
    Display(mat);
}
}
 
// This code is contributed by 29AjayKumar

Python3

# Python program for the above approach
 
# Dimensions of a matrix
N = 5
 
# Structure of the efficient matrix
class Matrix:
    def __init__(self, size):
        self.size = size
        self.A = [None] * (self.size)
 
# Function to set the
# values in the Matrix
def Set(mat, i, j, x):
    if i >= j:
        mat.A[i * (i - 1) // 2 + j - 1] = x
 
# Function to store the
# values in the Matrix
def get(mat, i, j):
    if i >= j:
        return mat.A[i * (i - 1) // 2 + j - 1]
    return 0
 
# Function to display the
# elements of the matrix
def display(mat):
    # Traverse the matrix
    for i in range(1, mat.size + 1):
        for j in range(1, mat.size + 1):
            if i >= j:
                print(mat.A[i * (i - 1) // 2 + j - 1], end=" ")
            else:
                print(0, end=" ")
        print()
 
# Function to generate an efficient matrix
def create_matrix(Mat):
    # Declare efficient Matrix
    mat = Matrix(N)
    # Initialize the Matrix
    mat.A = [None] * ((mat.size * (mat.size+1)) // 2)
    # Set the values in matrix
    for i in range(1, mat.size + 1):
        for j in range(1, mat.size + 1):
            Set(mat, i, j, Mat[i - 1][j - 1])
    # Return the matrix
    return mat
 
 
if __name__ == '__main__':
 
    Mat = [[1, 0, 0, 0, 0],
           [1, 2, 0, 0, 0],
           [1, 2, 3, 0, 0],
           [1, 2, 3, 4, 0],
           [1, 2, 3, 4, 5]]
 
    mat = create_matrix(Mat)
    display(mat)
 
# This code is contributed by Tapesh (tapeshdua420)

C#

// C# program for the above approach
using System;
public class GFG
{
 
  // Dimensions of a matrix
  static int N = 5;
 
  // Structure of the efficient matrix
  class Matrix {
    public int[] A;
    public int size;
  };
 
  // Function to set the
  // values in the Matrix
  static void Set(Matrix mat,
                  int i, int j, int x)
  {
    if (i >= j)
      mat.A[i * (i - 1) / 2 + j - 1] = x;
  }
 
  // Function to store the
  // values in the Matrix
  static int Get(Matrix mat, int i, int j)
  {
    if (i >= j) {
      return mat.A[i * (i - 1) / 2 + j - 1];
    }
    else {
      return 0;
    }
  }
 
  // Function to display the
  // elements of the matrix
  static void Display(Matrix mat)
  {
    int i, j;
 
    // Traverse the matrix
    for (i = 1; i <= mat.size; i++) {
      for (j = 1; j <= mat.size; j++) {
 
        if (i >= j) {
          Console.Write("{0} ",
                        mat.A[i * (i - 1) / 2 + j - 1]);
        }
        else {
          Console.Write("0 ");
        }
      }
      Console.Write("\n");
    }
  }
 
  // Function to generate an efficient matrix
  static Matrix createMat(int [,]Mat)
  {
 
    // Declare efficient Matrix
    Matrix mat = new Matrix();
 
    // Initialize the Matrix
    mat.size = N;
    mat.A = new int[(mat.size*(mat.size + 1)) / 2];
    int i, j;
 
    // Set the values in matrix
    for (i = 1; i <= mat.size; i++)
    {
      for (j = 1; j <= mat.size; j++)
      {
        Set(mat, i, j, Mat[i - 1,j - 1]);
      }
    }
 
    // Return the matrix
    return mat;
  }
 
  // Driver Code
  public static void Main(String[] args)
  {
    int [,]Mat = { { 1, 0, 0, 0, 0 },
                  { 1, 2, 0, 0, 0 },
                  { 1, 2, 3, 0, 0 },
                  { 1, 2, 3, 4, 0 },
                  { 1, 2, 3, 4, 5 } };
 
    // Stores the efficient matrix
    Matrix mat = createMat(Mat);
 
    // Print the Matrix 
    Display(mat);
  }
}
 
// This code is contributed by 29AjayKumar

Javascript

// JavaScript program for the above approach
 
let N = 5;
 
class Matrix{
    A = new Array();
    size;
    constructor(){ }
}
 
function Set(mat, i, j, x){
    if (i >= j){
        mat.A[i * (i - 1) / 2 + j - 1] = x;
    }
}
 
function Get(mat, i, j){
    if (i >= j) {
        return mat.A[i * (i - 1) / 2 + j - 1];
    }
    else {
        return 0;
    }
}
 
function Display(mat){
    let i, j;
     
    // Traverse the matrix
    for (i = 1; i <= mat.size; i++) {
        for (j = 1; j <= mat.size; j++) {
 
            if (i >= j) {
                console.log(mat.A[i * (i - 1) / 2 + j - 1] + " ");
            }
            else {
                console.log("0 ");
            }
        }
        console.log("<br>");
    }
}
 
function createMat(Mat){
    var mat = new Matrix();
     
    mat.size = N;
     
    mat.A = new Array((mat.size*(mat.size + 1))/2);
    let i, j;
     
    // Set the values in matrix
    for (i = 1; i <= mat.size; i++)
    {
        for (j = 1; j <= mat.size; j++)
        {
            Set(mat, i, j, Mat[i - 1][j - 1]);
        }
    }
 
    // Return the matrix
    return mat;
}
 
let Mat = [ [1, 0, 0, 0, 0],
               [1, 2, 0, 0, 0],
               [1, 2, 3, 0, 0],
               [1, 2, 3, 4, 0],
               [1, 2, 3, 4, 5] ];
 
var mat = createMat(Mat);
Display(mat);
 
// This code is contributed by lokesh.

Output:

Time Complexity: O(N²)
Auxiliary Space: O(N²)

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Efficient method to store a Lower Triangular Matrix using row-major mapping

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