2D Array-1¶

Agenda¶

Intro to 2D Arrays
Indexing and taking Input
Print matrix row by row and column by column
print matrix in wave form
Max of matrix.
Max of every row

Introduction¶

Two-dimensional arrays can be defined as arrays within an array. 2D arrays is a collection of 1d Arrays.

Syntax:¶

datatype name[][] = new datatype[rows][cols]

Note: When we create 2D matrix by int default all values are equal to 0.

Question¶

What is N representing in the line given below? int[][] mat = new int[N][M];

Choices¶

Number of Column
Number of Row
Total Element

Question¶

What is M representing in the line given below?

int[][] mat = new int[N][M];

Choices¶

Number of Column
Number of Row
Total Element

Question¶

How to create a matrix with 2 rows and 5 columns?

Choices¶

int[][] mat = new int[2] [5];
int[] mat = new int[2] [5];
int[][] mat = new int[5] [2];

Question¶

How to create a matrix with 5 columns and 7 rows?

Choices¶

int mat[ ] [ ] = new int[7] [5];
int mat[ ] = new int[5] [7];
int mat[ ] [ ] = new int[5] [7];

Question¶

How to create a matrix with size M * N having M = 5 and N = 7

Choices¶

int mat[ ] [ ] = new int[7] [5];
int mat[ ] = new int[5] [7];
int mat[ ] [ ] = new int[5] [7];

Indexing and Properties:¶

We can access ith row and jth column of matrix mat[][] by: mat[i][j]

If we iterate on a row, column changes and if we iterate on a column, row changes. For example, in above figure we can see that if we iterate on ith row, column number changes from [0,M - 1].
Similarly, in above figure we can see that if we iterate on jth row, row number changes from [0,N - 1].
In a matrix mat[][], mat.length will be equal to total number of rows in a matrix and mat[0].length will be equal to total number of columns.

Number of rows = array.length

Number of columns = array[0].length

Question¶

What will be the index of top left cell in a given matrix, mat of size N * M?

Choices¶

mat[0][M]
mat[0][0]
mat[top][left]
mat[N][0]

Question¶

What will be the index of top right cell in a given matrix, mat of size N * M?

Choices¶

mat[0][M-1]
mat[N - 1][0]
mat[N][0]
mat[bottom][left]

Question¶

What will be the index of bottom right cell in a given matrix, mat of size N * M?

Choices¶

mat[N - 1][0]
mat[N - 1][M - 1]
mat[N][M]
mat[bottom][right]

Question¶

What will be the index of bottom left cell in a given matrix, mat of size N * M?

Choices¶

mat[N - 1][0]
mat[N - 1][M - 1]
mat[N][M]
mat[bottom][right]

Taking input from user¶

Create a matrix having N rows and M columns fill the matrix by taking input from the user

Input: rows = 3, columns = 4

Code:

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        // Enter the number of rows
        int N = scanner.nextInt();

        // Enter the number of columns 
        int M = scanner.nextInt();

        int[][] matrix = new int[N][M];

        for (int i = 0; i < N; i++) {
            for (int j = 0; j < M; j++) {
                matrix[i][j] = scanner.nextInt();
            }
        }
    }

Question¶

Print the 0^th index row of the given matrix.

1  2  3  4
5  6  7  8 
9 10 11 12

Choices¶

1 2 3 4
1 5 9
1 2 3 4 5 6 7 8 9 10 11 12

Printing 0^th Row¶

Given a matrix, you are required to print its 0^th row.

Observation¶

To print the 0^th row of the matrix, we can directly access the elements in the 0^th row and print them.

Example¶

mat :

index	0	1	2	3
0	1	2	3	4
1	5	6	7	8
2	9	10	11	12

The 0^th row of the matrix would be: 1 2 3 4

Pseudocode¶

void printZeroRow(int mat[][]) {
    int n = mat.length;

    for (int c = 0; c < n; c++) // columns
    {
        System.out.print(mat[0][c] + " ");
    }
}

Print Matrix Row by Row and Column by Column¶

Given a matrix, print every row in new line.

Example¶

mat :

index	0	1	2	3
0	1	2	3	4
1	5	6	7	8
2	9	10	11	12

Output :

1  2  3  4
5  6  7  8 
9 10 11 12

Code¶

void printmat(int mat[][]){
    int n = mat.length;
    int m = mat[0].length;

    for(int r = 0; r < n; r++)//rows
    {
        for(int c = 0; c < m; c++) //columns
        {
            System.out.print(mat[r][c] + "  ");
        }
        System.out.println();
    }
}

Printing Row in wave form¶

Given a matrix, print rows and column in wave form.

Observation¶

First we will rum loop for rows from index 0 to n-1 where n is the number of rows. Inside this loop we will run another loop for columns from 0 to m-1, where m is total number of columns. Inside this loop we will print the value at row i and column j.

Example 1¶

mat :

index	0	1	2	3
0	21	41	17	9
1	11	14	24	30
2	29	7	35	16
3	32	50	6	10
4	15	18	49	4

Output :

21 41 17  30 24 14 11 29 7 35 16 10 6 50 32 15 18 49 4

Observation¶

For even rows we will traverse columns from 0 to m - 1 index.
For odd rows we will traverse columns from m - 1 to 0 index.

Pseudocode¶

void printWaveArray(int mat[][]){
    int n = mat.length;
    int m = mat[0].length;

    for(int r = 0; r < n; r++)//rows
    {
        if(r % 2 == 0){
            for(int c = 0; c < m; c++) //columns
            {
                System.out.print(mat[r][c] + " ");
            }
        }
        else{
            for(int c = m - 1; c >= 0; c--) //columns
            {
                System.out.print(mat[r][c] + " ");
            }
        }          
    }
}

Question¶

Print the 0^th index column of the given matrix.

1  2  3  4
5  6  7  8 
9 10 11 12

Choices¶

1 2 3 4
1 5 9
1 2 3 4 5 6 7 8 9 10 11 12

Print 0^th column¶

Given a matrix, print 0^th column.

Example¶

mat :

index	0	1	2	3
0	1	2	3	4
1	5	6	7	8
2	9	10	11	12

The 0^th col of the matrix would be: 1 5 9

Observation¶

We will run a single loop for i for rows from index 0 to n-1, where n is total number of rows and will print matrix[i][0].

Pseudocode¶

void printZeroCol(int mat[][]){
    int n = mat.length;
    int m = mat[0].length;

    for(int r = 0; r < n; r++)//rows
    {
        System.out.print(mat[r][0] + " ");
    }
}

Print every column¶

Given a matrix, print every column in new line.

Exmaple 1¶

mat[4][3] = {
    {21,16,17,14},rows
    {7,8,10,1},
    {6,11,13,21}
}

Ans = {
    {21, 7, 6}
    {16, 8, 11}
    {17, 10, 13}
    {14, 1, 21}
}

21	16	17	14
7	8	10	1
6	11	13	21

Observation¶

First we will rum loop for columns from index 0 to m - 1 where m is the number of columns. Inside this loop we will run another loop for rows from 0 to n - 1, where n is thw total number of columns. Inside this loop we will print the value at row i and column j.

Pseudocode¶

void printmat(int mat[][]){
    int n = mat.length;
    int m = mat[0].length;

    for(int c = 0; c < m; c++)//rows
    {
        for(int r = 0; c < n; c++) //columns
        {
            System.out.print(mat[r][c] + " ");
        }
        System.out.println();
    }
}

Printing Column in Wave Form¶

Given a matrix, you are required to print its elements in wave form by columns.

Observation¶

To print the matrix in wave form by columns, we can iterate through the columns of the matrix. For even columns, we start from the top and move downward; for odd columns, we start from the bottom and move upward. This way, we print the elements in a zigzag pattern along the columns.

Example¶

Consider the following matrix:

mat :

index	0	1	2	3
0	21	16	17	14
1	7	8	10	1
2	6	11	13	21
3	32	50	6	10
4	15	18	49	4

The elements in wave form by columns would be: 21 7 6 32 15 18 50 11 8 16 17 10 13 6 49.

Pseudocode¶

void printWaveArrayByColumn(int mat[][]) {
    int n = mat.length;
    int m = mat[0].length;

    for (int c = 0; c < m; c++){ // columns
        if (c % 2 == 0) {
            for (int r = 0; r < n; r++){ // rows
                print(mat[r][c] + " ");
            }
        } else {
            for (int r = n - 1; r >= 0; r--){ // rows
                print(mat[r][c] + " ");
            }
        }
    }
}

Max of matrix¶

Given a 2D Array A[][], return max element from this matrix.

Example:¶

mat :

index	0	1	2	3
0	12	65	89	74
1	22	44	12	30
2	10	12	97	19

Output:

Max element of matrix is 97

Idea:¶

Iterate on every element of row and column.
compare mat[i][j] with max element.
return max element.

Psuedo Code:¶

public class Solution {
    public int solve(int[][] A) {
        int max = Integer.MIN_VALUE;
        for(int i = 0; i < A.length; i++) {
            for(int j = 0; j < A[0].length; j++) {
                if(max < A[i][j]) {
                    max = A[i][j];
                }
            }
        }
        return max;
    }
}

Max of Every Row¶

Given a matrix and row number, return an array containing max of all elements in that row.

Example 1¶

mat :

index	0	1	2	3	max
0	21	16	17	14	21
1	7	8	10	1	10
2	6	11	13	21	21
3	32	50	6	10	50
4	15	18	49	4	49

ans :

index	0	1	2	3	4
ans	21	10	21	50	49

Question¶

What will be the max of every row for the given matrix?

1 2 3 13 4
5 6 17 8 9
19 0 1 2 21

Choices¶

15 19
4 9 21
13 17 21

Question¶

What should be the size of array to store max in every row for a matrix of size N * m

Choices¶

N + M
N
M
N * M

Observation¶

Size of ans array = total no of Rows

Create ans array
Iterate on every row and find max
Store the max of ith row at ans[i]

Dry Run wrt Above Example:

i	Initial MAX	Iterate on ith row: j -> 0 to m-1	Max in Row	ans[i] = max
0	- INF	Iterate on 0^th row: j -> 0 to m-1	21	ans[0] = 21
1	-INF	Iterate on 1^st row: j -> 0 to m-1	10	ans[1] = 10
2	-INF	Iterate on 2^nd row: j -> 0 to m-1	21	ans[2] = 21
3	-INF	Iterate on 3^rd row: j -> 0 to m-1	50	ans[3] = 50
4	-INF	Iterate on 4^th row: j -> 0 to m-1	49	ans[4] = 49

Pseudocode¶

int prinRowMax(int mat[][], int r){
    int n = mat.length;
    int m = mat[0].length;
    int[] ans = new int[n];

    int sum = 0;
    for(int i = 0; i < n; i++)//rows
    {
    int max = Integer.MIN_VALUE;
    for(int j = 0; j < m; j++){
        if(mat[i][j] > max)
        {
            max = mat[i][j];
        }
    }
    ans[i] = max;
   }
return ans;
}

2D Array-1¶

Agenda¶

Introduction¶

Syntax:¶

Question¶

Choices¶

Question¶

Choices¶

Question¶

Choices¶

Question¶

Choices¶

Question¶

Choices¶

Indexing and Properties:¶

Question¶

Choices¶

Question¶

Choices¶

Question¶

Choices¶

Question¶

Choices¶

Taking input from user¶

Question¶

Choices¶

Printing 0th Row¶

Observation¶

Example¶

Pseudocode¶

Print Matrix Row by Row and Column by Column¶

Example¶

Code¶

Printing Row in wave form¶

Observation¶

Example 1¶

Observation¶

Pseudocode¶

Question¶

Choices¶

Print 0th column¶

Example¶

Observation¶

Pseudocode¶

Print every column¶

Exmaple 1¶

Observation¶

Pseudocode¶

Printing Column in Wave Form¶

Observation¶

Example¶

Pseudocode¶

Max of matrix¶

Example:¶

Idea:¶

Psuedo Code:¶

Max of Every Row¶

Example 1¶

Question¶

Choices¶

Question¶

Choices¶

Observation¶

Pseudocode¶

Printing 0^th Row¶

Print 0^th column¶