CODEWITHSUNDEEP
javamultidimensional-array
Bry
Bry

Reputation: 65

How to sum a row in a matrix

Write the method:

public int sumRow(int[][] matrix, int row)

that sums row row in the 2D array called matrix.

Given:

public void run()
{
    System.out.println(sumRow(new int[][]{{70,93,68,78,83},{68,89,91,93,72},{98,68,69,79,88}}, 2));
    System.out.println(sumRow(new int[][]{{1,1,1}, {2,2,2}, {3,3,3}}, 0));
    System.out.println(sumRow(new int[][]{{2,4,6,8,10}, {1,2,3,4,5}, {10,20,30,40,50}}, 2));
}

So far I have:

public int sumRow(int[][] matrix, int row)
{
    int sum = 0;
    for(int i = 0; i < matrix.length; i++)
    {
        for(int j = 0; j < matrix.length; j++)
        {
            sum = sum + matrix[j][i];
        }   
    }
    return sum;
}

The outputs I get are 714, 18, and 78 when they should be 402, 3, and 150. What am I doing wrong?

Upvotes: 2

Views: 18827

Answers (6)

dreamcrash
dreamcrash

Reputation: 51443

With Java Streams can be done very elegantly:

public static int sumRow2(int[][] matrix, int row) {
    return  Arrays.stream(matrix[row]).sum();
}

Upvotes: 1

Chris Gong
Chris Gong

Reputation: 8229

You're currently trying to sum all of the elements in the 2D array when you were asked to sum a specific row within the 2D array. In this case, you only need one for loop to traverse a single row like you would traverse a single array. The loop would start at the first element, matrix[row][0] and run until the last element, matrix[row][matrix[row].length - 1] since matrix[row].length is the number of columns/elements in that specific row of the matrix. Therefore, matrix[row].length - 1 would be the index of the last element in matrix[row]. Here's what it should look like,

public int sumRow(int[][] matrix, int row)
{
    int sum = 0;
    for(int i = 0; i < matrix[row].length; i++)
    {
        sum += matrix[row][i];
    }
    return sum;
}

Upvotes: 6

Selim Ajimi
Selim Ajimi

Reputation: 344

public int sumRow(int[][] matrix, int row)
{
    int sum = 0;

    int colSize = matrix[row].length;


    for(int j = 0; j < colSize; j++){
        sum += matrix[row][j];
    }   

    return sum;
}

HINT

Length of row:

int row = matrix.length;

Length of column :

int col = matrix[0].length;

Upvotes: 2

Mohsen_Fatemi
Mohsen_Fatemi

Reputation: 3391

there is a problem with your second for loop's condition , matrix.length is the length of first dimension , for the second dimension it looks like matrix[i].length

for(int i = 0; i < matrix.length; i++){
     for(int j = 0; j < matrix[i].length; j++){
         sum = sum + matrix[i][j];
     }   
}

i prefer to use sum+=matrix[i][j] instead of sum = sum + matrix[i][j]

calculating for one row :

for(int j = 0; j < matrix[row].length; j++){
    sum = sum + matrix[row][j];
}

just note that row's range is from 0 to matrix.length-1

Upvotes: 0

Locke
Locke

Reputation: 8944

You need to reference the second dimension of the array for j.

//ex: first dimension is matrix.length
//second dimension is matrix[any index in the first dimension].length
//and this cycle would continue with more and more [num] on the end

public int sumRow(int[][] matrix, int row)
{
    int sum = 0;
    for(int i = 0; i < matrix.length; i++)
    {
        for(int j = 0; j < matrix**[0]**.length; j++)
        {
            sum = sum + matrix[j][i];
        }   
    }
    return sum;
}

Upvotes: -1

ollaw
ollaw

Reputation: 2193

In the internal loop, modify the condition to: 

for(int j = 0; j < matrix[i].length; j++)

and then switch i and j in the sum

Upvotes: 0

Related Questions