How to sum a row in a matrix

Question

Write the method:

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

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

Given:

Answer 1

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;
}

Answer 2

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;

Answer 3

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

Answer 4

With Java Streams can be done very elegantly:

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

Answer 5

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

Answer 6

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;
}