Adding columns in an array that is created using command line arguments

Question

My goal for this program is to create a 2D-array ratings whose size is specified by the first two arguments from the command line. args[0] would represent the number of rows r and arg[1] would represent the number of columns c. The next arguments that follow would all be used to fill in the array. So if the command-line argument was 3 2 5 2 3 3 4 1. I would hope that the array would be 3 rows by 2 columns. The value 5 would be in ratings[0][0], the value 2 would be in ratings[0][1], the value 3 would be in ratings[1][0], etc. After that, I want to compute the sum of each column. So column 0 would be 12 and column 1 would be 6 in this scenario.

public static void main(String[] args) {
    int r = Integer.parseInt(args[0]);
    int c = Integer.parseInt(args[1]);
    int[][] ratings = new int[r][c];
    int z = 2;
    int y = 3;
    int x = 0;
    int counting = 0;
    int sum = 0;
    for (int rows = 0; rows < ratings.length; rows++ ) {
        for (int column = 0; column < ratings[c].length; column++){
            ratings[rows][column] = Integer.parseInt(args[z]);
            sum += ratings[rows][column];
            z++;
        }
        System.out.println(sum);
        sum = 0;
    }
    //System.out.println(movieRating);
}

This is my attempt at summing the columns but this right now just sums 5 and 2, 3 and 3, 4 and 1. I want the columns not the rows to be summed but do not know how to fix it. Thank you

Answer 1

You seem to be adding the values to the 2D array correctly. What you're missing is an additional nested loop to print out the column totals:

for (int i = 0; i < ratings[c].length; i++) {
   int colSum = 0;
   for (int j = 0; j < ratings.length; j++) {
      colSum += ratings[j][i];
   }
   System.out.println(colSum);
}

Add that where you currently have that //System.out.println(movieRating); line. Since you were adding the numbers to the array row-wise, you need to flip the for loops to be able to sum the columns.

Answer 2

Things you did right

You correctly initialized the ratings 2D-array with the values given on the command line. Let me re-write this below without your attempt at computing the columns' sum. Note that I renamed the variables so that the indices used in the for loop are single letter variable.

public static void main(String[] args) {
    int rows = Integer.parseInt(args[0]);
    int columns = Integer.parseInt(args[1]);
    int[][] ratings = new int[rows][columns];
    int argIndex = 2;
    
    for (int r = 0; r < ratings.length; r++ ) {
        for (int c = 0; column < ratings[r].length; c++){
            ratings[r][c] = Integer.parseInt(args[argIndex]);
            argIndex++;
        }
    }
}

Thing you didn't get right

The ratings array is filled row by row. In the code you posted, you compute in variable sum the sum of the elements inserted in the same row. This is the reason why it doesn't print the results you expected.

To compute the sum of each columns, I would recommend you create a new array in which to store this result. Integrating it with the code above:

public static void main(String[] args) {
    int rows = Integer.parseInt(args[0]);
    int columns = Integer.parseInt(args[1]);
    int[][] ratings = new int[rows][columns];
    int[] columnSums = new int[columns];
    int argIndex = 2;
    
    for (int r = 0; r < ratings.length; r++ ) {
        for (int c = 0; column < ratings[r].length; c++){
            ratings[r][c] = Integer.parseInt(args[argIndex]);
            columnSums[c] += ratings[r][c];
            argIndex++;
        }
    }

    // array columnSums contains your results
}

Answer 3

I have changed your original code with a simpler version. Let me know if you have problems understanding the solution.

public static void main(String[] args)
{
    int row = Integer.parseInt(args[0]);
    int col = Integer.parseInt(args[1]);
    
    int arrayLength = row * col;            // use a single dimension array, for simplicity
    int[] ratings = new int[arrayLength];   // the array size is based on the rows and columns
    
    // add data to the 'ratings' array
    // This is not actually needed because the conversion can be done directly when the columns are summed up
    for(int i = 0; i < arrayLength; i++)
    {
        ratings[i] = Integer.parseInt(args[i + 2]);
    }
    
    // sum up the columns using the % operator
    int[] result = new int[col];
    for(int i = 0; i < arrayLength; i++)
    {
        result[i % col] += ratings[i];
    }
    
    // print result
    for(int i = 0; i < result.length; i++)
    {
        System.out.println(String.format("Movie %d rating is %d", i, result[i]));
    }
}

PS: you are missing the validation around the String to int conversion and checks around the correctness of the user input