二维数组问题

Question

Okay, so I have an assignment in class to write a method that returns the location of the largest element in a 2D array. The return value is a 1D array that contains two elements that indicate the row and column of the largest element of the 2D array.

public static void main(String[] args) {
    int[] loc=new int[2];
    Scanner scan=new Scanner(System.in);
    int x,y;
    System.out.print("Enter the size of your two-dimensional array. Length then height: ");
    x=scan.nextInt();
    y=scan.nextInt();
    double [][]matrix=new double[x][y];
    System.out.println("Enter your two-dimensional array row by column: ");
    for (int i=0;i<y;i++){
        for (int j=0;j<x;j++){
            matrix[j][i]=scan.nextDouble();
            System.out.print("i: "+i+" j: "+j + " ");
        }
        System.out.println();
    }
    System.out.println();
    for (int i = 0; i<x; i++){
        for (int j = 0; j<y; j++){

            System.out.print(matrix[i][j] + ("i: "+i+" j: "+j + " "));

        }
         System.out.println();
    }
    loc=locateLargest(matrix,x,y);
        System.out.println(matrix[loc[0]][loc[1]]);


}
public static int[] locateLargest(double[][] a ,int x, int y){
    int m=0,n=0;
    int[] location={0,0};
    double largest=a[0][0];
    for (int i=0;i<x;i++){
        for (int j=0;j<y;j++){
            System.out.println("X: "+i+" Y: "+j +" value: " + a[i][j]);
            if (a[i][j]>largest){

                location[0]=i;
                m=i;
                location[1]=j;
                n=i;
            }
        }
    }
    System.out.println("the location of the largest element of you two-dimensional array is : ("  + location[0] + ", " + location[1] + ")");
    System.out.println(a[m][n]);
    return location;
}

This is my code. It's pretty sloppy and may not make a whole lot of sense. I included both the main(String[] args) and my method because both are needed to set it up. When I run it I am getting the wrong answer and I am trying to troubleshoot by printing out the values(I have left these print lines in my code), The answer I'm getting is wrong. I belive it is something to do with how the elements of my array are being input.

The array I'm using is 23.5 35 2 10 4.5 3 45 3.5 35 44 5.5 9.6 and the answer I keep getting is (2,2) which is 5.5 if I am reading it correctly. The value of X is 3 and Y is 4.

Any help as to why my code is returning the wrong answer would be greatly appreciated. Thank you.

Edit: I added the values of X and Y.

Answer 1

You need to reassign the variable largest after you find a bigger value in the block:

        if (a[i][j]>largest){
            largest=a[i][j]; //current value is largest
            location[0]=i;
            location[1]=j;
        }

Delete variables m and n , they are redundant.

Answer 2

I think the way you used the loops is right to find the largest number, the only problem is re-assignment of largest is not done in locateLargest function,

public static int[] locateLargest(double[][] a ,int x, int y)
{
    int m=0,n=0;
    int[] location={0,0};
    double largest=a[0][0];
    for (int i=0;i<x;i++)
    {
        for (int j=0;j<y;j++)
        {
            System.out.println("X: "+i+" Y: "+j +" value: " + a[i][j]);
            if (a[i][j]>largest)
            {
                location[0]=i;
                m=i; //Why you need this extra variables 
                location[1]=j;
                n=i; //Why you need this extra variables 
                largest = a[i][j];
            }
        }
    }
    System.out.println("the location of the largest element of you two-dimensional array is : ("  + location[0] + ", " + location[1] + ")");
    System.out.println(a[m][n]);
    return location;
}

Answer 3

The question has been answered but I think it's also worth you thinking about how you can solve this sort of problem without having to ask on Stack Overflow. Even if your assignment does not ask for it I STRONGLY recommend you look at starting to do Test Driven Development (TDD) - http://www.javaworld.com/article/2073090/testing-debugging/getting-started-with-test-driven-development.html

In the absence of that at least try and "reduce your feedback loop". For example the following code block allows you to easily (and visually) build test arrays that you can work with (rather than having to take user input every time you want to test your code).

This would allow you to build 5 slightly different arrays in code and see which ones are failing which should help you work out why.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class LargestNumber {

    public static void main(String[] args){
        TableBuilder<Integer> tableBuilder = new TableBuilder<Integer>()
                .row(1,   2, 3)
                .row(3,   2, 1)
                .row(3, 100, 1);
        print(tableBuilder.build());
    }


    static class TableBuilder<T>{
        List<List<T>> table = new ArrayList<List<T>>();

        TableBuilder<T> row(T... rowData){
            table.add(Arrays.asList(rowData));
            return this;
        }

        T[][] build(){
            T[][] tableAsArray = (T[][]) new Object[table.size()][];
            int arrayIndex = 0;
            for(List<T> row: table){
                tableAsArray[arrayIndex++] = (T[])row.toArray() ;
            }
            return tableAsArray;
        }
    }

    static void print(Object[][] table){
        for(Object[] row:table){
            for(Object cell:row){
                System.out.print(String.format("%1$" + 5 + "s", cell));
            }
            System.out.println();
        }

    }
}