processing an array via method

Question

is it possible to use a reference and pass it to a method as a argument and do the initialization in the method body in java? what i mean is this: i have a method that get's a array as parameters:

public static void arrayreader(int [] [] m){
int rows,cols;
rows=input.nextInt();cols=input.nextInt();
m = new int [rows][cols];
for(int i=0;i<rows;i++){
    for(int j=0;j<cols;j++){
       m[i][j] = input.nextInt();
       }
   }
}

i want to do the initialization of array in the method body not in the main method but java dont allow that and says it's not initialized

No, you cannot do that, all variables should be initialized before using it — Pradeep Simha
– Pradeep Simha, Commented Dec 27, 2012 at 8:31
@PradeepSimha all local variables and final should be initialized before using them , not instance vars — PermGenError
– PermGenError, Commented Dec 27, 2012 at 8:32
@GanGnaMStYleOverFlowErroR..yes, but I think OP hasn't mentioned in question. Thanks.. :) — Pradeep Simha
– Pradeep Simha, Commented Dec 27, 2012 at 8:35
No you cannot do that. Because Java dosent know you are initializing in your passing method or not. — Jayamohan
– Jayamohan, Commented Dec 27, 2012 at 8:36

Thilo · Accepted Answer · 2012-12-27 08:33:31Z

3

If you made that array a return value instead of a parameter, your main method can look like this:

int[][] m = arrayreader();

answered Dec 27, 2012 at 8:33

Thilo

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Comments

xxxxxxxxx · Accepted Answer · 2012-12-27 08:33:30Z

1

You can't, because when you say:

m = new int [rows][cols];

You are effectively assigning a new array to "m" reference(pointer), thus you are losing the reference to the array passed as argument.

answered Dec 27, 2012 at 8:33

xxxxxxxxx

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4 Comments

mahdi mahzouni Over a year ago

tnx but why this happening?u mean java will generate 2 reference's that the first one will be garbage collected and the 2nd one will be used?

xxxxxxxxx Over a year ago

Do you understand the notion of pointer (like in C language)? If so, "m" is a pointer. In main() you point m to a memory zone containing an array, but then in arrayreader() you point it to a new zone containing a new array. Thus, what you say it's true, the first array will be garbage collected if no one else "points" at it (if you, for example, have assigned it to a variable in main() it won't be garbage collected)

xxxxxxxxx Over a year ago

Also: parameters in Java are always passed by-value. When you pass a reference to an object (an array is also an object) you pass the reference by-value. That means, that if you assign something to the parameter, you are not reassigning the reference object, but the reference itself: you are pointing the reference to a new object inside the method, but that does not affect the outside-of-the-method world.

xxxxxxxxx Over a year ago

You could then mark the answer as the solution if you feel it is the valid one.

Allan Spreys · Accepted Answer · 2012-12-27 08:36:14Z

1

if you would like to initialise an array in the method, than there is no need to pass it as a parameter

you can do something like this instead

public static int[][] arrayreader()
{
   int rows,cols;
   int[][] result = new int[rows][cols];

   rows=input.nextInt();
   cols=input.nextInt();

   for(int i=0;i<rows;i++)
   {
       for(int j=0;j<cols;j++)
       {
           result[i][j] = input.nextInt();
       }
   }

   return result;
}

answered Dec 27, 2012 at 8:36

Allan Spreys

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1 Comment

Rohit Jain Over a year ago

What is m in m[i][j]? And you forgot to return the array.

vishal_aim · Accepted Answer · 2012-12-27 08:46:24Z

0

you can also create the array object in main like:

int rows,cols;
rows=input.nextInt();cols=input.nextInt();
m = new int [rows][cols];

and then can have the function like:

public static void arrayreader(int [] [] m, int rows, int cols){
 for(int i=0;i<rows;i++){
   for(int j=0;j<cols;j++){
      m[i][j] = input.nextInt();
   }
 }
}

answered Dec 27, 2012 at 8:46

vishal_aim

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5 Comments

Allan Spreys Over a year ago

Java doesn't pass an int as a reference, it uses values instead. I don't think your code is going to work... Unless you remove the "m" parameter from the second method and make the 1st initialisation of "m" as global

vishal_aim Over a year ago

did you try the similar code (as I mention)? It'll work as we are just assigning the values to the same array object

Allan Spreys Over a year ago

then you don't need to pass it as a parameter

vishal_aim Over a year ago

if both (main and this function) are not in the same class, we may need to pass it

vishal_aim Over a year ago

or if array is declared in the main method, we do need to pass it

Andrei · Accepted Answer · 2016-03-14 09:10:47Z

Taking into account that I spent one day due to errors like:

Exception in thread "main" java.util.NoSuchElementException at java.util.Scanner.throwFor(Scanner.java:907)

I implemented all the ideas from above in the code bellow which works like a charm for matrix multiplication:

import java.util.*;

public class MatmultC
{
private static Scanner sc = new Scanner(System.in);
  public static void main(String [] args)
  {
    int m = sc.nextInt();
    int n = sc.nextInt();
    int a[][] = new int[m][n];
    arrayreader(a,m,n);
    printMatrix(a);

    int[][] b = readMatrix();
    printMatrix(b);

    int[][] c=mult(a,b);
    printMatrix(c);

  }

   public static void arrayreader(int [][] m, int rows, int cols) {
       for (int i = 0; i < rows; i++) {
           for (int j = 0; j < cols; j++) {
              m[i][j] = sc.nextInt();
           }
       }
   }


   public static int[][] readMatrix() {
       int rows = sc.nextInt();
       int cols = sc.nextInt();
       int[][] result = new int[rows][cols];
       for (int i = 0; i < rows; i++) {
           for (int j = 0; j < cols; j++) {
              result[i][j] = sc.nextInt();
           }
       }
       return result;
   }


  public static void printMatrix(int[][] mat) {
  System.out.println("Matrix["+mat.length+"]["+mat[0].length+"]");
       int rows = mat.length;
       int columns = mat[0].length;
       for (int i = 0; i < rows; i++) {
           for (int j = 0; j < columns; j++) {
               System.out.printf("%4d " , mat[i][j]);
           }
           System.out.println();
       }
       System.out.println();
   }

   public static int[][] mult(int a[][], int b[][]){//a[m][n], b[n][p]
   if(a.length == 0) return new int[0][0];
   if(a[0].length != b.length) return null; //invalid dims

   int n = a[0].length;
   int m = a.length;
   int p = b[0].length;

   int ans[][] = new int[m][p];

   for(int i = 0;i < m;i++){
      for(int j = 0;j < p;j++){
         for(int k = 0;k < n;k++){
            ans[i][j] += a[i][k] * b[k][j];
         }
      }
   }
   return ans;
   }

}

where as input matrix we have inC.txt

in unix like cmmd line execute the command:

$ java MatmultC < inC.txt > outC.txt

and you get the output

outC.txt

Matrix[4][3]
   1    2    3 
  -2    0    2 
   1    0    1 
  -1    2   -3 

Matrix[3][2]
  -1    3 
  -2    2 
   2    1 

Matrix[4][2]
   1   10 
   6   -4 
   1    4 
  -9   -2

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