Java: for / forEach run through int matrix

Question

This is not a 'how to do sth.' question, but a 'why can't I do it that way' question.

Obviously I have missed something important here (sth. about references I guess) to understand this problem and it would be nice if someone could explain it to me.

I did find the other posts where this question was dealt with before, but none of them explained it.

I want to assign new values to every element of an int matrix.

I have a piece of code like this:

public static void main(String[] args)
{
    int[][] tileMatrix = new int[5][5];

    System.out.println("New Tile Values:");
    for ( int[] tileLine : tileMatrix)
    {
        for ( int tile : tileLine)
        {
            tile = (int) (Math.random() * 39);
            System.out.print(tile + "  ");
        }
        System.out.println("");
    }

    System.out.println("Check Values");
    for ( int[] tileLine : tileMatrix)
    {
        for ( int tile : tileLine)
        {
            System.out.print(tile + "  ");
        }
        System.out.println("");
    }
}

That leads to that:

New Tile Values: 22 17 29 20 5
12 13 38 35 19
1 9 10 23 27
24 3 36 3 19
37 4 5 18 26
Check Values 0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0

So obviously I can't change the values like that. I can't find out whether that is generaly impossible in foreach loops, or whether I'm just doing it wrong. I could fix my problem with a normal for loop, I know that, but why do I have to?

Answer 1

= only affects the variable you are currently interacting with

A very simple example shows why your loop doesn't change the values stored in the array

int a=5;
int b=a;
b=10;
System.out.println(a); //prints 5

Your code is obviously more complex than this, but it is the same at heart. Each value from the array is copied into a variable and then that variable is manipulated.

for ( int[] tileLine : tileMatrix)
{
    for ( int tile : tileLine)
    {
        tile = (int) (Math.random() * 39); //<---tile is the same as b in my example
        System.out.print(tile + "  ");
    }
    System.out.println("");
}

Variables that refer to objects hold memory addresses

You asked why the following does affect the underlying 2D array

for ( int[] tileLine : tileMatrix){
    for (int i=0;i<tileLine.length;i++){
        tileLine[i] = (int) (Math.random() * 39);
    }
}

This is because tileMatrix does not contain 1D arrays in each of its elements, it contains memory addresses (huge oversimplification). I'm going to refer to memory addresses as #120 for the 120th region of memory.

for ( int[] tileLine : tileMatrix){
    //tileLine is a memory address, perhaps #130, it is not an int[] itself
    //we only take a copy of the memory address, not a copy of the array itself 

    for (int i=0;i<tileLine.length;i++){
           //tileLine points to the piece of memory within the original 
           //array

           //tileLine[i] actually means get the array at memory address `#120` 
           //and put `(int) (Math.random() * 39);` in its i-th entry
           tileLine[i] = (int) (Math.random() * 39);
    }
}

So to conclude when you use a variable java (behind the scenes) deferences it and gets the real object for you to work with.

tileMatrix is actually 1D matrix (pretending to be a 2D matrix). It may contain the following

#120
#130
#1180
#2021
#2022

Then when we look at one of those memory addresses, say #120, we find it contains the following

1
5
5
1
4

So you see if we have the memory address #120 it doesn't matter where we are, we're refering to the same place.

^{the memory model here is not supposed to be accurate, only conseptually useful, its more complicated than this in reality}

Answer 2

I know you mentioned you didn't want to know how to do something, but you might want to look at the JDK8 Arrays parallelPrefix method

Answer 3

Enhanced for loop will only take the values of the elements in the array and store them inside variables. With values, this mean a copy of the primitive or a copy of the reference value stored in the array. If you reassign a new value to this variable, you're just modifying the value of this variable, not the value in the array.

This explain why this for loop doesn't work:

for ( int tile : tileLine) {
    tile = (int) (Math.random() * 39);
    System.out.print(tile + "  ");
}

The for statement above behaves like this:

for (int i = 0; i < tileLine.length; i++) {
    int tile = tileLine[i];
    //you modify the local variable tile, not the element in the array
    tile = (int) (Math.random() * 39);
    System.out.print(tile + "  ");
}

If you want to modify the values in the array, you should modify the value directly in the array:

for (int i = 0; i < tileLine.length; i++) {
    tileLine[i] = (int) (Math.random() * 39);
    System.out.print(tileLine[i] + "  ");
}