(Java)what should I return in an int function, that I made to randomize the element of 2D array

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Hey guys

this might be too basic, but i really find it hard to understand about return in function(i kinda understand it in a way, but for some cases its confusing), lets say i make a function to to generate a 2D array with randomized elements, and then i need to print and use that function in the main, what should i return? given there is no parameters that i throw from the main, i just want to use this function in the main

Here is my code:

What I have tried:

public class Main {

    public static void main(String[] args) {
        Main main = new Main();


        System.out.println(main.map());

    }

    public int map(){
        int[][] grid = new int[10][10];
        int i=0;
        int j=0;
        int max=0;

        for(i =0; i<grid.length; i++){
            for(j = 0; j<grid[i].length; j++){
                grid[i][j] = ((int)Math.random()*10);
                System.out.print(grid[i][j]);
            }
            System.out.println("");
        }

        /*ignore this part
        if(max > grid[i][j])
        {
            max = grid[i][j];
        }
        System.out.println("");

        for(i=0; i<grid.length; i++){
            for(j=0; j<grid.length; j++){
                if(grid[i][j] == max){
                    int a = grid[i][0];
                    int b = grid[0][i];
                    System.out.println("["+a+"]"+"["+b+"]");
                }
            }
        }
        */
        return 0;
    }

}

Posted 31-Oct-19 22:17pm

chandra liquidzey rapier

Updated 31-Oct-19 22:30pm

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OriginalGriff · Answer 1 · 2019-10-31T22:31:00

Solution 1

YOu've defined the function map as returning an integer value - so that is all it can pass back to the caller when it finishes. If you want it to return more data, you need to change the way it's declared and return a different datatype.

Think of like this: you go to the cinema and then want $15 per person. You hand them a $20 note, and they return $5 and let you in to see the movie.
That's a fucntion returning an integer:

Java

public int pay movie(int payment, int persons) {
   return payment - (persons * 15);
   }

But if you go to the supermarket, you expect the shop to let you leave with a bag of groceries and your change - and that doesn't fit in an integer value at all, so you need to return a class instance that holds an array of products, and an integer change value.

If you want your map function to pass a grid back to what called it, then you need it to return an array:

Java

public int[][] map() {
   ...
   return grid; 
   }

And change your code that calls it to handle a different return type.

Posted 31-Oct-19 22:31pm

OriginalGriff

Comments

chandra liquidzey rapier 1-Nov-19 5:00am

Okay, thanks for the explanation, i didnt know you can even make a function for 2Darray like that,
but now the problem is, if i were to return the grid that i had randomized the element inside a for loop
how do i return that grid, with the elements that was filled in that loop? because when i return grid[i][j] or grid, its only zero because i can't use the i and j outside the loop

OriginalGriff 1-Nov-19 5:14am

No you can't - but if you return the whole array to the caller, then it can use it's own i and j to access the array in any way it needs to.
Think about it: "map" doesn't know what the caller is going to do with the grid, so it has no idea what i or j values it should return anyway, and there is no point in creating and populating a grid inside the function if you don't pass the whole thing back to the caller when it's done.

What I would do is make the map function take two integer parameters which specify the x and y size of the grid (instead of using the fixed sizes (10, 10) you do) and return the whole grid that size for the caller to do what they will with.

Make sense?

chandra liquidzey rapier 1-Nov-19 5:26am

I kinda understand, but i dont know where to start :(
and i dont know how to pass it, maybe you can make an example?

OriginalGriff 1-Nov-19 5:32am

public int[][] map(int x, int y) {
   ...
   return grid;
   }

And then

int[][] grid = map(10, 20);

chandra liquidzey rapier 1-Nov-19 5:38am

i wanted to pass my code here but it doesnt let me because a bit too long

soo what i did is according to you, i made the function like this

public int map(int[][] grid, int i, int j)

i defined the grid[][] size based on i and j, that i declared in the main, is that the same thing?
but it still doesnt print out the grid with randomized elements that was being for looped in the function

OriginalGriff 1-Nov-19 5:48am

Why pass the grid in at all? Let the map function handle that and return the complete grid to main.

But if you want to print the grid, you need to loop through all the elements, printing each out - just as you would if you were printing them by hand!
So write a second function that takes a grid (and uses its sizes) that holds two nested loops: one for lines, and the inner one for columns within the line. Inside the loops, you print each element.
public void printMap(int[][]map) {
...
}
You can get the number of rows with map.length, and the number of columns for each row with map[y].length
http://ice-web.cc.gatech.edu/ce21/1/static/JavaReview-RU/Array2dBasics/a2dLoop.html

chandra liquidzey rapier 1-Nov-19 6:17am

so no matter what, i cant just return the elements that was randomized in a function into the main without another function to print it out first?
thanks for the explanation, really appreciated, im really sorry if i cant seem to understand, its a bit confusing to me on how actually passing and returning functions work in some complicated cases

OriginalGriff 1-Nov-19 6:31am

You can return the whole array, but if you want to return a subset of that, you would need to create a new, smaller, array and return that.

What are you trying to achieve with this? It may be that you are trying to go down the wrong route entirely, but I can't tell from here! :laugh:

chandra liquidzey rapier 1-Nov-19 7:05am

No purpose honestly, only just fooling around the function and 2d Array, im trying to learn it piece by piece, just trying to understand it fully and using functions in some many complex cases

OriginalGriff 1-Nov-19 7:15am

Don't try to learn "piece by piece" - you'll miss too much that you don't even know exists!

Get a book (or better a course) and follow that from start to end. Addison Wesley do some good ones, as do Wrox. A course is better because a tutor can try a different way to explain something if you don't understand. But trying to learn it just by trying to get things working isn't a good method as you never know what you haven't met that might make it easier and / or better - or why it works when it does!

chandra liquidzey rapier 1-Nov-19 7:55am

Thanks man, appreciate the advices
is there any free book though? i've been trying to get books too, but mostly are really costly

OriginalGriff 1-Nov-19 8:07am

For Java, I'm not sure - for C# there is the Yellow Book, but I don't know of a Java equivelant.
You might try your local library service - if they don't have them, they can generally order them from the regional headquarters?