Syntax Passing and Array into a function as reference

Programming Questions

gennylk

Syntax Passing and Array into a function as reference

in Programming Questions • 6 months ago

Hi There,

I am writing a function which reads a 2D int array, and outputs a trasformed value to a new similar dimension array. My problem is how do I pass the 2nd array (below) such that it returns the values.

This is an extract.

The two Arrays are defined as

final int TANK_LENGTH = 17; //Tank Row

final int TANK_WIDTH = 9; //Tank Col

int[][] cctank = new int[TANK_LENGTH][TANK_WIDTH]; // tank with binary image

int[][] pxtank = new int[TANK_LENGTH][TANK_WIDTH]; // tank with sorted blobs

In program they are called as

LabelPoints(cctank,pxtank);

in function they are defined as :

void LabelPoints(int[][] InputArray, int[][]OutputArray) {....................code of function..........}

Is all passing in Processing as reference ?.

Replies(7)

ramin

Re: Syntax Passing and Array into a function as reference

6 months ago

Porcessing (or Java, because Processing is actually a Java library with some additional sugar )

primitives are passed as values and Objects, which includes Arrays are passed as reference.

Here a little example

void setup() {
int a[] = {
1, 2, 3
};
doubleIt(a);
println(a[0]); // will print 2, cause 1 is doubled
int b = 1;
doubleNum(b);
println(b); // will print 1, b is not doubled
}
void doubleIt(int a[]) {
for (int i=0; i < a.length;i++)
a[i] = a[i]*2;
}
void doubleNum(int n) {
n *=2;
}

GoToLoop

Re: Syntax Passing and Array into a function as reference

6 months ago

Primitives are passed as values...

... and Objects, which includes Arrays are passed as reference.

That is a very controversial issue between C/C++ vs Java/C# camps!

Officially, Java's standards claim that everything is always passed by value!

And C peeps argue it's a false claim!

In C, we have a choice to pass an argument as either reference or value

by prefixing or not unary operator & in front of that argument!

Java got no such feature!!!

Java always pass a copy of the value of an argument to a function call's parameters.

Confusion arises from the fact that value can be:

numerical, boolean or a reference pointer (memory address).

But even if it's a reference pointer, it's still a value!!!

To be "by reference", Java outghta be able to pass the reference belonging to the argument variable itself,

rather than the reference already stored in it!

P.S.: I like both C & Java. But I don't let any bias to overcome the facts. At least I try!

gennylk

Re: Syntax Passing and Array into a function as reference

6 months ago

Ramin, thank you for the example. However, in that case, if I needed to pass two variables (not arrays) as reference, I guess there is no way, other than use it in an temporary array perhaps ?

PhiLho

Re: Syntax Passing and Array into a function as reference

6 months ago

If the purpose is to call a function and to get several values as output, the preferred Java way is to make a class encapsulating these values and returning it (ie. returning an instance of this class).

gennylk

Re: Syntax Passing and Array into a function as reference

6 months ago

Thanks. Got the idea now...

GoToLoop

Re: Syntax Passing and Array into a function as reference

6 months ago

Values stored in local primitive variables can't be modified

by another method when passed as arguments for it.

Java doesn't have parameters by reference as I have explained above!

So a method can't directly manipulate a passed variable's memory address itself!

Only a copy of the value stored in it is sent; be it a number or a reference pointer!

However you can assign a returned value to it:

void setup() {
  int a = 10;

  println(a);
  a = function(a);
  println(a);

  exit();
}

int function(int a) {
  return a + 10;
}

GoToLoop

Re: Syntax Passing and Array into a function as reference

6 months ago

Another proof that Java doesn't pass arguments as reference:

void setup() {
  PVector a = new PVector(10, 20, 30);
  println(a);

  function(a); // Sends PVector instance
  println(a);  // But its memory address can't be changed by a method

  exit();
}

void function(PVector a) {
  // tries to assign another PVector instance to passed parameter:
  a = new PVector(-50, 100, .5);
  println(a);
}

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