Indexing for (datatype element : array) loops

Processing Beginners
1

I’ve been trying to figure out if there’s a simple way to extract the index of the iteration through a for-loop when using the for(elements in an array) style iteration.

It’s simple enough using the for(init; test; update) iteration:

for (int i = 0; i < limit; i++)
{
   someArray[i]; // *just use i to get the item in that array index*
}

But if we look at the Bouncy Bubbles Example we iterate through the elements of the array of Balls:

  Ball[] balls = new Ball[numBalls];
.
.
.
  for (Ball ball : balls)
  {
    someArray[???]; // *how would we get the index the for-loop has iterated through here?*
    ball.collide();
    ball.move();
    ball.display();  
  }

Of course, in the Bouncy Bubbles example, the Ball class has an int variable called id that gets indexed as it’s constructed, so we could do something like:

  for (Ball ball : balls)
  {
     someArray[ball.id];
  }

But what if you’re working with an array of objects that don’t have the id variable? Is there a way to get the iteration count from the for-loop? Or would it make more sense to switch to either an init/test/increment for loop, or implement an ID variable to the class’s objects?

Thank you?

2

Hi @RFullum

The for-each loop that not provide a counter by itself, but you can provide your own counter.
The reason for this is that the for-each loop internally does not have a counter; it is based on the Iterable interface, i.e. it uses an Iterator to loop through the “collection” - which may not be a collection at all, and may in fact be something not at all based on indexes (such as a linked list).

So, the solution is to either have your own counter, like:

int index = 0;
for(Ball ball : balls) {
    System.out.println("Current index is: " + (index++));
}

or make the class possess the id.

Best regards

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3

There is an example here:
https://processing.org/reference/for.html
https://processing.org/reference/Array.html
https://processing.org/reference/ArrayList.html It is called an enhanced for loop if you want to search for references to this.

:)

1 Like
4

Thanks @MiguelSanches and @glv

Yah. If the for-each-loop (enhanced for-loop) doesn’t have an internal counter and i have to introduce the iterator, I might as well just structure the for-loop to use the init-test-update model or introduce an ID variable to the class. Seems cleaner than introducing the iterator.

A lot of what i’m doing is sending values of FFT bands to instances of classes within arrays of that object. My current implementation is:

for (Ball ball : balls)
  {
    float fftVelocity = spectrum[ball.id] * 10.0 + 1;   // THIS RIGHT HERE
    
    ball.resize(fftDiameter[ball.id]);
    ball.collide();
    ball.move(fftVelocity);
    ball.display();
  }

Alternately, i’ll just structure it with an init/test/update for-loop in the future.

Was mostly curious if the enhanced for-loop has that internal counter. But you two gave me the info i needed. Thanks again! Cheers!

3 Likes
5

Rather than some id field in class Ball you could instead use a vanilla for ( ; ; ) loop and cache each index of array balls[] in a variable:

for (int len = balls.length, i = 0; i < len; ++i) {
  final Ball ball = balls[i];

  ball.resize(fftDiameter[i]);
  ball.collide();
  ball.move(spectrum[i] * 10 + 1);
  ball.display();
}
2 Likes
6

@GoToLoop Yah. For this example, the id variable is also used for other things within the class so it’s a handy identifier. And I was just having fun modifying Processing’s Bouncy Ball example. But in the future, yeah, if i need some sort of iterator i’ll just use the vanilla for ( ; ; ) loop. I usually do. Was mostly curious if there was some sort of counter within the enhanced for loop.

Cheers!

2 Likes