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Question about code in "learning processing" book. (Read 1090 times)

diskclaimer YaBB Newbies Offline Posts: 24	Question about code in "learning processing" book. Aug 9^th, 2009, 8:46am In Shiffmans book on page 187 near the top, he has a boolean intersect function with the argument of (Drop d) The Drop is refering to the class, but what is the d refering to? I can not see anywhere where it is initalized.

antiplastik YaBB Moderator Offline Posts: 699 France	Re: Question about code in "learning processing" book. Reply #1 - Aug 9^th, 2009, 9:15am I guess d refers to the instance of the class? Could you give the line of code - for those who didn't get the book... yet

diskclaimer YaBB Newbies Offline Posts: 24	Re: Question about code in "learning processing" book. Reply #2 - Aug 9^th, 2009, 10:40am Sorry I'm on my iPhone today otherwise I would give more info. Here is the code: boolean intersect (drop d) { float distance = dist(x,y,d.x,d.y); if (distance < r + d.r) { return true; } else { return false; } }

NoChinDeluxe

Re: Question about code in "learning processing" book.
Reply #3 - Aug 9^th, 2009, 1:04pm

This is a custom function called intersect(). When you define a function, you'll see two meaningful pieces of information surrounding the function name. The item on the left of the name says "boolean." This tells us that this function will return a boolean variable. You can see that happen at the end when you return false. The item inside the parentheses is called the "argument." This function takes one argument, so when you call intersect() in your code, you need to pass in a variable of the type specified. Here, that variable type is of class "drop." The variable "d" is going to represent the drop object that is passed through once it is in the function. So you'll notice now that in the next line, Shiffman calls d's x and y members (d.x & d.y).

So in this case, "d" is actually a reference to the drop object that gets passed in. So anything you do with "d" inside of the function effects the actual object that's getting passed in when the function is called.

So to actually use this function, the code might looking something like this:
Code:


void setup(){
     size(100, 100);
     drop c = new drop();
     intersect(c);
}

In the above code we declare a drop object called "c", then we pass that object into the intersect function. Once in the function, the object will get assigned to the variable "d", and we can manipulate that however we like. Understand?

diskclaimer YaBB Newbies Offline Posts: 24	Re: Question about code in "learning processing" book. Reply #4 - Aug 9^th, 2009, 3:21pm Thanks for the reply. I'm getting there. But even in the "final code" for this example 10-10 on page 187 he never defines Drop d he just uses it as an argument. I can't even get example 10-3 to run. I suspect because Ball b is never initialized.

blindfish

Re: Question about code in "learning processing" book.
Reply #5 - Aug 9^th, 2009, 3:59pm

I haven't got the book, but the source code is downloadable from here.

I'm not sure you've properly followed NoChinDeluxe's explanation.

Code:

boolean intersect(Drop d) {  
  //
}

is a method definition that accepts one parameter, in this case a Drop. The 'd' is simply a label used to represent *any* drop passed to the method. A Drop 'd' isn't created in the main code, nor does it need to be. However a Drop does need to be included in any call to the method and in fact is in this line at the bottom of example_10_10:

Code:

if (catcher.intersect(drops[i])) {

'drops' is an array of Drop objects created at the top of example_10_10 ("Drop[] drops; // An array of drop objects").
The above condition is in a for loop, so 'drops[i]' passes each Drop from the array as a parameter to the 'intersect' method.

Where is the what if the what is in why?

NoChinDeluxe

Re: Question about code in "learning processing" book.
Reply #6 - Aug 10^th, 2009, 6:37am

How about a simpler example? Let's define a function called addTwoNumbers();

Code:


//here's the function definition:
int addTwoNumbers(int a, int b){
     int c = a + b;
     return c;
}

//Here is how we would use it in code:
int x = 1;
int y = 2;
int sum = addTwoNumbers(x, y); //sum now equals 3

Notice in the above code that we never actually define ints a or b (the function arguments). They simply get assigned the value of ints x and y when we call the function. So in essence, they are just copies of the two integers we passed into the function. It will be the same with your Drop d example. There is no need to define or initialize Drop d, because it's simply an exact copy of some other object that is being passed in to the function. So if it's an exact copy of an object that's already been initialized, why initialize it again? No need, we just assign it the value of the other initialized object being passed in.

diskclaimer YaBB Newbies Offline Posts: 24	Re: Question about code in "learning processing" book. Reply #7 - Aug 10^th, 2009, 9:33am Thanks everyone !! It finally clicked..

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