**FIRST EMAIL**/Java and Orbit Balls *****JAVA DISCUSSION***

[The Lucas]

Originally Posted by Jared341

What is it that you want to do with pointers that Java's pass by reference system can't do exactly?

Pointer arithmetic. It can be useful if used properly. However, I wouldn't encourage students new to C++ to use pointer arithmetic on the robot, unless of course, you want them to experience the frustration of the dreaded "Seg Fault".

[Jared Russell]

Originally Posted by The Lucas

Pointer arithmetic. It can be useful if used properly. However, I wouldn't encourage students new to C++ to use pointer arithmetic on the robot, unless of course, you want them to experience the frustration of the dreaded "Seg Fault".

Proper pointer arithmetic can result in small and efficient implementations of some data structures and algorithms, but given (a) the scope of a typical FIRST program and (b) the fact that we now have hardware and compilers that often get the job done just as fast and without the difficult-to-debug errors, I would probably expand "students new to C++" to "just about anyone without a really convincing reason".

[Chris27]

Originally Posted by The Lucas

Pointer arithmetic. It can be useful if used properly. However, I wouldn't encourage students new to C++ to use pointer arithmetic on the robot, unless of course, you want them to experience the frustration of the dreaded "Seg Fault".

char *ptr = malloc(SIZE);

ptr + i is equivalent to &ptr[i]. Both cases boil down to just a load effective address instruction. Besides the obvious that Java hides the virtual address of all values in the symbol table, I don't see how you are missing anything by not being able to explicitly use pointer arithmetic. The point of pointer arithmetic is to compute the address of an element in an array. the "[]" operator retrieves that element and many people find this interface more intuitive. There is no performance advantage of using pointer arithmetic and there is nothing you can't do with the "[]" that you can do with pointer arithmetic (as the "[]" operator just encapsulates pointer arithmetic).

[The Lucas]

Originally Posted by Chris27

char *ptr = malloc(SIZE);

ptr + i is equivalent to &ptr[i]. Both cases boil down to just a load effective address instruction. Besides the obvious that Java hides the virtual address of all values in the symbol table, I don't see how you are missing anything by not being able to explicitly use pointer arithmetic. in an array. the "[]" operator retrieves that element and many people find this interface more intuitive. There is no performance advantage of using pointer arithmetic and there is nothing you can't do with the "[]" that you can do with pointer arithmetic (as the "[]" operator just encapsulates pointer arithmetic).

[SIDE RANT] I think you are missing the point (sorry for the pun). The point of pointer arithmetic is to compute the address of an element (the array data structure is optional). If you can get that address by simply adding to a pointer (stored in a register) that is more efficient than computing that address by loading a symbol and adding the result of a multiplication (indexing an array using a variable).[/SIDE RANT]

I actually used a bit of pointer arithmetic this year to make a simple circular buffer for Camera Report Structures. Of course I could make a circular buffer class in Java too. Again, I am not recommending pointer arithmetic or saying it's vital, I am just saying it can be useful, simple and intuitive.

My real concern with Java is not the pointer stuff but rather the runtime performance. Java uses just in time (JIT) compilation and garbage collection which should add additional (non deterministic) load to the programs run (of course I need to read more about how theses features are implemented). That being said, I think additional language options are a good thing and all of my programing students are taught Java (not C) in high school for the AP exam.

[Alan Anderson]

Originally Posted by The Lucas

[SIDE RANT] I think you are missing the point (sorry for the pun). The point of pointer arithmetic is to compute the address of an element (the array data structure is optional). If you can get that address by simply adding to a pointer (stored in a register) that is more efficient than computing that address by loading a symbol and adding the result of a multiplication (indexing an array using a variable).[/SIDE RANT]

The efficiency is not what you suggest it is. Pointer arithmetic adds the result of a multiplication too: addition and subtraction are scaled by the size of the structure pointed to.

[AustinSchuh]

Originally Posted by Alan Anderson

The efficiency is not what you suggest it is. Pointer arithmetic adds the result of a multiplication too: addition and subtraction are scaled by the size of the structure pointed to.

But, at least in C, the size of the structure is a known quantity at compile time, so I would imagine that most of the time that multiplication can be eliminated by the compiler. And then on top of that, sometimes the multiplication will be by a power of 2, so the compiler will use a cheap shift operation.

[Alan Anderson]

Originally Posted by AustinSchuh

But, at least in C, the size of the structure is a known quantity at compile time, so I would imagine that most of the time that multiplication can be eliminated by the compiler. And then on top of that, sometimes the multiplication will be by a power of 2, so the compiler will use a cheap shift operation.

This does not describe a difference between array indexing in Java and pointer arithmetic in C.

[Chris27]

Originally Posted by The Lucas

[SIDE RANT] I think you are missing the point (sorry for the pun). The point of pointer arithmetic is to compute the address of an element (the array data structure is optional). If you can get that address by simply adding to a pointer (stored in a register) that is more efficient than computing that address by loading a symbol and adding the result of a multiplication (indexing an array using a variable).[/SIDE RANT]

And as I stated in my earlier post, both cases boil down to a simple lea instruction in assembly

See, here is a small piece of code I just wrote.

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr = array + i;
11 
12   return 0;
13 }

and its assembly

0000000000000000 <main>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	48 83 ec 20          	sub    $0x20,%rsp
   8:	bf 28 00 00 00       	mov    $0x28,%edi
   d:	e8 00 00 00 00       	callq  12 <main+0x12>
  12:	48 89 45 f8          	mov    %rax,0xfffffffffffffff8(%rbp)
  16:	c7 45 ec 00 00 00 00 	movl   $0x0,0xffffffffffffffec(%rbp)
  1d:	83 7d ec 09          	cmpl   $0x9,0xffffffffffffffec(%rbp)
  21:	7f 19                	jg     3c <main+0x3c>
  23:	8b 45 ec             	mov    0xffffffffffffffec(%rbp),%eax
  26:	48 98                	cltq   
  28:	48 c1 e0 02          	shl    $0x2,%rax
  2c:	48 03 45 f8          	add    0xfffffffffffffff8(%rbp),%rax
  30:	48 89 45 f0          	mov    %rax,0xfffffffffffffff0(%rbp)
  34:	48 8d 45 ec          	lea    0xffffffffffffffec(%rbp),%rax
  38:	ff 00                	incl   (%rax)
  3a:	eb e1                	jmp    1d <main+0x1d>
  3c:	b8 00 00 00 00       	mov    $0x0,%eax
  41:	c9                   	leaveq 
  42:	c3                   	retq

Alternate code

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr = &array[i];
11 
12   return 0;
13 }

its assembly

0000000000000000 <main>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	48 83 ec 20          	sub    $0x20,%rsp
   8:	bf 28 00 00 00       	mov    $0x28,%edi
   d:	e8 00 00 00 00       	callq  12 <main+0x12>
  12:	48 89 45 f8          	mov    %rax,0xfffffffffffffff8(%rbp)
  16:	c7 45 ec 00 00 00 00 	movl   $0x0,0xffffffffffffffec(%rbp)
  1d:	83 7d ec 09          	cmpl   $0x9,0xffffffffffffffec(%rbp)
  21:	7f 19                	jg     3c <main+0x3c>
  23:	8b 45 ec             	mov    0xffffffffffffffec(%rbp),%eax
  26:	48 98                	cltq   
  28:	48 c1 e0 02          	shl    $0x2,%rax
  2c:	48 03 45 f8          	add    0xfffffffffffffff8(%rbp),%rax
  30:	48 89 45 f0          	mov    %rax,0xfffffffffffffff0(%rbp)
  34:	48 8d 45 ec          	lea    0xffffffffffffffec(%rbp),%rax
  38:	ff 00                	incl   (%rax)
  3a:	eb e1                	jmp    1d <main+0x1d>
  3c:	b8 00 00 00 00       	mov    $0x0,%eax
  41:	c9                   	leaveq 
  42:	c3                   	retq

Absolutely no difference whatsoever.

[The Lucas]

Originally Posted by Alan Anderson

The efficiency is not what you suggest it is. Pointer arithmetic adds the result of a multiplication too: addition and subtraction are scaled by the size of the structure pointed to.

Originally Posted by AustinSchuh

But, at least in C, the size of the structure is a known quantity at compile time, so I would imagine that most of the time that multiplication can be eliminated by the compiler. And then on top of that, sometimes the multiplication will be by a power of 2, so the compiler will use a cheap shift operation.

Originally Posted by Chris27

And as I stated in my earlier post, both cases boil down to a simple lea instruction in assembly

See, here is a small piece of code I just wrote.

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr = array + i;
11 
12   return 0;
13 }

and its assembly

0000000000000000 <main>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	48 83 ec 20          	sub    $0x20,%rsp
   8:	bf 28 00 00 00       	mov    $0x28,%edi
   d:	e8 00 00 00 00       	callq  12 <main+0x12>
  12:	48 89 45 f8          	mov    %rax,0xfffffffffffffff8(%rbp)
  16:	c7 45 ec 00 00 00 00 	movl   $0x0,0xffffffffffffffec(%rbp)
  1d:	83 7d ec 09          	cmpl   $0x9,0xffffffffffffffec(%rbp)
  21:	7f 19                	jg     3c <main+0x3c>
  23:	8b 45 ec             	mov    0xffffffffffffffec(%rbp),%eax
  26:	48 98                	cltq   
  28:	48 c1 e0 02          	shl    $0x2,%rax
  2c:	48 03 45 f8          	add    0xfffffffffffffff8(%rbp),%rax
  30:	48 89 45 f0          	mov    %rax,0xfffffffffffffff0(%rbp)
  34:	48 8d 45 ec          	lea    0xffffffffffffffec(%rbp),%rax
  38:	ff 00                	incl   (%rax)
  3a:	eb e1                	jmp    1d <main+0x1d>
  3c:	b8 00 00 00 00       	mov    $0x0,%eax
  41:	c9                   	leaveq 
  42:	c3                   	retq

Alternate code

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr = &array[i];
11 
12   return 0;
13 }

its assembly

0000000000000000 <main>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	48 83 ec 20          	sub    $0x20,%rsp
   8:	bf 28 00 00 00       	mov    $0x28,%edi
   d:	e8 00 00 00 00       	callq  12 <main+0x12>
  12:	48 89 45 f8          	mov    %rax,0xfffffffffffffff8(%rbp)
  16:	c7 45 ec 00 00 00 00 	movl   $0x0,0xffffffffffffffec(%rbp)
  1d:	83 7d ec 09          	cmpl   $0x9,0xffffffffffffffec(%rbp)
  21:	7f 19                	jg     3c <main+0x3c>
  23:	8b 45 ec             	mov    0xffffffffffffffec(%rbp),%eax
  26:	48 98                	cltq   
  28:	48 c1 e0 02          	shl    $0x2,%rax
  2c:	48 03 45 f8          	add    0xfffffffffffffff8(%rbp),%rax
  30:	48 89 45 f0          	mov    %rax,0xfffffffffffffff0(%rbp)
  34:	48 8d 45 ec          	lea    0xffffffffffffffec(%rbp),%rax
  38:	ff 00                	incl   (%rax)
  3a:	eb e1                	jmp    1d <main+0x1d>
  3c:	b8 00 00 00 00       	mov    $0x0,%eax
  41:	c9                   	leaveq 
  42:	c3                   	retq

Absolutely no difference whatsoever.

Since I'm done my work for the day, I'll continue this rant. What I (and I think Austin too) am getting at is this example (in same style as Chris' examples):

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr = array;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr++;
11 
12   return 0;
13 }

That is different and removes the need for the shift left op (since as Austin said the sizeof(int) is known at compile time and can just be added). Cant do that without explicit pointers. If the multiplication by type size is bothersome you can use an unsigned int and typecast it to a pointer. Also useful if you have an array of structs and are only working with same element in each struct (you can just add the size of the struct to the pointer).

Plus explicit pointers make functions like swap (commonly used in bubblesort simple) simpler using C pass by reference. Again not vital, just something that gets abstracted away in a higher level language.

[Alan Anderson]

Originally Posted by The Lucas

Since I'm done my work for the day, I'll continue this rant. What I (and I think Austin too) am getting at is this example (in same style as Chris' examples):

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr = array;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr++;
11 
12   return 0;
13 }

That is different and removes the need for the shift left op (since as Austin said the sizeof(int) is known at compile time and can just be added). Cant do that without explicit pointers. If the multiplication by type size is bothersome you can use an unsigned int and typecast it to a pointer. Also useful if you have an array of structs and are only working with same element in each struct (you can just add the size of the struct to the pointer).

I don't understand the point (!) of your example. What part of it should I focus on? None of it appears to show me a reason for preferring to use explicit pointers.

Plus explicit pointers make functions like swap (commonly used in bubblesort simple) simpler using C pass by reference...

Isn't pass by reference exactly what Java lets you do in such cases? I guess I'm completely failing to understand what you're trying to explain here.

[The Lucas]

Originally Posted by Alan Anderson

I don't understand the point (!) of your example. What part of it should I focus on? None of it appears to show me a reason for preferring to use explicit pointers.

I generated assembly for both of the examples with the relevant instructions bolded

Originally Posted by Chris27

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr = array + i;
11 
12   return 0;
13 }

.L2:
        cmpl    $9, -12(%ebp)
        jg      .L3
        movl    -12(%ebp), %eax
        sall    $2, %eax
        addl    -4(%ebp), %eax
        movl    %eax, -8(%ebp)
        leal    -12(%ebp), %eax
        incl    (%eax)
        jmp     .L2
.L3:

Originally Posted by The Lucas

 1 #include <stdlib.h>
 2 
 3 int main()
 4 {
 5   int *array = malloc(10 * sizeof(int));
 6   int *ptr = array;
 7   int i;
 8 
 9   for(i = 0; i < 10; i++)
10     ptr++;
11 
12   return 0;
13 }

.L2:
        cmpl    $9, -12(%ebp)
        jg      .L3
        leal    -8(%ebp), %eax
        addl    $4, (%eax)
        leal    -12(%ebp), %eax
        incl    (%eax)
        jmp     .L2
.L3:

So 4 instructions are necessary for ptr = array + i; and only 2 instructions for ptr++; in this assembly language.

Originally Posted by Alan Anderson

Isn't pass by reference exactly what Java lets you do in such cases? I guess I'm completely failing to understand what you're trying to explain here.

I Googled "java swap" and this site explains it pretty well. Basically when you pass by reference to a function in Java it gives the function a new (different) reference to the object and there is no way to change the original reference in the passing function. In C++, the pointer is like an absolute reference (memory location) that is the same everywhere and you can even use pointers to pointers (no references to references in Java).

[Chris27]

Originally Posted by The Lucas;860118
So 4 instructions are necessary for [B

ptr = array + i;[/b] and only 2 instructions for ptr++; in this assembly language.

So what's your point? Of course an expression that contains multiple variables will generally be more complex then an expression that contains a single variable. ptr = (&(ptr[0])) + 1 will again compile to the same assembly. Array indexing is just an encapsulation for pointer arithmetic as I explained earlier. Any pointer arithmetic expression will have a matching array index expression and both of these expressions will compile to the exact same instructions.

I Googled "java swap" and this site explains it pretty well. Basically when you pass by reference to a function in Java it gives the function a new (different) reference to the object and there is no way to change the original reference in the passing function. In C++, the pointer is like an absolute reference (memory location) that is the same everywhere and you can even use pointers to pointers (no references to references in Java).

If you want to change a reference to an object, just have the method return a new reference. If you want a series of references, just encapsulate the references in objects. If you absolutely need to have access to the virtual address of an object, then Java just isn't the right language for the job.

Here is swap with references:

public class MyInt {
	
	int data;
	public MyInt(int x)
	{
		this.data = x;
	}
	
	public static void swap(MyInt a, MyInt b)
	{
		a.data ^= b.data;
		b.data ^= a.data;
		a.data ^= b.data;
	}

}

[The Lucas]

Originally Posted by Chris27

ptr = (&(ptr[0])) + 1 will again compile to the same assembly.

You can't dereference (&) and do pointer arithmetic like that in Java.

Originally Posted by Chris27

Array indexing is just an encapsulation for pointer arithmetic as I explained earlier. Any pointer arithmetic expression will have a matching array index expression and both of these expressions will compile to the exact same instructions.

There is no matching array index expression for this:

ptr = 365;

Array indexing requires a symbol (base of array) and a type (size of element). When an integer can be cast to a pointer, pointer arithmetic can be done without an array.

Originally Posted by Chris27

If you want to change a reference to an object, just have the method return a new reference. If you want a series of references, just encapsulate the references in objects. If you absolutely need to have access to the virtual address of an object, then Java just isn't the right language for the job.

Here is swap with references:

public class MyInt {
	
	int data;
	public MyInt(int x)
	{
		this.data = x;
	}
	
	public static void swap(MyInt a, MyInt b)
	{
		a.data ^= b.data;
		b.data ^= a.data;
		a.data ^= b.data;
	}

}

I know you can create a new class, a similar example was on the page I linked. I just said it was simpler in C. Creating new classes or returning multiple references doesn't sound simple to me.

Again, I understand both languages and the reasons why Java doesn't allow explicit pointers (source of bugs, reference counting for garbage collecting, sercurity, etc..).

Lets revisit how this whole conversion happened (with me paraphrasing)
Billy: Yuck Java. Pointers ftw
Jared: What cant you do with Java references?
Me: Pointer Arithmetic.
Jared: Granted, but you better have a good reason.
<Long posts involving example code by Chris, me and Alan>
I thought twice before pressing Submit Reply on my first post because I knew it wouldn't be my last on this thread Programming debates are interesting (for me atleast) but time consuming. Maybe I should have just let Billy reply or learned from C++ vs LV threads.

[Alan Anderson]

Originally Posted by The Lucas

You can't dereference (&) and do pointer arithmetic like that in Java.

Right. The whole discussion here is based on the lack of pointer arithmetic in Java. But I'm still trying to find out why some think that pointer arithmetic is more efficient than using arrays. My understanding is that they compile to exactly the same thing, since in C a[i] is just syntactical sugar for *(a+i). I used to confuse people by writing i[a] instead, which gives exactly the same result.

There is no matching array index expression for this:

ptr = 365;

Without significant context, I'm at a loss to understand why you'd want to write it. With the exception of using memory-mapped I/O, I have yet to see any examples anywhere where using absolute pointers gives any improvement in efficiency over using arrays.

[Bharat Nain]

Originally Posted by Alan Anderson

Right. The whole discussion here is based on the lack of pointer arithmetic in Java. But I'm still trying to find out why some think that pointer arithmetic is more efficient than using arrays.

More importantly, how much does it matter in our application? What difference will it make?