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

[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.