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Nov 9th, 2008

array indexing vs pointer arithmetic

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C++ Syntax (Toggle Plain Text)
int x[5];
x[4] = 10;
*(x + 4) = 10;
int x[5];
x[4] = 10;
*(x + 4) = 10;

Array indexing has a cleaner syntax but (as I've read) pointer arithmetic is faster. My question is, is it worth to use pointer arithmetic? Is there a noticeable difference?

Last edited by minas1; Nov 9th, 2008 at 7:55 am.

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minas1

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Nov 9th, 2008

Re: array indexing vs pointer arithmetic

Quote ...

Array indexing has a cleaner syntax

You said it yourself! So why bother and make your life difficult. When you come back later to your code it will be more easy to understand then if you used pointer arithmetic. If you have a good C++ compiler he would translate both options the same way!

Last edited by ddanbe; Nov 9th, 2008 at 8:28 am.

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Nov 9th, 2008

Re: array indexing vs pointer arithmetic

Alright, thanks!

minas1

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Re: array indexing vs pointer arithmetic

>but (as I've read) pointer arithmetic is faster
Prove it. In your example, the two are quite likely compiled to exactly the same machine code. Array subscripting is literally syntactic sugar for the equivalent pointer offset calculation. A more likely example is this:

C++ Syntax (Toggle Plain Text)
int x[N];
int *p = x;
 
for ( int i = 0; i < N; i++ )
  process ( x[i] );
 
for ( int i = 0; i < N; p += i )
  process ( *p );
int x[N];
int *p = x;

for ( int i = 0; i < N; i++ )
  process ( x[i] );

for ( int i = 0; i < N; p += i )
  process ( *p );

The latter could very well be faster (though I would question why you're worried about such micro-optimizations) due to the fact that the body of the loop is only performing a dereference rather than an addition and a dereference.

My advice is not to worry about it. Use whichever more clearly displays your intentions.

Last edited by Narue; Nov 9th, 2008 at 10:32 am.

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Nov 9th, 2008

Re: array indexing vs pointer arithmetic

From the C++ Standard:

Quote ...

The expression E1[E2] is identical (by definition) to *((E1)+(E2)).

Sometime pointer arithmetics code is evidently cleaner than "subsripted" one:

C++ Syntax (Toggle Plain Text)
int countDigits(const char* str)
{
    int n = 0;
    if (str)
    while (int c = *str++)
        if (c >= '0' && c <= '9')
            ++n;
    return n;
}
int countDigits(const char* str)
{
    int n = 0;
    if (str)
    while (int c = *str++)
        if (c >= '0' && c <= '9')
            ++n;
    return n;
}

Sometimes we have no choice: as usually (but not always) pointer arithmetics code is faster than "subscripted" one on embedded systems where we (often) must save on every microsecond...

It's a far-fetched problem. It's not a language problem...

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