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Loops

General and Gameplay Programming Programming

Started by jfclavette April 02, 2003 04:47 PM

4 comments, last by jfclavette 21 years ago

jfclavette

1,058

Author

April 02, 2003 04:47 PM

hypothetical: Would

  
long foo=0
long bar=0

long array[10000][5]= {3,6,2,6,8}

for (foo=0; foo<=9999; foo++)
{
  for (foo=0; foo<=5; foo++)
  {
    array[foo][bar]++;
  }
}

be faster than

  
long foo=0
long bar=0

long array[10000][5]= {3,6,2,6,8}

for (foo=0; foo<=9999; foo++)
{
  array[foo][0]++;
  array[foo][1]++;
  array[foo][2]++;
  array[foo][3]++;
  array[foo][4]++;
}

Assuming the situation is rigorously the same, but a little bit more complicated. Would the later be much faster?

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Russell

118

April 02, 2003 04:55 PM

Theoretically the second should be faster. Also, I think this might be a little faster than the first example you gave.

  long foo=0;long bar=0;long array[10000][5]= {3,6,2,6,8};for (bar=0; bar<=5; bar++) {  for (foo=0; foo<=9999; foo++) {      array[foo][bar]++;  }}

All I did was switch the inner and outer loops. Putting the loop with more iterations as the inner loop should be faster, I think. Also it might be possible to write it quite a bit faster using pointers rather than indexes into a 2D array. Maybe like this.

  long array[10000][5]= {3,6,2,6,8};long * p, * end = array + 49999;for (p=array; p != end; p++)    (*p)++;}

[edited by - Russell on April 2, 2003 6:00:16 PM]

civguy

308

April 02, 2003 05:08 PM

foo<=5;

This goes to 5.. meaning that it''ll try to access index [5] which is illegal.

Shannon Barber

1,684

April 02, 2003 05:29 PM

use < not <=, and yes the second case would be marginally faster, depending on what goes on inside the second loop.

It would also be faster to flip it around:

for(int j=0; j<5; ++j)   for(int i=0; i<10000; ++i)      stuff();

- The trade-off between price and quality does not exist in Japan. Rather, the idea that high quality brings on cost reduction is widely accepted.-- Tajima & Matsubara

superdeveloper

188

April 02, 2003 10:38 PM

Careful everyone,

read up on multidimensional arrays in your C++ help index!

array [ a ] implies a-rows by b-colums. the b is direct indexing, so your loop should be (optimized): for(a…) { for(b…) { array[ a ] = xxx; }; }; and not the other way around like this: for(b…) { for(a…) { array[ a ] = xxx; }; }; This second example is an order of magnitude slower. www.cppnow.com [edited by - superdeveloper on April 2, 2003 11:58:44 PM]

-silencer-

144

April 03, 2003 01:42 AM

Don't forget about loop inversion*.. some machines are faster with decrementing and comparing with zero than incrementing and comparing with a certain maximum. Text both loops with your machine to see which runs faster.

    foo = 10000while (--foo){   array[foo][4]++;   array[foo][3]++;   array[foo][2]++;   array[foo][1]++;   array[foo][0]++;}

* "C Unleashed" by SAMS is a great book.

(silencer)

[edited by - -silencer- on April 3, 2003 2:43:44 AM]

Loops

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