c++ - Conditional large flat array traversal and surprisingly short loop execution time -

i in need of explanation have discovered experience. have large flat array of type char. array in total 500x500x500 = 125e+6 bytes long. inside cells keep number between 0 , 255. luckily when traversing array interested in cells having non-zero value!

now here goes question. found out experimenting doing smallest operation on cells takes huge amount of time when going through whole 0 , non-zero array whereas, if use condition similar 1 below,

while( index < 125000000 ) {     if( array[ index ] > 0 )     {         // stuff     }      index++; } 

the execution time drastically shorter. in fact can go through whole array , perform operations on non-zero cells in few seconds rather half hour execution time of approach no conditions.

what need explanation of why works! need explain phenomena in thesis report , best if can relate scientific paper or similar.

thank in advance!

best regards, omid ariyan

it expect char unsigned, hence capable of holding values in range [0,255], in fact signed, holding values in range [-128, 127] (assuming two's complement). number of cases array[ index ] > 0 smaller expect, because elements assigned values larger 127 have negative value.

note claim check non-zero values, checking positive ones.

you can check range of char on platform:

#include <limits> #include <iostream>  int main() {   std::cout << static_cast<int>(std::numeric_limits<char>::min()) << std::endl;   std::cout << static_cast<int>(std::numeric_limits<char>::max()) << std::endl;    char c = 234;   std::cout << static_cast<int>(c) << std::endl; // 234 if unsigned, -22 if signed }