This question already has an answer here:
Note:the current allege duplicate question I think is mostly related to "<" and ">" comparison, but not "==" comparision and hence not answer the question that about performance of "==" operator.
At long time I believe "processing" a sorted array should be faster than unsorted array. At first I think using "==" in sorted array should be faster than in unsorted array because I guess how branch prediction works:
UNSORTEDARRAY:
5 == 100 F
43 == 100 F
100 == 100 T
250 == 100 F
6 == 100 F
(other elements to check)
SORTEDARRAY:
5 == 100 F
6 == 100 F
43 == 100 F
100 == 100 T
(no need to check other elememt,so all are F)
so I guess SORTEDARRAY should be faster than UNSORTEDARRAY,but today I use the code to generate 2 arrays in a header to test ,branch prediction seems not work as I guess.
I generate a unsorted array and a sorted array to test:
srand(time(NULL));
int UNSORTEDARRAY[524288];
int SORTEDARRAY[sizeof(UNSORTEDARRAY)/sizeof(int)];
for(int i=0;i<sizeof(SORTEDARRAY)/sizeof(int);i++){
SORTEDARRAY[i]=UNSORTEDARRAY[i]=rand();
}
sort(SORTEDARRAY,SORTEDARRAY+sizeof(SORTEDARRAY)/sizeof(int));
string u="const int UNSORTEDARRAY[]={";
string s="const int SORTEDARRAY[]={";
for(int i=0;i<sizeof(UNSORTEDARRAY)/sizeof(int);i++){
u+=to_string(UNSORTEDARRAY[i])+",";
s+=to_string(SORTEDARRAY[i])+",";
}
u.erase(u.end()-1);
s.erase(s.end()-1);
u+="};\n";
s+="};\n";
ofstream out("number.h");
string code=u+s;
out << code;
out.close();
to test,just count if value is == RAND_MAX/2 like this:
#include "number.h"
int main(){
int count;
clock_t start = clock();
for(int i=0;i<sizeof(SORTEDARRAY)/sizeof(int);i++){
if(SORTEDARRAY[i]==RAND_MAX/2){
count++;
}
}
printf("%f\n",(float)(clock()-start)/CLOCKS_PER_SEC);
}
run 3 times:
UNSORTEDARRAY
0.005376
0.005239
0.005220
SORTEDARRAY
0.005334
0.005120
0.005223
it seems rare performance difference, I don't believe and then try to change "SORTEDARRAY[i]==RAND_MAX/2" to "SORTEDARRAY[i]>RAND_MAX/2" to see difference:
UNSORTEDARRAY
0.008407
0.008363
0.008606
SORTEDARRAY
0.005306
0.005227
0.005146
this time have much difference.
Is "==" in sorted array not faster than unsorted array? If yes, why ">" in sorted array is faster than unsorted array but "==" is not?
stringis not a standard type in C, and using the+=operator with one operand of typestringand the otherchar *makes no sense. Are you sure this isn't C++ code? – Seb Aug 18 '15 at 5:29