Something weird in for loop speed - objective-c

Question

here is a part of my program code:
int test;
for(uint i = 0; i < 1700; i++) {
test++;
}
the whole program takes 0.5 seconds to finish, but when I change it to:
int test[1];
for(uint i = 0; i < 1700; i++) {
test[0]++;
}
it will takes 3.5 seconds! and when I change the int to double, it will gets very worse:
double test;
for(uint i = 0; i < 1700; i++) {
test++;
}
it will takes about 18 seconds to finish !!!
I have to increase an int array element and a double variable in my real for loop, and it will takes about 30 seconds!
What's happening here?! Why should it takes that much time for just an increment?!
I know a floating point data type like double has different structure from a fixed point data type like int, but is it the only cause for such a big different time? and what about the second example which is also an int array element?!
Thanks

Answer 1

You have answered your question yourself.
float (double) operations are different from integer ones. Even if you just add 1.0f.
Your second example takes longer than the first one just because you added some pointer refernces. An array in C is -bottom down- not much different from a pointer to the first element. Accessing any element, even the first one, would cause the machine code to load the starting address of the array multiply the index (0 in this case) with the length of each member (4 or whatever bytes int has) and add that (0) to the pointer. Then it has to dereference the pointer, meaning to acutally load the value at that very address. Add one and write back the result.
A smart modern compiler should optimize this a bit. When you want to avoid this optimization, then modify the code a bit and don`t use a constant for the index.
I never tried that with a modern objective-c compiler. But I guess that this code would take much loger than 3.5s to run:
int test[2];
int index = 0;
for(uint i = 0; i < 1700; i++) {
test[index]++;
}
If that does not make much of a change then try this:
-(void)foo:(int)index {
int test[2];
for(uint i = 0; i < 1700; i++) {
test[index]++;
}
}
and then call foo:0;
Give it a try and let us know :)