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For loop optimization


AggrorJorn

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Today I learned a pretty basic but very usefull programming principle. It is about declaring multiple initialisers in a for loop. I am working with a mobile application and speed and memory are more of an issue than on computer right now. My application is finished when you look at its functionality and construction, but it has to be optimized.

 

Here is a narrowed down example of what I am doing:

 

for(int i = 0; i < veryLongArray.length(); i++)
{
 DoSomething(i);
}

This is very basic coding right here that can be found in many language.

 

 

What I didn't know is that you can declare multiple initialisers. It is very simple but has increased the speed of the for loop (which is pretty huge) immensly. This is how it looks now:

for(int i = 0, j = veryLongArray.length(); i < j ;i++)
{
 DoSomething(i);
}

By initialising the j variable at the start of the for loop, we can save a little time. Instead of checking the length every loop we can simply refer to the j variable. ofcourse you can store the length of the array outside the loop as well, but this is so much nicer. One catch though: If the length of the array is dynamicly changed during the loop, then you shouldn't be using this.

 

So simple, yet, I have never come accross it before.

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7 Comments


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I have another little optimization for you.

Always use pre incrementing values where possible, like ++i instead of i++.

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So simple, yet, I have never come accross it before.

 

If you think about its very logical. You called each for iteration a function compared to call it once and save the result temporarly.

 

Another good optimization is for two dimensional array, take care of your traversing order e,g.

// creation
unsigned char** myarray = new unsigned char*[1024];
for (int i = 0; i < 1024; ++i)
{
   myarray[i] = new unsigned char;
}

// update
for(int y = 0; y < 1024; ++y)
{
for(int x = 0; x < 1024; ++x)
{
 myarray[y][x] = 0;  // swapping x and y results in decrease/increase of nearly 50% on my system .... its because cache misses and miss prediction of cpu.
}
}

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Guest Red Ocktober

Posted

i have a hard enough time keeping track of a single iterator...

 

:)

 

--Mike

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The i++ version creates a temporary copy of i, increases i by one and returns the copy.

The ++i version just increases the value of i by one and returns it.

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