Iterating performance question

Programming Questions

sn333p

Iterating performance question

in Programming Questions • 3 years ago

I have a question about looping through an array, in Processing but also programming in general.
Lets say I have 2 arrays with large amount of data. I need to loop through them, something like this:

for (int i = 0; i < myArray1.length; i++) {
for (int j = 0; j < myArray2.length; j++) {
...
}
}

If I'm correct int i & j are recreated every time this code runs, and the test expression (array length) is collected every iteration. So instead, would this be faster?

int i;
int j;
int length1 = myArray1.length;
int length2 = myArray2.length;
for (i = 0; i < length1; i++) {
for (j = 0; j < length2; j++) {
...
}
}

Thanks in advance.

Replies(4)

bjordal

Re: Iterating performance question

3 years ago

in this case, no. first of i & j are not recreated (that wouldn't make any sense). Secondly myArray1.length is a variable and it should (unless i'm mistaken) take just as long time to access as any other variable. However if the code was something like this:

for(int i = 0; i<mySomething.someComplexMethodThatTakesAMinuteToReturnAValue();i++){

where someComplexMethodThatTakesAMinuteToReturnAValue() is a complex method that takes a minute to return a value,
it would be alot faster to do this (like your example):

int holder = mySomething.someComplexMethodThatTakesAMinuteToReturnAValue();
for(int i = 0; i<holder;i++){

sn333p

Re: Iterating performance question

3 years ago

Thanks, that makes sense.

What I meant with i & j being recreated is they are reinitialized by resetting int j = 0; every iteration (maybe I'm wrong).
The way I saw it is that it's reusing the same space/place in the memory both ways, but instead of "recreating" it changes its value. I'm working with a large dataset (100.000+ record) and I need every performance boost I can get ;)

If I want to test the performance for these approaches, what the best approach to print the time it took to complete?
I'm familiar with the millis(); function, that's all I can think of.

Jo Wood

Re: Iterating performance question

3 years ago

In the case of loop variables, in your example i only gets declared and initialised once as Bjordal says. j gets declared and initialised myArray1.length times, but there really is no appreciable overhead in doing this for simple variables. Much better not to worry about these things and instead concentrate on clarity and algorithm efficiency. See also this thread on local and global variables .

If you wish to monitor frame rates while developing your sketches, you may wish to have a look at the giCentre Utilities library which has a FrameTimer class for keeping timing code simple (see the FrameTimer API documentation for details). Here is a simple example:

import org.gicentre.utils.FrameTimer;

FrameTimer timer;

void setup()
{
  size(400,400);
  timer = new FrameTimer();    
}

void draw()
{
  background(255);
  
  // Add your drawing code here that you wish to time.
  
  // This will display the frame rate of your sketch.
  timer.displayFrameRate();
}

sn333p

Re: Iterating performance question

3 years ago

Cheers, I'll give it a try.

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