This is from a session I did with the UBC R Study Group. Loops can be convenient for applying the same steps to big/distributed datasets, running simulations, and writing your own resampling/bootstrapping analyses. Here are some ways to make them faster.
1. Don’t grow things in your loops.
2. Vectorize where possible. i.e. pull things out of the loop.
3. Do less work in the loop if you can.
Frame-blending is a great way to illustrate animal behaviour and other things that change over time. This got me thinking about ways to animate time series data. In R, the animation package has lots of options, but you can also build your own just by plotting over the same device window. If you save each iteration in a loop, the resulting images can be used as frames in a video or gif.
Click the image to see a larger version
Here is an example using recordings that track hummingbirds flying in our tunnel here at UBC. This animation shows a bird’s eye view of 50 flights by 10 birds. In half of the flights (the red ones), the birds had horizontal stripes on their left side and vertical stripes on their right, and the other half (blue) had the reverse. The subtle difference between the red and blue trajectories (red ones tend to have more positive y values) shows that on average, birds tend to deviate away from vertical stripes, and towards horizontal ones. The histogram that builds up on the right side of the figure shows the mean lateral (y) position for each trajectory as it finishe
My learning curve with the statistical software R has been a long one, but one of the steepest and most exciting times was learning how to write functions and loops. Suddenly I could do all kinds of things that used to seem impossible. Since then, I’ve learned to avoid for loops whenever possible. Why? Because doing things serially is slow. With R, you can almost always reduce a big loop to just few lines of vectorized code.
But there’s one situation where I can’t avoid the dreaded for loop. Recently, I learned how to make for loops run 100s of times faster in these situations.