logical matrix divided by double matrix is slow

a=logical(12,90,111);
b=randi(100,[12,90,111]);
c = a ./ (b+1e-10).

This line seems a very simple operation but it is so time-consuming. I am wondering if any speed up can be done?

Answer 1

[Walter Roberson]

You are executing that line 8675040 times and it is taking 84367.53 seconds to do so, which is an average of 0.00972531884579207 seconds per iteration. Is that slow? 12*90*111 = 119880 divisions are done each time, for an average of 8.1125449164098e-08 seconds per division. Is that slow?

The code internally has to do

double(a) ./ (b+1e-10)

which is an extra operation compared to the case where a is already double. Those extra operations take time.

That said, my tests do tend to suggest that doing the double() yourself is faster, by up to a factor of 5. Try

c = double(a) ./ (b+1e-10);

[Chaoyang Jiang]

Oh, yes, you are right. the double operation speed up the code by 5 times. Thank you very much!

Answer 2

[Image Analyst]

What is the last line in your post???

When I run your code I get this:

Error using logical
Too many input arguments.
Error in test2 (line 1)
a=logical(12,90,111);

When I do this:

a=false(12,90,111);
b=randi(100,[12,90,111]);
tic;
c = a ./ (b+1e-10);
toc

I get

Elapsed time is 0.000511 seconds.

which seems pretty fast to me. What are you doing differently?

[Chaoyang Jiang]

1 For the error, the first line should be changed to a=zeros(12,90,111,'logical') to make the code run.

2 This line will run 8675040 times, which takes a longer time.

[Image Analyst]

Well change it to false, which is also a logical, and it will run so much faster.