# logical matrix divided by double matrix is slow

3 ビュー (過去 30 日間)
Chaoyang Jiang 2018 年 5 月 21 日
コメント済み: Image Analyst 2018 年 5 月 21 日
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?

サインインしてコメントする。

### 採用された回答

Walter Roberson 2018 年 5 月 21 日
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);
##### 1 件のコメント表示非表示 なし
Chaoyang Jiang 2018 年 5 月 21 日
Oh, yes, you are right. the double operation speed up the code by 5 times. Thank you very much!

サインインしてコメントする。

### その他の回答 (1 件)

Image Analyst 2018 年 5 月 21 日
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?
##### 2 件のコメント表示非表示 1 件の古いコメント
Image Analyst 2018 年 5 月 21 日
Well change it to false, which is also a logical, and it will run so much faster.

サインインしてコメントする。

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by