Code optimization by way of selective computations
显示 更早的评论
I have the following code:
clc; clearvars;
Ts = 1e-1; t = 0:Ts:1-Ts
flag = mod(1:length(t),2)
s_t = exp(1i*2*pi*t)
I am looking to only execute the 
computation only when the corresponding value for 
is logical 1. I wish to do this without iterating as the final vector to process is 
long and is about 97% zero values in the result.
computation only when the corresponding value for is logical 1. I wish to do this without iterating as the final vector to process is long and is about 97% zero values in the result.采纳的回答
Jiri Hajek
2022-11-22
Hi, you need to convert your flags to logical array, initialize the results with zeros and then apply the function to the flagged positions like this:
flag = logical(mod(1:length(t),2));
s_t = zeros(size(t));
s_t(flag) = exp(1i*2*pi*t(flag));
3 个评论
This definitely works, but the computation time ends up being longer... I thought that if I avoid the 
computaitons I could reduce overall processing time.
computaitons I could reduce overall processing time.clc; clearvars;
Ts = 1e-7; t = 0:Ts:1-Ts;
tic
s_t = exp(1i*2*pi*t);
toc
tic
flag = logical(mod(1:length(t),2));
s_t = zeros(size(t));
s_t(flag) = exp(1i*2*pi*t(flag));
toc
Jiri Hajek
2022-11-22
Yes, the logical indexing probably does add to the execution time, as basic functions are quite optimized... But you mentioned that your non-zeros should be only about 3% of the array size, whereas in this test you have 50% nonzeros... So the test dis not really fair, I think.
Perhaps you can also look at sparse arrays, but truth is, their usefullness is also limited by the fraction of zero elements.
Howard Wilton
2022-11-22
更多回答（1 个）
Ts = 1e-7; t = 0:Ts:1-Ts;
tic
s_t = exp(1i*2*pi*t(1:2:end));
toc
or if your condition is more complicated:
tic
s_t = exp(1i*2*pi*t(mod(1:length(t),2)==1));
toc
类别
在 帮助中心 和 File Exchange 中查找有关 Operating on Diagonal Matrices 的更多信息
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!
Translated by 
