Assigning values to the array of Enumeration Class seems to be very slow

Question

pingping zhang 2020-1-20

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https://ww2.mathworks.cn/matlabcentral/answers/501017-assigning-values-to-the-array-of-enumeration-class-seems-to-be-very-slow

评论： pingping zhang 2020-1-20

Hi, I met this problem when parsing a gps log.

I made an Enumeration Class GpsState as follows.

classdef GpsState
    enumeration
        Lost
        Single
        RTD
        RTK_Float
        RTK_Fix
    end
end

Then I created an array of GpsState to record all the states of the log. The process was painfully slow.

Finally, I found out what the problem was caused by the assign of Enumeration Class array.

I simplified the code as follows, the process takes about 4.6s.

tic
test_num = 1e5;
gps_state = repmat(GpsState.Lost,test_num,1);
for ii = 1:test_num
    gps_state(ii) = GpsState.RTK_Fix;    
end
toc

Is there any problem with my usage mode of Enumeration Class?

Thanks for your reading.

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Answer 1

Steven Lord 2020-1-20

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https://ww2.mathworks.cn/matlabcentral/answers/501017-assigning-values-to-the-array-of-enumeration-class-seems-to-be-very-slow#answer_410948

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Why are you initializing all the elements in gps_state to GpsState.Lost in one call them immediately overwriting them with GpsState.RTK_Fix?

If you oversimplified your problem, and you wanted say) to only overwrite certain elements of gps_state, there's no need for a for loop here. Just overwrite them in one fell swoop, something like this untested code.

test_num = 1e5;
gps_state = repmat(GpsState.Lost, test_num, 1);
OneThousandRandomPoints = randperm(test_num, 1000);
gps_state(OneThousandRandomPoints) = GpsState.RTK_Fix; % scalar expansion

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pingping zhang 2020-1-20

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Hey, there is some misunderstanding,that is my issue.

In actual fact, the original code contains a process of parsing and judging gps status.

I read gps status words from the log line by line, and assign gps_state values one by one.

It's something like this.

for ii = 1:test_num
    ....
    switch gps_status_word(ii)
        case "00"
            gps_state(ii) = GpsState.Lost;  
        case "04"
            gps_state(ii) = GpsState.RTK_Fix;  
            ...
    end
end

I found the process was particularly slow, and the root cause is the the assign of Enumeration Class array.

If I replace the Enumeration of GpsState by another tye like int8, for exmaple, 'gps_state(ii) = 0' , it will be much faster.

In order to describe the problem more simply, I use the code above. It shows us the elapsed time spent when assigning values to the array of Enumeration Class directly.

Now I can temporarily use int8 instead, but still want to make sure how to use Enumeration Class effectively.

Thanks for your advice.

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