Replace percentage of data in a for loop

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Tracy Campbell 2017-6-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/343551-replace-percentage-of-data-in-a-for-loop

评论： Walter Roberson 2017-6-14

Hello,

I'm hoping to find out how to replace a defined percentage of data? I'm currently using a for loop with an if statement and would like to add another component quantifying the percentage replaced.

This is my current code:

for i=1:53280
LULC1=data(i,21);
LCC=data(i,20);
if LULC1==12||LULC1==15
    data(i,3)=16;
end
end

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Geoff Hayes 2017-6-6

Tracy - please clarify what you mean by how to replace a defined percentage of data. In your above code, is it a certain percentage of the elements of data that will be replaced? So once you reach that percentage then you can exit the for loop?

Tracy Campbell 2017-6-6

I am hoping to replace 15%, 25%, and 50% of the data that fits the conditions listed in the if statements. Any help is appreciated!

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Answer 1

Walter Roberson 2017-6-6

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/343551-replace-percentage-of-data-in-a-for-loop#answer_269745

在 MATLAB Online 中打开

percent_to_replace = 17.3;    %for example
row_matches = find( ismember(data(:,21), [12 15]) );
num_matches = length(row_matches);
num_to_replace = round(num_matches * percent_to_replace / 100);
which_to_replace = row_matches( randperm(num_matches, num_to_replace) );
data(which_to_replace, 3) = 16;

This uses the percentage as a fixed percentage to replace, that to within round-off, exactly that portion will be replaced. There is an alternative to that, which is to treat the percentage as a probability that any given one will be replaced.

percent_to_replace = 17.3;    %for example
which_to_replace = ismember(data(:,21), [12 15]) & (rand(size(data,1),1) < percent_to_replace/100);
data(which_to_replace, 3) = 16;

This would replace the given percentage on average

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Tracy Campbell 2017-6-14

Thank you so much for the answer. I have a follow-up question. When I run the script, it provides me with the information in which rows fit the description, but does not actually replace the data. Am I missing something? Thank you.