loop for finding a threshold and change corresponding number

Question

khan 2017-1-8

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/319471-loop-for-finding-a-threshold-and-change-corresponding-number

评论： khan 2017-1-10

hi, i have a dataset in the form [2000x730], where my variable of interest is 730. i want to use the loop and find the following equation.

aa=(data)-min(data)/max(data)+min(data);

which i already did with following code:

[m,n]=size(data);
% aa=zeros(m,n);
for i=1:m;    
    aa(i,:)=(data(i,:)-min(data(i,:)))/(max(data(i,:))-min(data(i,:)));
end

in second part since i have output file aa[2000x730]. i want to find the number where my data meets the required threshold i-e 0.76,

any idea how to loop for it,

regards and thnx in advance

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Guillaume 2017-1-9

在 MATLAB Online 中打开

Whichever version of matlab, you're using the loop in your example is totally unnecessary.

In r2016b:

aa = (data - min(data, [], 2)) ./ (max(data, [], 2) - min(data, [], 2));

Prior to R2016b:

aa = bsxfun(@rdivide, bsxfun(@minus, data, min(data, [], 2)),  max(data, [], 2) - min(data, [], 2));

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Guillaume 2017-1-9

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/319471-loop-for-finding-a-threshold-and-change-corresponding-number#answer_249855

在 MATLAB Online 中打开

As per my comment to the question, the loop you've shown is unnecessary:

aa = (data - min(data, [], 2)) ./ (max(data, [], 2) - min(data, [], 2));  %in R2016b

I'm assuming that you want to find where aa first crosses the threshold for each row. If not, see ImageAnalyst's answer.

[r, c] = find(aa >= 0.76)
crossthreshold = accumarray(r, c, [size(aa, 1), 1], @min)

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

khan 2017-1-9

Thank You sir, your codes are really helpful and saved me a lot of time, bundles of respect for all who helps from this platform.

请先登录，再进行评论。

Answer 2

Image Analyst 2017-1-8

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/319471-loop-for-finding-a-threshold-and-change-corresponding-number#answer_249788

在 MATLAB Online 中打开

Why loop? Why not do it vectorized in a single line:

atOrAboveThreshold = aa >= 0.76;

5 个评论
显示 3更早的评论隐藏 3更早的评论

Image Analyst 2017-1-9

在 MATLAB Online 中打开

The code I gave you was vectorized. atOrAboveThreshold is a logical vector of 1 or 0 depending on if the element is above or below the threshold. If you want a count of the number above, you can do

numAbove = sum(atOrAboveThreshold);

If you want to extract only the numbers above or below the threshold you can do

aaAbove = aa(atOrAboveThreshold);
aaBelow = aa(~atOrAboveThreshold);

These will have fewer numbers of elements.

If you want to mask the ones below, which will keep the same number of elements but just set the "below" elements to zero, you can do

aa(~atOrAboveThreshold) = 0; % Set below values to 0

You should try to learn about logical indexing. It is one of the most powerful features of MATLAB and the key to writing vectorized code.

khan 2017-1-10

thank you sir, your code really changed my way of thinking and writing codes for my research. i hope i can learn more from you. Respect and regards sir.

请先登录，再进行评论。