Converting values to 0 and 1 and then counting occurrences!

Hello, I am stuck with a problem.

I have a 200x1 matrix containing values ranging from 0 to 30.

Now I want to convert the values which are less than 20 to zero and values greater than zero to 1.

After doing that I want to count how many occurrences are there when 1 is followed by zero and give the counting as an output!

[adi kul]

More information:

I am having 5 .mat files. Each mat file contains 5 parameters. A,B,C,D,E. The 5 files are named Trial1.mat, ....., Trial5.mat

I want to take B variable which is of a size 200x1 from each mat file, convert values less than 20 to 0 and greater than 20 to 1.

0 shows system is OFF 1 Shows system is ON

Now I want to check how many times 1 is followed by 0.

I want to count this occurrence for each .mat file and give an output file which will give number of count per mat file.

Here is the for loop available to put the code:

myFolder = 'C:\Users\adi\Documents\MATLAB\';
if ~isdir(myFolder)
  errorMessage = sprintf('Error: The following folder does not exist:\n%s', myFolder);
  uiwait(warndlg(errorMessage));
  return;
end
filePattern = fullfile(myFolder, '*.mat');
matFiles = dir(filePattern);
for k = 1:length(matFiles)

end

I hope now you got my question!

Answer 1

[Thorsten]

 A(A<20) = 0;
 A(A>0) = 1;
 cnt = sum(diff(A) == -1);

[adi kul]

As per my earlier question which you answered correctly,

I want to do this under that for loop. Actually the 0 and 1 will indicates "On" and "off" activity of my system. So I want to count the number of time 0 followed by 1.

So for 5 .mat files I want to get an output file which will give me this count per mat file.

Can you tell me how to do that?

[Thorsten]

You just have to replace A by B and use cnt(k) instead of cnt; so inside your for loop, after you have loaded the mat files, use:

B(B<20) = 0;
B(B>0) = 1;
cnt(k) = sum(diff(B)) == -1);

[adi kul]

and where to get output? I mean I should get a file where 1 column will be the name of mat file and 2nd column will be the count of the occurrence. Currently I am testing it on 5 files but I have more than 100 such files!

[adi kul]

and also, is it B(B>0)=1 or B(B>20)=1

[Thorsten]

The name of the matfiles is stored in your variable MatFiles. The count of occurrences is stored in cnt.

In your question you specified: Now I want to convert the values which are less than 20 to zero and values greater than zero to 1.

This is a bit strange since values between 20 and 0 are mapped to 0 or 1, depending on which rule you use first. Also, a value of 20 would be left as is and not mapped at all. It would probably make more sense to define a treshold T = 20 (or some other value) and then use

 B(B < T) = 0;
 B(B >= T) = 1;

[adi kul]

This helped but can we count only change??

I mean instead of 0 followed by 1 there are some 1 followed by zero at the end which are not being counted. So what to do to count total 0 to1 and 1 to 0 changes???

Answer 2

[Sebastian Castro]

Suppose your original data is named x. The following line will create a matrix the same size as your original, where the elements are 1 if the logical condition is met and 0 otherwise:

xThresh = x>=20

Then, you can use the diff function to get the difference between consecutive elements. You know that a transition from 1 to 0 is a difference of -1, so you can find all the -1s and add them up:

xDiff = diff(xThresh)
numTransitions = sum(xDiff==-1)

- Sebastian

[Image Analyst]

I recommend (and voted for) Sebastian's straightforward single-step way, over the other two 2-step solutions offered, though they will also work. In addition to being shorter and more direct, this thresholding method doesn't change your original matrix.

To process a bunch of files, like 5 mat files, use one of the two code snippets in the FAQ: http://matlab.wikia.com/wiki/FAQ#How_can_I_process_a_sequence_of_files.3F

[adi kul]

I tried this. But problem is, when there is not "B" column available, it still gives me some value!!

Answer 3

[Andrei Bobrov]

out = numel(strfind(A(:)' >= 20,[1, 0]));

Answer 4

[Ingrid]

is this what you mean as it is not clear to what end you would like to do this

    x(x<20) = 0;
    x(x>=20) = 1;
    temp = diff(x);
    answer = sum(temp == -1);