MATLAB: How to create an indexing program that removes numbers between values

indexing

I have a large data set of numbers that I would like to import so I need help writing a program that can do the following.
1. Find the first negative number within the set. 2. Once first negative number is found, note the positive number that came right before the first negative number found, and remove all data entries until that positive number is found again.
Some examples are…
test1 = [9,-7,7,1,10,-9,9] would print newtest1 = [9 9]
or test2 = [1, 2, 4, -2, 7, 8, 4] would print newtest2 = [1, 2, 4, 4]

Best Answer

Hello Andrew,
You can accomplish your task by using logical indexing and successively calling "ismember" and "find" functions. The documentation for these functions can be found here:
https://www.mathworks.com/help/matlab/ref/ismember.html
https://www.mathworks.com/help/matlab/ref/find.html
The documentation for logical indexing can be found here:
https://www.mathworks.com/help/matlab/math/matrix-indexing.html
The idea is to find the index of the first negative number in the array "test". From that, you can determine the index of the previous (positive number), ind1. You can then use "ismember" and "find" functions to determine the index, ind2, of the next occurrence of the positive number. After that, you can concatenate the two arrays "test(1:ind1)" and "test(ind2:end)". You can then repeat this process on the new array until all negative numbers are weeded out.
The code that implements this can be found below:
close all
clear all
clc
testOriginal = [9,-7,7,1,10,-9,9,-3,7,1,10,-9,9];
test = testOriginal;
if test(1)<0
    % If the first element is a negative number, return an empty array.
    test = [];
else
    % Find all the negative numbers.
    negativeNumber = test(test<0);
      % The while loop will be entered only if negative numbers are present.
      while ~isempty(negativeNumber)
          % Find the index of the first negative number in the updated array
          % test.
          indNegativeNumber = find(ismember(test, negativeNumber(1)), 1);
          % Determine the positive number that appears before the first
          % occurrence of the negative number.
          previousPositiveNumber = test(indNegativeNumber - 1);
          % Determine the index of the next occurrence of that positive
          % number in test(indNegativeNumber:end).
          indNextPositiveNumber = find(ismember(test(indNegativeNumber:end), previousPositiveNumber), 1);
          if ~isempty(indNextPositiveNumber)
              % Determine the index of this positive number in test.
              indNextPositiveNumber = indNextPositiveNumber + indNegativeNumber - 1;
              % Concatenate the matrices.
              test = [test(1:indNegativeNumber - 1) test(indNextPositiveNumber:end)];
          else
              % Case when there is no next occurrence of the positive number.
              test = test(1:indNegativeNumber - 1);
          end
          % Find all the negative numbers in the updated array test.
          negativeNumber = test(test<0);
      end
  end
testOriginal
test
Executing the above code results in the following output:
testOriginal =
       9    -7     7     1    10    -9     9    -3     7     1    10    -9     9
test =
       9     9     9
Harish
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