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I have a fairly large vector named blender. I have extracted n elements for which blender is greater than x (irrelevant). Now my difficulty is the following:

I am trying to create a (21 x n) matrix with each element of blender plus 10 things before, and the 10 things after. 

element=find(blender >= 120);

I have been trying variations of the following:

for i=element(1:end)
    Matrix(i)= Matrix(blender(i-10:i+10));
end

then I want to plot one column of the matrix at the time when I hit Enter. This second part I can figure out later, but I would appreciate some help making the Matrix

Thanks

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2 Answers 2

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First, you can use "logical indexing" of your array, which uses a logical expression do address your vector. With blender = [2, 302, 35, 199, 781, 312, 8], it could look like this:

>> b_hi = blender(blender>=120)
b_hi =
       302  199  781  312

Second, you can concatenate arrays like in b_padded = [1, 2, b_hi, 3, 4]. If b_hi was a column vector, you'd use semicolons instead of commas.

Third, there is a function reshape that allows you to turn the resulting vector into a matrix. doc reshape will tell you details. For example, to turn b_padded into a 2-by-4 matrix,

>> b_matrix = reshape(b_padded, 4, 2)
b_matrix =
           1   302   781     3
           2   199   312     4

will do. This means you can do all of the job without any for-loop. Note that transposing the result of reshape(b_padded, 2, 4) will give you the other possible 2-by-4 matrix. You obtain the transpose of a matrix A by A'. You will find out which one you want.

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6 Comments

+1: obviously the OP is experiencing difficulties with vectorization. I suggest laying out the final solution for clarity.
I see, but how do I get the 10 before and after. Say bleder is 1:1:100. And I extracted 60, also element 60. I want 50:70 as one column, then the 10 before and after of the second extracted element
I apologize I did not understand your question right. @shoelzer's approach is correct. If your index bounds are valid downwards, another problem to consider is the upper bound. See comment below.
I don't understand how indexing is even a problem since k is an element number (integer and positive from 608 to 22,543), blender is my raw data, 0 from up to 153.7444
@shoelzer's point is that blender may be less than 22,543 + 10 = 22,553 long. What is length(blender)?
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You need to create a new matrix, and use two indices so that Matlab knows it is assigning to a column in a 2D matrix.

NewMatrix = zeros(21, length(element));
for i = 1:length(element)
    k = element(i);
    NewMatrix(:,i)= Matrix(blender(k-10:k+10));
end
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7 Comments

I understand this answer the most. When I try to run it I get "Subscript indices must either be real positive integers or logicals"
I'm guessing that k-10 ends up being negative so it's not a valid index. You need to decide what should happen in that case.
No, the smallest k I have is 608, I even tried with blender(k:k+10), same error.
Then maybe the values in blender are not valid indices for Matrix.
that could be it, since blender does not have integers
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