How to use logical indexing to return elements of a signal?

Hi all,

I have a signal like this (attached). My ultimate aim is to return elements of the signal between parts of the signal that are empty and put them in seperate cells as depicted on the image on the right. My approach and a problem is below. Would you mind helping?

load 'signal'

% Replace nan values with 0

force_y_r(isnan(force_y_r))=0;

% find peaks of the both sides of the flat section of the signal

idx_first = islocalmin(force_y_r, 'FlatSelection','first', 'MinSeparation',500, 'MinProminence',750);
idx_last = islocalmin(force_y_r, 'FlatSelection','last', 'MinSeparation',500, 'MinProminence',750);

% THE PROBLEM: find column numerical values in force_y_r that corresponds to idx_first (green) & % idx_last (red)

[row_idx_first,col_idx_first] = find(size(force_y_r),idx_first);
[row_idx_last,col_idx_last] = find(size(force_y_r),idx_last);

this gives me:
Error using find
Second argument must be a positive scalar integer.

% extract parts of the signal between col_idx_first(1) and col_idx_last(1)% col_idx_first(2) and col_idx_last(2) 
% and so on

%if col_idx_last occured before col_idx_first

col_idx_last(1) = []

n_start = numel(col_idx_first)-1;

force_y_r_cycle = cell(n_start,1) ;

for i = 1:n_start
    force_y_r_cycle{i} = force_y_l(col_idx_first(i):col_idx_last(i+1)) ;
end

Answer 1

[Star Strider]

Try this —

StartIndices = find(Lv2);

StartIndices = StartIndices(1:end-1);

EndIndices = find(Lv1);

EndIndices = EndIndices(2:end);

for k = 1:numel(StartIndices)

Force{k,:} = force_y_r(StartIndices(k):EndIndices(k));

end

Force

Force1 = Force{1}(1:10) % First 10 Rows

SegmentIndices = table(StartIndices, EndIndices)

producing:

Force =

8×1 cell array

{367×1 double}

{402×1 double}

{402×1 double}

{392×1 double}

{379×1 double}

{379×1 double}

{397×1 double}

{370×1 double}

They must be stored in cell arrays, since they are not all the same sizes. It would be possible to store them in a matrix, however the unused matrix elements for the shorter vectors would be NaN or 0 or whatever the matrix was preallocated to be.

SegmentIndices =

7×2 table

StartIndices EndIndices

____________ __________

1649 2482

2883 3628

4029 4706

5097 5852

6230 6946

7324 8036

8432 9149

To retrieve the contents of the cell arrays:

Force1 = Force{1}

Force1 =

0.0000

31.4198

45.7187

69.0982

82.0834

95.3331

103.3904

114.8121

129.8821

147.0273

.

Answer 2

[dpb]

>> whos -file signal
  Name              Size            Bytes  Class     Attributes

  force_y_r      9607x1             76856  double              

>> 

Your signal is just a vector, not a 2D array so all you need is one-element addressing -- the column is always 1. And, since islocalmin returns a logical array, everything is zero except the true locations and so

idx1=find(idx_first);
idx2=find(idx_last);

is the set of indices you're looking for. I'd not bother with the second variable here, probably, but just overwrite the two you already have. Or, you could write

idx_first=find(islocalmin(force_y_r, 'FlatSelection','first', 'MinSeparation',500, 'MinProminence',750));

from the start.