MATLAB: How to compare a group of values in a data set to each other using vectorization

vectorization

Hi!

I have two very large vectors: X and T, where one is measured power and the other one is time of each measurement. First I want to apply conditions to X, namely finding drops in power. I then want to apply constraints to these drops must be longer than t time. The time between each data point is equal.

The first part could be done like this

diff_X = diff(X);
drops_X = diff_X < x; % drops_X now contains all the drops in power
% Now I want to filter out all the drops that are shorter in duration than t, and it must be vectorized.

Thanks

Best Answer

Basically you want to find the duration of each run of power drop. As you've already established finding the power drops is a simple diff:

ispowerdrop = diff(X) < x;

which produces a logical vector. In that logical vector a sequence of [0 1] will indicate the start of a power drop, and a sequence of [1 0] will indicate the end. Other possible sequences are [0 0] (out of a power drop) or [1 1] (in a power drop). The diff of these sequences is 1, -1, 0 and 0 respectively, so finding the location of 1 and -1 in the sequence is all you need to find the start and end of the drops.

transitions = diff([false, ispowerdrop, false]);  %replace , by ; if X is a column vector
startdrop = find(transitions == 1) + 1;
enddrop = find(transitions == -1);

I surrounded ispowerdrop by false to make sure you detect the start and end of power drop if they are at the beginning/end of the vector.

The duration of each power drop is then:

powerdropduration = T(enddrop) - T(startdrop);

Related Solutions

MATLAB: Any suggestion for the method I should use for calculating the sum of the event which has zero values and its duration with matlab

Looks like you use excel macros, not VBA. In any case, no you should not do the same method at all in matlab. Matlab is a lot more powerful but requires a different way of thinking.

Giving us a file we can play with would have been more useful than screenshots. Anyway, first thing you need to do is import your data in matlab. For that use readtable with appropriate options (possibly none). Something like:

flowtable = readtable('c:\somewhere\somefile.xlsx');

If necessary rename the columns of the table to something useful. In the following I assume the flow column is called Flow. You can then detect dry periods, their beginnings and ends:

dryperiod = flowtable.Flow == 0;
transitions = diff([false, dryperiod, false]); %is 1 at the start of a dry period, -1 after the end
drystarts = find(transitions == 1);
dryends = find(transitions ==  -1) - 1;

You can then build your result table. I'm assuming that your flowtable has a single column named DateTime of type datetime

drytable = table(flowtable.DateTime(drystarts), ...
                 flowtable.DateTime(dryends), ...
                 flowtable.DateTime(dryends) - flowtable.DateTime(drystarts), ...
                 'VariableNames', {'Start', 'End', 'Duration'})

MATLAB: Bachelor Thesis Help! Dealing with NaN when calculating absolute difference

I think this will give you the idea. You can fix it up to suit your needs.

First of all, use isnan(y) to determine whether y is NaN. Even if y = NaN, y == NaN is false (don't blame me, I didn't make the rules).

Assume you have your data in the vector x.

diff_x = abs(diff(x)); % differences data and takes abs, but you will be left with NaN values
diff_x = diff_x(~isnan(diff_x)); % removes NaN values

Example:

>> x = [1 2 NaN 3 4 NaN NaN 5 NaN 4 2 NaN 7 2 NaN 2 NaN NaN NaN 5]
x =
     1     2   NaN     3     4   NaN   NaN     5   NaN     4     2   NaN     7     2   NaN     2   NaN   NaN   NaN     5
>> diff_x = diff(x)
diff_x =
     1   NaN   NaN     1   NaN   NaN   NaN   NaN   NaN    2   NaN   NaN    5   NaN   NaN   NaN   NaN   NaN   NaN
>> diff_x = diff_x(~isnan(diff_x))
diff_x =
     1     1    2    5

Best Answer

Related Solutions

MATLAB: Any suggestion for the method I should use for calculating the sum of the event which has zero values and its duration with matlab

MATLAB: Bachelor Thesis Help! Dealing with NaN when calculating absolute difference

Related Question