MATLAB: Grabbing sections of a matrix by using two doubles as the index

gaindexinglogical indexing

Hello everyone,

I am trying to access sections of an array such as:

data = [10 10.5 11.5 13 15 16 20.5 24];

by using the following two arrays:

indStart = [1 0 0 0 1 0 0 0]; indEnd = [0 0 1 0 0 0 0 1];

The final output should look something like this:

result = { [10 1.5 11.5] }

{ [15 16 20.5 24] }

This is what i have tried, along with little variations of it:

result{ : } = data(indStart : indEnd);

The results of my method is either an error, or i can access 1:3, but not get 5:8.

I know i can do this quite easily with a for loop, but i am trying to accomplish this using indexing and logical statements so that processing time stays down. The final script needs to be processed by ga(), so any cut down on processing time is very valuable.

Thank you to anyone who can help!

Best Answer

E.g.,

result = arrayfun(@(x1,x2)data(x1:x2),find(indStart),find(indEnd),'uni',false);

This assumes of course that the indStart and indEnd are consistent and have the appropriate number of 1's etc.

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MATLAB: How to separate complex data from a .txt file

Hi!

It seems all your 6 fields in a row are separated with ",", right? The easiest is to read in the whole file and process each line. I used the sample line (all in one line!)

str = '"2013-11-13 08:12:33",1239169,3.9675,28.4041,"1.115000;1.117000;1.116000","1.118000;1.117000;1.117000"';

The code is

% positions of ','
ind = strfind(str, ',');
% start of each column
indstart = [1 ind+1];
% end of each column
indend = [ind-1 length(str)];
% get all columns
DateString = str(indstart(1):indend(1));
idString = str(indstart(2):indend(2));
temperatureString = str(indstart(3):indend(3));
irradianceString = str(indstart(4):indend(4));
currentString = str(indstart(5):indend(5));
voltageString = str(indstart(6):indend(6));
% convert current and voltage to numeric array
currentString = regexprep(currentString, '"', '');
currentArray = str2num(currentString);
voltageString = regexprep(voltageString, '"', '');
voltageArray = str2num(voltageString);

This can of course be done for multiple rows in the file and in vectorised form. It is just to get you started.

MATLAB: Slow Triple for loop

The very first thing is that you should not modify the loop index LatCounter within the loop. ( EDIT: the following sentence is wrong! see Iain's comment) What you are doing with the -1 will create an infinite loop blocked on LatCounter == length(handles.Latitude).

Then you can probably get rid of the two outer loops by creating a meshgrid of longitudes and latitudes. Example:

 lat = 1:2:7
 lon = 10:2:14
 [LAT, LON]  = meshgrid(lat, lon)
 normCoordSq = LAT.^2 + LON.^2
 cumResult = zeros(size(LAT)) ;
 data = [2, 7, 5, 9] ;
 for dataId = 1 : length(data)
    mask = sqrt(normCoordSq + cumResult + data(dataId)^2) < 16 ;
    cumResult(mask) = cumResult(mask) + data(dataId)^2 ;
 end
 cumResult

This leads to the following output:

 lat =
     1     3     5     7
 lon =
    10    12    14
 LAT =
     1     3     5     7
     1     3     5     7
     1     3     5     7
 LON =
    10    10    10    10
    12    12    12    12
    14    14    14    14
 normCoordSq =
   101   109   125   149
   145   153   169   193
   197   205   221   245
 cumResult =
    78    78    78    78
    78    78    78    53
    53    29    29     4

As you can see, there is only one loop, and the conditional statement is replaced by logical indexing based on the outcome of a relational operation. Then you could probably get rid of the last loop using ARRAYFUN, CELLFUN, or BSXFUN, but I doubt that it would bring much improvement (in the sense that these are essentially hidden loops which bring more conciseness than efficiency in term of computation time).

Best Answer

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