Compare column vector to column in a Matrix and extract data

I have a column vector A=1x5 (timestamps) and a Matrix B=20x5 (time + pressure readings). I want to look at B(:,1) and find where the values in A match up- then extract the data up to this point. For example

A = [5; 10; 15; 20; 25] B(:,1) = [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]

value of A(1,1) = value of B(5,1) so I want to make 3rd matrix: D(:,1) = [1; 2; 3; 4; 5] (+ pressure readings in other columns) then the value of A(2,1) = value of B(10,1) so I want 4th matrix E(:,1) = [6; 7; 8; 9; 10]

So far I know you will need two for loops... and an if/while statement maybe? Also not too sure how to store the data each time etc.

Any help is much appreciated, thanks!

Answer 1

[Stephen23]

Do not use loops for this. MATLAB is a high-level langauge, it is not C++.

It is not completely clear what you expect to get, but here is one approach that groups the data into a cell array, using histc and accumarray:

>> A = [5;10;15;20;25];
>> B = [1;2;3;4;5;6;7;8;9;10;11;12;13;14;15;16;17;18;19;20];
>> [~,idx] = histc(B,0.5+[0;A]);
>> C = accumarray(idx,B(:,1),[],@(v){v});
>> C{:}
ans =
    1
    2
    3
    4
    5
ans =
    6
    7
    8
    9
   10
ans =
   11
   12
   13
   14
   15
ans =
   16
   17
   18
   19
   20

From your description, it seems that you should probably be using a table to store your data, which would make it easy to group data and apply functions to the groups.

[Silver]

Hello Stephen !

I have a similar problem , Actually I have two kind of data :

• netcdf file (contain latitude , longitude , time , and chlorphyle as main parameter)
• xlsx file

The main parameter data (Chl) is having this format 818x839x120 double : 3D

The xlsx data form a 2D line like the photo below (longitude, latitude and chlorophyle as main parameter) :

Actually I want to extract chlorophyle data from the netcdf file while its latitude and longitude of correspond of those from xlsx file . So the idea is to extract the range of the coincidance points between long and lat from both files into a vector.

I used this code that does not work as I wish so :

 B=[]; %vector of range of similitude points
 % latitudebis :  latitude from ferrybox data file 
 % longitudebis : longitude from ferrybox data file 
 % m : latitude from satellite data file
 % n :  longitude from satellite data file 
 
 for b=1:818   % 818 is the size of the latitude and longitude
     if ((m(1,b)== latitudebis(1,b)) && (n(1,b)== longitudebis(1,b))) == 0 
         B (1,b) = b 
     end 
 end

I will appreciate your help again !

Answer 2

[KSSV]

ncfile = youfilename ;
lon = ncread(ncfile,'lon') ; nx = length(lon) ;
lat = ncread(ncfile,'lat') ; ny = length(lat) ;
t = ncread(ncfile,'time') ; nt = length(t) ;
[X,Y] = meshgrid(lon,lat) ;
% Let loni,lati be your locations from the excel for which you want the extract chlorophyll data for each time 
iwant = cell(nt,1) ;
for i = 1:nt
    chl =  ncread(ncfile,'chlorophyll',[1,1,i],[nx,ny,1]) ;
    % surf(lon,lat,chl') ; shading interp ; colorbar ; drawnow 
    chli = interp2(X,Y,chl',loni,lati) ;
    iwant{i} = chli ;
end

The above code should do what you wanted. I have assumed the netCDF variables as lon,lat, time and chlorophyll. If they vary in your file, do change them.