MATLAB: How to calculate mean of submatrices in a large matrix and print out a new matrix

meansubmatrix

Hi,

I have a matrix 4×4

M =[

11 12 13 14

21 22 23 24

31 32 33 34

41 42 43 44]

I want to calculate the mean of submatrix 2×2

M1 = [11 12

21 22]

M2 = [31 32

41 42]

…

The final result is a new matrix 2×2 containing the mean value computed from the submatrices

Thanks in advance.

Best Answer

>> M = [11,12,13,14;21,22,23,24;31,32,33,34;41,42,43,44;51,52,53,54;61,62,63,64]
M =
   11   12   13   14
   21   22   23   24
   31   32   33   34
   41   42   43   44
   51   52   53   54
   61   62   63   64

Using convolution is simple and relatively efficient:

>> A = conv2(M,ones(2,2),'valid')/4;
>> A = A(1:2:end,1:2:end)
A =
   16.500   18.500
   36.500   38.500
   56.500   58.500

Or you could rearrange the matrix before calling mean:

>> S = size(M)/2;
>> A = mean(reshape(permute(reshape(M,2,S(1),2,S(2)),[2,4,1,3]),S(1),S(2),[]),3)
A =
   16.500   18.500
   36.500   38.500
   56.500   58.500

Or split the matrix into submatrices:

>> A = cellfun(@(m)mean(m(:)),mat2cell(M,[2,2,2],[2,2]))
A =
   16.500   18.500
   36.500   38.500
   56.500   58.500

Or use blockproc (requires the image toolbox).

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I suggest http://www.mathworks.com/help/matlab/ref/spline.html

x = 1 : length(psdn);
x128 = linspace(x(1), x(end), 128);
p128 = spline(x, psdn, x128);
plot(x128, p128);

MATLAB: Conversion of 3d array to 2d array in text file

textscan returns empty arrays is because it could not match the provided format string with the actual data in the file. It cannot match the file data because you have changed the file format since Cedric Wannaz wrote that code. When you change the file, then the same formatting command will not work any more.

Cedric Wannaz defined a format string like this:

fSpec = ['Temperature_TqJ_A[%f][%f]', repmat( ', %f', 1, 20 )]

but the first line of your file looks like this:

Longitude, 60.5, 61.5, 62.5, 63.5, 64.5, 65.5, 66.5, 67.5, 68.5, 69.5, 70.5, 71.5, 72.5, 73.5, 74.5, 75.5, 76.5, 77.5, 78.5

This format string does not match the very first line of the file (the string literal is different, there are different number of numeric terms), so it stops looking and returns some empty arrays.

This correctly reads your sample data file:

% Read the data file:
fid = fopen('temp.txt','rt');
Longitude = cell2mat(textscan(fid,['%*s',repmat('%f',1,19)],1,'Delimiter',','));
Latitude  = cell2mat(textscan(fid,['%*s',repmat('%f',1,17)],1,'Delimiter',','));
StdPress  = cell2mat(textscan(fid,['%*s',repmat('%f',1,24)],1,'Delimiter',','));
C = textscan(fid,['%s',repmat('%f',1,19)],'Delimiter',',','CollectOutput',true);
fclose(fid);
% Parse indices of Pressure and Latitude into numeric (not used):
idx = cellfun(@(s)sscanf(s,'Temperature_TqJ_A[%f][%f]'),C{1},'UniformOutput',false);
idx = [idx{:}];
% Calculate first two columns:
[lat,lon] = meshgrid(Latitude,Longitude);
% Write output files:
for k = 0:23
    idy = k==idx(1,:);
    %mat = [lat(:),lon(:),reshape(C{2}(idy,:).',[],1)].'; % unsorted
    mat = sortrows([lat(:),lon(:),reshape(C{2}(idy,:).',[],1)]).'; % sorted
    fid = fopen(sprintf('Output_%02d.txt',k),'wt');
    fprintf(fid,'latitude\tlongitude\tGP_value\n');
    fprintf(fid,'%.3f\t%.3f\t%.3f\n',mat);
    fclose(fid);
end

This code works for your sample data file, here:

Best Answer

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