Extracting matrix from mat file

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Rock Interpreter 2020-8-11

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评论： hosein Javan 2020-8-12

I am working with 3 matrices of size 997*2160, which has been extracted from available mat file; First two matrices are x and y coordinates and last one data matrix:A. Now I would like to get the data matrix A corresponding to some specific part of x & y matrices. I have checked the range of x matrix that I require and it is (349:589,:), varying vertically and y matrix as (:,1381:1681), varying horizontally. I would require the data matrix A corresponding to these stated range.

I tried this:

x=xmat(349:589,:); % this generated 241*2160 
y=ymat(:,1381:1681); % this generated 997*301
A=Amat(349:589,1381:1681); %this generated 241*301

Am I doing in right way? Please suggest if this is not correct. Your help will be very much appreciated.

Best regards,

Sumit G

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maiaL 2020-8-11

If you tried that, what did you get as a result/error? At a first glance, it looks plausible to me.

Rock Interpreter 2020-8-12

@Leonardo Maia

I didn't receive any error, just want to ensure whether doing correctly or not?

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Answer 1

hosein Javan 2020-8-11

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在 MATLAB Online 中打开

I assume you have an "x" and a "y" rectangular grid point coordinates, and "A" contains "f(x,y)" data. in this case "x" is a row-vector repeated and "y" is a column vector repeated. now you're trying to extract coordinates ranging in these intervals: "349<x<589" and "1381<y<1681". if that is the case, you're code should look like:

idx_x = 349<x & x<589; % index of extracted x values
idx_y = 1381<y & y<1681; % index of extracted x values
x_ext = x(idx_x); % extracted x values
y_ext = x(idx_y); % extracted y values
A_ext = A(idx_x & idx_y) % extracted A values

what you were doing was to unnoticably replace data range by matrix index while they have to be calculated first as was shown.

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hosein Javan 2020-8-12

编辑：hosein Javan 2020-8-12

在 MATLAB Online 中打开

yes, I have set an example in which X varys vertically and Y varies horizontally like yours. this example shows what you're looking for. in here we are extracting "1<x & x<5" and "2<y & y<6".

the only difference you have to make is that vectors "x" and "y" that here have been defined in first line of the code, you must create yourself by extracting one column and one row of Xmat and Ymat respectively.

x = 0:6
y = 0:7
[Y,X] = meshgrid(y,x)
A = X.^2 + Y.^2
idx_x = find(1<x & x<5) % linear index of x values
idx_y = find(2<y & y<6) % linear index of y values
A_ext = A(idx_x,idx_y) % extracted A corresponding to 1<x<5 and 2<y<6

results:

Y =
   1     2     3     4     5     6     7
   1     2     3     4     5     6     7
   1     2     3     4     5     6     7
   1     2     3     4     5     6     7
   1     2     3     4     5     6     7
   1     2     3     4     5     6     7
   1     2     3     4     5     6     7
X =
   0     0     0     0     0     0     0
   1     1     1     1     1     1     1
   2     2     2     2     2     2     2
   3     3     3     3     3     3     3
   4     4     4     4     4     4     4
   5     5     5     5     5     5     5
   6     6     6     6     6     6     6
A =
   1     4     9    16    25    36    49
   2     5    10    17    26    37    50
   5     8    13    20    29    40    53
  10    13    18    25    34    45    58
  17    20    25    32    41    52    65
  26    29    34    41    50    61    74
  37    40    45    52    61    72    85
idx_x =
   4     5
idx_y =
   5     6
A_ext =
  20    29
  25    34
  32    41

you should be able to do the same for your matrix.

I think in your case it will be:

x = Xmat(:,1)
y = Ymat(1,:)
idx_x = find(349<x & x<589) % linear index of x values
idx_y = find(1381<y & y<1681) % linear index of y values
A_ext = A(idx_x,idx_y) % extracted A corresponding to 1<x<5 and 2<y<6

Rock Interpreter 2020-8-12

Hi Javan,

Yes, this is it. Thanks a ton for your support; appreciated it.

Best regards.

hosein Javan 2020-8-12

your welcome. best wishes.

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