Extract values from a matrix using a for loop

I am trying to get a 3x20 matrix populated with the eigenvalues from the eig function. The first values are populated in the matrix, that is, column one has three rows of numbers but the other 19 columns are not populating.

The error reads

Index in position 1 exceeds array bounds (must not exceed 3).

my code:

A = [3x60 double]
Eig=zeros(3,20)

for i=1:20
  A1=A(i:i+2,i:i+2)
  Eig(:,i)=eig(A1) 
end

Any help would be greatly appreciated. Thanks.

[noor alkhudher]

how to do function in for loop for matrix

[dpb]

A = [3x60 double]
...
for i=1:20
  A1=A(i:i+2,i:i+2)

What is value of i:i+2 when i >=1?

Don't know what the intent here is, but the reason for the error is clear; you cannot address A(2:4,2:4) when size(A)==>3,60.

Answer 1

[dpb]

for i=1:20
  A1=A(i:i+2,i:i+2);
  ...

The first indices into A are the same as the second expression.

i  i:i+2  A subscripts
1   1:3    A(1:3,1:3)
2   2:4    A(2:4,2:4)
3   3:5    A(3:5,3:5)
...

Probably what you intended to write was

for i=1:20
  A1=A(:,i:i+2);
  ...

to pick up all three rows for the sequential series of columns...

[David]

Thanks. This works well.

A1=A(:,i:i+2);

However, I didn’t explain it well. I would like to analyze the first 3x3 group and then the second 3x3 group which starts at the fourth column and so on. Currently, the code you provided is using the first 3x3 group and then going to the second column and using that 3x3 group.

[dpb]

Excuse me? That is the code logic YOU provided, not me! :)

What would the algebraic expression be that would increment the index by 3? Or, alternatively, use the optional increment value to iterate by a non-default value.

Alternatively yet, if you have the Image Processing Toolbox, it contains the function blockproc which applies a function to a defined area of an array and puts the result into an output array...