MATLAB: Matrix manipulation: update columns from stored variables

matrix manipulation

Suppose I have a set of variables (15) as column vectors, called A from 1-15. Now I would like to make a matrix of all those variables. They have 12501 rows if that helps.

I thought about creating first a zeros matrix (12501,15), and then append each column with my variables.

is there other way to do it? I read about cell2mat but I don't get it. I am truly beginner..

How could I do it in a loop for n variables?

Any help will be very appreciated! Thanks

Best Answer

‘I have a set of variables (15) as column vectors, called A from 1-15’

If they are ‘A1’, ‘A2’, ... ‘A15’, and you only need to do this once, just type it manually:

A_Matrix = [A1 A2 A3 A4 A5 ... A15];

I will let you fill in the others.

Then save it as a ‘.mat’ file so you can simply load it when you need it.

Related Solutions

MATLAB: Matrix manipulation: Changing Diagonal values

>> S = randi(9,3,5)
S =
     4     3     9     4     4
     6     4     2     2     9
     6     1     1     5     9
>> idx = 1==eye(size(S));
>> new = -S;
>> new(idx) = sum(new,2) + 2*S(idx)
new =
   -16    -3    -9    -4    -4
    -6   -15    -2    -2    -9
    -6    -1   -20    -5    -9

And checking:

 NewMatrix(2,2) = S(2,2)-S(2,1)-S(2,3),...S(2,n)
>> S(2,2)-S(2,1)-S(2,3)-S(2,4)-S(2,5)
ans =
   -15
>> new(2,2)
ans =
   -15

MATLAB: How to compute cross terms

Easy enough to do it with a loop. All that you need are the exponents. If you really want the symbolic forms later, trivial for that too.

function XT = xterms(N,nv)
% compute all cross terms of order N for nv variables.
XT = zeros(1,nv);
for n = 1:N
  XTnew = zeros(0,nv);
  for m = 1:nv
    XTm = XT;
    XTm(:,m) = XTm(:,m) + 1;
    XTnew = [XTnew;XTm];
  end
  XT = unique(XTnew,'rows');
end

So:

xt = xterms(2,3)
xt =
     0     0     2
     0     1     1
     0     2     0
     1     0     1
     1     1     0
     2     0     0

If you want a symbolic form for each term, (it is a REALLY bad idea to create numbered variables, so I do not advise it.) But this is trivial too.

syms a1 a2 a3
prod(repmat([a1 a2 a3],size(xt,1),1).^xt,2)
ans =
  a3^2
 a2*a3
  a2^2
 a1*a3
 a1*a2
  a1^2

As I said, the exponent array is all that you really need for any computations.

Best Answer

Related Solutions

MATLAB: Matrix manipulation: Changing Diagonal values

MATLAB: How to compute cross terms

Related Question