Want to insert a matrix in a for loop and change it with a variable

Hello,

So, I have a variable phi(1x41) changing with lambda(1x41) and there is matrix (2x2) inside a for loop that needs to be changed with phi but the dimension of matrix should be (2x2x41) instead of (2x42) as it shown in workspace. Can anyone help me in this?

d1 = 0.1077;
lam = 3:0.1:7;
th = 0;
n1 = 2.32;

for j = 1:length(lam)
    phi1 =2*pi.*(d1./lam).*sqrt((n1).^2 - sind(th).^2);
    P1 = [exp(1i.*phi1) 0; 0 exp(-1i.*phi1)];
end

[Dyuman Joshi]

"there is matrix (2x2) inside a for loop that needs to be changed"

There is no 2x2 matrix in the given loop. phi1 is 1x41 and P1 is 2x42.

And the for loop doesn't make sense, you are just over-writing phi1 and P1 with each iteration. No variable is varying with the loop.

[chicken vector]

d1 = 0.1077;
lam = 3:0.1:7;
th = 0;
n1 = 2.32;

phi1 = 2*pi.*(d1./lam).*sqrt((n1).^2 - sind(th).^2);

P1 = zeros(2,2,length(lam));
P1(1,1,:) = exp(1i.*phi1);
P1(2,2,:) = exp(-1i.*phi1)];

This is just one of the many ways you could build your matrix.

Other methods may involve the use of reshape.

Note that the for loop changes the values of the variable j, which your are not using so it is completely useless.

At each iteration you overwrite the variable P1 with the same values since:

x = 1:3;
M = [x 0; 0 x];

Produces:

M = [1 2 3 0 ; 0 1 2 3];

To effectively use the loop I suggest you to initialise the variable outside the loop and use j to change indexing inside P1.

P1 = zeros(2,2,length(lam));
phi1 =2*pi.*(d1./lam).*sqrt((n1).^2 - sind(th).^2);

for j = 1 : length(lam)
   P1(:,:,j) = [exp(1i.*phi1(j)) 0 ; 0 exp(1i.*phi1(j))];
end

Anyway I don't reccomend you doing this because vectorisation is much faster than looping and you would notice a great difference with large datasets.

[Ashi Khajotia]

Let us suppose lam has only one value, then P1 is a 2x2 matrix. but as I want to change lam, phi1 will change and correspondingly I want P1 to be changed with each iteration. I hope I am clear now.

Answer 1

[Dyuman Joshi]

d1 = 0.1077;
lam = 3:0.1:7;
th = 0;
n1 = 2.32;

%Define phi1 outside the loop as it is not varying with the loop
phi1 = 2*pi.*(d1./lam).*sqrt((n1).^2 - sind(th).^2);
numlam = numel(lam);
%Preallocation
P1 = zeros(2,2,numlam);

for j = 1 : numlam
   P1(:,:,j) = [exp(1i.*phi1(j)) 0; 0 exp(1i.*phi1(j))];
end

[Ashi Khajotia]

Thanks a lot for your help :D