How to create an mxn matrix with a for-loop

Hello all,

I would like to display (x(i),y(j)) in separate columns according to the x(i)'s.

i.e.:

(0,0) (1,0) (2,0)
(0,1) (1,1) (2,1)
(0,2) (1,2) (2,2), etc.

I am currently using a nested for-loop to calculate all (x(i),y(j)) as follows:

for i = 1:3
  a = i-1;
  for j = 1:3
    b = j-1;
    X = [a b];
    disp(X);
  end
end

I get the output as follows:

       0     0
       0     1
       0     2
       1     0
       1     1
       1     2
       2     0
       2     1
       2     2

I tried using reshape to display the output as above, however in fiddling with the program, I discovered that MATLAB considers each line of output to be a separate 1x2 matrix. Do you have any suggestions as to how to get MATLAB to recognize the outputs as a 9x2 matrix? This is an extremely scaled-down version of what I need the program to do.

Thank you!!

[Stephen23]

@Dominique Brasee: why waste time and space writing nested loops? MATLAB is a high-level language, so you don't need to rely on loops to solve all of your tasks (see dpb's comment how).

There is also no point in reinventing the wheel: you can use the excellent FEX submission permn

>> permn(0:2,2)
ans =
   0   0
   0   1
   0   2
   1   0
   1   1
   1   2
   2   0
   2   1
   2   2

[dpb]

Nice find,Stephen...I should check FEX more often, meself... :)

[ABISHAI JOY]

How will you put cos (x+y) in to a matrix and it should generate random values with x, y between [0,2pi].

[dpb]

Use rand(), maybe???

Answer 1

[dpb]

As you've written it, you only have a 1x2 (row) vector at any one time, you overwrite X on each pass. There are certainly more efficient ways to generate this, but the issue with respect to building an array is simple to handle; I presume there's something much more than this going on in the real problem...

N=3;
k=0;
for i=0:N-1
  for j=0:N-1
    k=k+1;
    X(k,:) = [i j];
  end
end
disp(X)

[Dominique Brasee]

In the real program I will be computing a very large set of coordinates but I am trying to get a feel for a generic method (work asked me to do some modeling to fix some bugs in an existing program and I want to make sure I don't look like a complete idiot, since I am the intern haha).

Thank you for your help!

[dpb]

You recognize that looping isn't very Matlab-like here, I presume?

>> [reshape(repmat(0:N-1,N,1),[],1)  repmat(0:N-1,1,N).']
ans =
   0     0
   0     1
   0     2
   1     0
   1     1
   1     2
   2     0
   2     1
   2     2
>>

Answer 2

[Lam Nguyen Van]

Hi,

I want to creat matrices automatically with for loop in Matlab?

phi1=30; phi2=45; phi3=90;

After running the code I want to have a matric A with the following elements:

A=[sin(phi1) cos(phi1) sin(phi1)*cos(phi1);

sin(phi2) cos(phi2) sin(phi2)*cos(phi2);

sin(phi3) cos(phi3) sin(phi3)*cos(phi3);]

[Rik]

Your answers look like questions. Please post them as questions. Have a read here and here. It will greatly improve your chances of getting an answer.

Answer 3

[Lam Nguyen Van]

https://www.mathworks.com/matlabcentral/answers/372036-can-you-create-a-matrix-using-a-for-loop?s_tid=mlc_ans_email_view&utm_source=zalo&utm_medium=zalo&utm_campaign=zalo&zarsrc=1303

Answer 4

[Lam Nguyen Van]

clear;

phi=[30, 45, 90];

n=length(phi);

A=zeros(n,3);

for i=1:n

[d]= ham_luong_giac (phi(i));

A(i,:)=[d];

end

A

function [xuat] = ham_luong_giac (phi)

a1=sin(phi);

a2=cos(phi);

a3=sin(phi).*cos(phi);

xuat= [a1, a2, a3];

end