Creating Dynamic matrices in MATLAB

Question

I have a problem with defining some matrices in MATLAB. I get three numbers x,y,z as an input from user, then I want to create y-1 empty matrices. For example consider x = 3, y = 4, and z = 2. The required y-1 matrices M1, M2, and M3 are:

size(M1) = [3,4] ~ [x,y]

size(M2) = [4,4] ~ [y,y]

size(M3) = [4,2] ~ [y,z]

The parameters x,y are not known before running the program. If y was 5, the matrices were:

size(M1) = [3,5] ~ [x,y]

size(M2) = [5,5] ~ [y,y]

size(M3) = [5,5] ~ [y,y]

size(M4) = [5,2] ~ [y,z]

Indeed the main problem is that the number of matrices is an input. Please guide me on how I can create a function loop to define this matrices.

Empty in MATLAB would be A=[], a zero matrix (e.g. `A = zeros(3)') still has some size. — Adriaan
– Adriaan, Commented Aug 29, 2015 at 14:22
@Jubobs, zero or one, or any other initial value is not important since the values will revised in the algorithm. Problem is that the number of matrices is an input. — Afshin Oroojlooy
– Afshin Oroojlooy, Commented Aug 29, 2015 at 14:31
You'll want to do this in a cell-class for sure. bsxfun and cellfun however cannot handle zeros. I'll keep digging. — Adriaan
– Adriaan, Commented Aug 29, 2015 at 14:40
If the number of variable is an input, you have to use a cell array, which will still have the benefit of matrix style indexing, or a structure, which is convenient when you need (or prefer) to name your variables. — Hoki
– Hoki, Commented Aug 29, 2015 at 14:41

Ali Ahmadvand · Accepted Answer · 2015-08-29 14:44:58Z

4

X = input('Enter X please: '); 
Y = input('Enter Y please: '); 
Z = input('Enter Z please: '); 
Cells={}
Cells{1}=zeros(X,Y);
for i=2:Y-1
 Cells{i}=zeros(Y,Y);   
end;
Cells{Y-1}=zeros(Y,Z);

answered Aug 29, 2015 at 14:44

Ali Ahmadvand

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5 Comments

Adriaan Over a year ago

I was just about to type this up. Kudoos

Daniel Over a year ago

Probably the solution. Depending on the logic it might be simpler to use Cells{end+1}=.. so there is no need for bookkeeping of indices.

Afshin Oroojlooy Over a year ago

How I can call the value of cell [i,j] in one of the matrices?

Ali Ahmadvand Over a year ago

you can use of this term "Cells{prob}(i,j)" or use of new matric same as tmp=Cells{prob};

Afshin Oroojlooy Over a year ago

Thanks a lot for the explanation .

Stewie Griffin · Accepted Answer · 2016-07-05 08:08:41Z

You could do this without using cells, but I strongly advice you not to, so: One way to do this, with each matrix being part of a cell:

dims = str2num(input('Type in selected x,y,z: ', 's'));

M = arrayfun(@(n) zeros(dims(n), dims(2)), [1 2*ones(1,y-1) 3], 'UniformOutput', 0)

%% In the command window:
Type in selected x,y,z: 3 4 2

M = 
    [3x4 double]    [4x4 double]    [2x4 double]

Note that with the str2num(input()) approach, you can input both: [4 3 2], [4, 3, 2], 4 3 2, 4, 3, 2 or even 4;3;2. It's basically impossible to make mistakes here!

The way this works is: arrayfun performs an operation for each elements of the vector [1 2*ones(1,y-1) 3]. The operation is to create a matrix of zeros, with the desired dimensions. UniformOutput is a parameter that must be set to false, or 0 if the output is something other than scalars.

To access, and make changes to any of the matrices:

When you type M{x}, you can think of that as the equivalent of just a matrix name, i.e. it's fine to use () to index the matrix, straight after the {}.

So, you can do:

M{1}(3,3) = 2;

which would assign the value 2 to the element (3,3) in matrix 1.

Adriaan · Accepted Answer · 2015-08-29 14:25:09Z

0

M1 = zeros(x,y);
M2 = zeros(y,y);
M3 = zeors(z,y);

Simple enough. Though why M2 and M3 in your question are the same I haven't figured out yet.

answered Aug 29, 2015 at 14:25

Adriaan

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3 Comments

Afshin Oroojlooy Over a year ago

The number of matrices is an input !,

Adriaan Over a year ago

So why are M2 and M3 exactly the same? You want M1 and M(end) to be (x,y) and (z,y) and everything else as (y,y)? I still do not understand that part.

Afshin Oroojlooy Over a year ago

Yes, the the first and last matrices are (x,y) and (z,y) and the remainder are (y,y). The problem is that there are "y-3" matrices so that I cannot simply write zeros(y,y). The number of those matrices is itself an input.

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