Is it necessary to make a for loop to make matricies of ones based on a matrix of dimensions? - matlab

Question

I would like to generate a set of all 1 matrices given a matrix of sizes, (dimensions in this example), but I have been having a hard time making the dimensions matrix return a size vector that ones can work with.
My first instinct is: dimensions(:,:)
I read this (incorrectly) as return a matrix of size vectors [x,y]
But this doesn't seem to work -- Is there any way to use dimensions to produce a matrix of size vectors?
I am tempted to use a loop to iterate, for i = 1 to 3 dimensions(i,:) , but I was wondering if this is the only way.
Code:
clear;
%3x2
dimensions = [32,40; %32x40 box of ones
20, 30; %20x30 box of ones
60, 10; %60x10 box of ones
];
Onesboxes = ones(dimensions(1,:));
%this works, but I really want OnesBoxes to be an array such that:
%OnesBoxes(1) = 32x40 box of ones
%OnesBoxes(2) = 20x30 box of ones
%OnesBoxes(3) = 60x10 box of ones
% if I try:
OnesBoxes = ones(dimensions);
%Error using ones: Size vector should be a row vector with real elements.
%what I want to do is pass in sizes as rows in dimensions
%passing in the size of the ones array as a single vector works:
%onessize dimensions: 1x2
onessize = [4,2];
%tTestOnes dimensions: 4x2
tTestOnes = ones(onessize);
%making dimensions a 2x3 matrix instead doesn't seem to make a difference
%(I was thinking that maybe matlab thinks of matricies as an array of
%columns instead of arrays of rows?)
%dimensions2 = [32,20,60; 40,30,10];
%tOnesBoxes2 = ones(dimensions2);
EDIT: Outputs:
Output of
onessize = [4,2];
%tTestOnes dimensions: 4x2
tTestOnes = ones(onessize);
is a 4x2 array of all ones
Output of
dimensions = [32,40; %32x40 box of ones
20, 30; %20x30 box of ones
60, 10; %60x10 box of ones
];
Onesboxes = ones(dimensions(1,:));
Is a 32x40 array of all ones
Output of
dimensions = [32,40; %32x40 box of ones
20, 30; %20x30 box of ones
60, 10; %60x10 box of ones
];
OnesBoxes = ones(dimensions);
Is an error
Error using ones Size vector should be a row vector with real
elements.

Answer 1

If you are okay with having a for loop, the simplest way I can think of is:
dimensions = [32,40; %32x40 box of ones
20, 30; %20x30 box of ones
60, 10; %60x10 box of ones
];
for i=1:size(dimensions,1)
OnesBoxes{i}=ones(dimensions(i,:));
end
This will create OnesBoxes like how you wanted like this:
OnesBoxes{1}% = 32x40 box of ones
OnesBoxes{2}% = 20x30 box of ones
OnesBoxes{3}% = 60x10 box of ones

Answer 2

You need to pass ones a vector that defines the matrix size. If you want several matrices, you need to call ones several times. To collect all resulting matrices you need a cell array because the matrices have different sizes.
You can do that using a for loop, or arrayfun, or cellfun. Here I'm using the latter:
dimensions = [ 2 3;
4 5 ]; %// input data. Each row defines a matrix size
dimCell = mat2cell(dimensions, ones(1,size(dimensions,1)), size(dimensions,2));
%// split each row into a cell, ready to be used as input for `cellfun`
result = cellfun(#(x) ones(x), dimCell, 'uniformoutput', false);
This gives a cell array of the desired matrices. In the example,
>> result
result =
[2x3 double]
[4x5 double]
>> celldisp(result)
result{1} =
1 1 1
1 1 1
result{2} =
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1