obtaining an array containing arrays

hello, suppose i am generating an array with every loop. I want a new array to store all the values of the array together but seperately as different elements. If I have A = [1 2 3 4] in the 1st loop and A = [2 4 6 8] when it completes the second loop i want an array B that will have B= [1 2 3 4 ; 2 4 6 8]. If anyone can help me with this, I will be grateful. Thank you.

Answer 1

[James Tursa]

E.g.,

m = number of loop iterations
B = zeros(m,4);
for k=1:m
    A = results of current iteration
    B(k,:) = A;
end

[Jan]

:-) Obviously the solution is obvious. +1

[shru s]

Thank you so much for the help. If i don't know that the array will contain 4 elements beforehand, how should i go about it?

[James Tursa]

You could set things up so the first iteration expands the matrix appropriately. E.g.,

m = number of loop iterations
B = zeros(m,0); % <-- Unknown column size, so start with 0
for k=1:m
    A = results of current iteration
    B(k,1:size(A,2)) = A;  % <-- 1st iteration will set the column size
end

[shru s]

Thank you so much for your help!

Answer 2

[Jan]

nLoop  = 5;
nValue = 4;
Result = zeros(nLoop, nValue);  % Pre-allocate!
for k = 1:nLoop
  A = randi(10, 1, nValue)      % Example data
  Result(k, :) = A;
end
disp(Result)

[shru s]

thank you! but if i cannot determine nValue beforehand, how should I go about it?

[Jan]

See James' answer. If nValue i changing between the elements, use a cell array:

nLoop  = 5;
Result = cell(nLoop, 1);  % Pre-allocate!
for k = 1:nLoop
  A = randi(10, 1, randi(5))      % Example data
  Result{k} = A;
end

An "implicite pre-allocation" works also, if you create the last element at first:

nLoop = 5;
for k = nLoop:-1:1        % Backwards for implicite pre-allocation
  A = randi(10, 1, 4)      % Example data
  Result(k, :) = A;
end

It is essential, that the output does not grow iteratively, because this requires an exponentially growing number of resources. With 5 iterations this cannot be measured, but the rule is to care about a pre-allocation in general as a good programming practice.

[shru s]

thank you so much for helping me out :)