Multiple Outputs of a function into a single vector

22 views (last 30 days)
I am a new Matlab programmer, and am familiar with functional languages such as python. In my university Matlab course, we are required to write test functions similar to this one:
function [x,y] = f(a, b)
x = a;
y = b;
This function is called with a number of different parameters, and I'd like to store them in one large array. Previous answers I've found lead me to this solution:
returns = zeros(4, 2);
[returns(0,0), returns(0,1)] = f(a0, b0);
[returns(1,0), returns(1,1)] = f(a1, b1);
[returns(2,0), returns(2,1)] = f(a2, b2);
[returns(3,0), returns(3,1)] = f(a3, b3);
but when "returns" has more than two outputs, this gets ugly very fast. The only other solution I've found instead breaks this into two lines:
returns = zeros(4, 2);
[A, B] = f(a0, b0);
returns(0, :) = [A, B];
[A, B] = f(a1, b1);
returns(1, :) = [A, B];
[A, B] = f(a2, b2);
returns(2, :) = [A, B];
[A, B] = f(a3, b3);
returns(3, :) = [A, B];
But if I merge those two lines, it broadcasts A into every element of the vector. In Python, I could re-cast the output as a numpy array with almost identical syntax. Is there a similar function in Matlab? Every output has the same data type. I'm aware this can be solved with for loops, but I'm interested if a functional solution exists.

Accepted Answer

Walter Roberson
Walter Roberson on 20 Sep 2022
There is no functional solution in MATLAB. The only way to capture multiple outputs is an assignement statement. The assignment can be to an expansion such as
[returns{1:5}] = f(a3, b3)
but you cannot gather the outputs "in-line" like
min({f{a3,b3)}) %will not work to capture multiple outputs
  1 Comment
Joseph on 20 Sep 2022
This works! Its unfortunate I can't use full functional syntax, but this solves my immediate problem in a very clean way.
returns = cell(4, 2);
[returns{1, 1:2}] = f(a1, b1)
[returns{2, 1:2}] = f(a2, b2)
[returns{3, 1:2}] = f(a3, b3)
[returns{4, 1:2}] = f(a4, b4)
returns = cell2mat(returns)
I want to highlight James's solution for anyone else stumbling on this answer. This one works for my workflow, but his is also a very good answer. I wish I could accept both.

Sign in to comment.

More Answers (1)

James Tursa
James Tursa on 20 Sep 2022
You could modify f( ) to return a vector. Or if you didn't want to modify f( ) you could create a helper function that does this. E.g.,
function v = f2(a,b)
[A,B] = f(a,b);
v = [A,B];
Then just call f2( ) instead of f( ).
Another approach is to have the helper function use the nargout/varargout feature to detect how many outputs are requested by the caller. E.g.,
function varargout = f2(a,b)
[A,B] = f(a,b);
if nargout == 2
varargout{1} = A;
varargout{2} = B;
varargout{1} = [A,B];
  1 Comment
Joseph on 20 Sep 2022
This is extremely handy. I will definitely be using this in the future. I wish I could accept both answers, because they're both very situational.

Sign in to comment.


Find more on Loops and Conditional Statements in Help Center and File Exchange




Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!