How to define several variables from vector at the same time

Hi

I am trying to define several variables at the same time. The data for each variable is saved in a vector. I have tried the following:

   x = [1 2 3];
   y = [4 7 9];
   [a b] = [x(end) y(end)]
   %Or alternatively
   [c d e] = [x(1) x(2) x(3)]

When I try this I receive the "Too many output arguments." error. Is there some easy way to do this?

I realize I could use:

   a = x(end)
   b = y(end)

But I would like a cleaner method.

Regards

Answer 1

[Jan]

[a, b, c] = deal(x(1), x(2), x(3))

I am not sure that this is "cleaner", if you do this for many variables. It is slightly slower than the direct and simple:

a = x(1);
b = x(2);
c = x(3);

deal and num2cell have a certain overhead. The code might be more compact, if you do this in one line instead of 3, but internally much more code lines are processed. I'm convinced, that the code will not suffer from additional lines, if they are such simple and clear: neither a human reader nor the Matlab interpreter will have any problems with them. Do not try to optimize the optical appearance of the code. It is more valuable to keep the code fast (to optimize the run time) and simple (to optimize the time for debugging and maintaining).

[Martin Olafsen]

My issue is that I calculate 17 variables in a for-loop. I need to store the previous values to calculate the next iteration, so the variables are stored in vectors. When I then try to return these values to a script, I only want the last iteration, but I was unable to specify which element of the vectors to return.

function [x(end), y(end)] = Cycle(fluid, massflow)

This produces the error "Unbalanced or unexpected parenthesis or bracket". So instead I planned to redefine the variables (the 17 vectors.) using:

   [x, y...] = deal(x(end), y(end)...]

And then to return x, y, etc.

[Jan]

@Martin: This is a completely different question. The solution is easy:

function [xf, yf] = Cycle(fluid, massflow)
... calculate x and y
xf = x(end);
yf = y(end);
end

as you have mentioned in your question already. This is clean, clear and fast. Searching for more complicated methods like deal() is a waste of time - for programming and for the processing.

Remember Donald Knuth's mantra:

 KISS - Keep it simple stupid

Perhaps the bigger problem is that you have 17 vectors in your code. Would a matrix or a cell array be smarter? Then the extraction of the last element could be done in a loop or by a simple indexing. Returning a vector instead of a bunch of variables might be an option also.

[Bill Tubbs]

From the title, I thought this would answer my problem but it doesn't. I have a vector params and I want to extract three parameters from it.

This doesn't work:

[b1, a1, a2] = deal(params(2:4));

deal(X) copies the single input to all the requested outputs.

But why the need for any function to do this? Wouldn't it be nice and logical if this worked:

>> [b1, a1, a2] = params(2:4);
Insufficient number of outputs from right hand side of equal sign to satisfy assignment.

I see there is a note in the documentation saying:

Beginning with MATLAB® Version 7.0 software, you can access the contents of cell arrays and structure fields without using the deal function. See Example 3, below.

Why not allow it for vectors as well?

Answer 2

[KL]

You can use cell arrays and get the comma separated lists,

xy = num2cell([x y])
[a b] = xy{1,[3 6]};
[c d e] = xy{1, 1:3};

Answer 3

[Walter Roberson]

[c, d, e] = deal(x(1), x(2), x(3));