MATLAB: Capture return of Sum() directly in to variables

sum

K=3×3 matrix when getting the sum it returns 1×3 matrix if try to capture it directly to variables given error

but same can be done with size() function

Best Answer

The size function has been implemented so it can return one output or multiple outputs. The sum function has not.

But even if sum had been implemented so it could return one output per element in the result of summing or one element per column, I recommend against trying to automatically create a dynamic number of variables with dynamic names. Read this Answer for a lengthy discussion about the problems that can occur when you do so.

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MATLAB: Dynamically assign structure names and loop them in fuction

Avoid creating dynamically named variables in MATLAB. This is poor practice as has been explained many times on this forum, and is not recommended by MATLAB themselves:

http://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

When you are a beginner it seems like a cunning and fast way to store information, but actually it is really bad practice to name your variables dynamically. MATLAB is also not intended for this kind of variable naming: if you continue to include data in the variable names then you will find yourself fighting many more of these battles against MATLAB.

However when you use more appropriate storage for your data (and meta-data) then you will suddenly find lots of MATLAB functions that do many useful operations for you, quickly and easily.

In your case a much more robust solution would be to use structures , where you can include fields for each kind of data (e.g. Process type, Flow data, Temperature data, Notes, Units, etc), or cell arrays . There are many functions that support working on structures and cell arrays, and can access these data easily, and they can also be used in vectorized code (which is something you need to learn about). And yes, you can even define structure fieldnames dynamically .

Placing your data in a structure or cell array also makes it much easier to pass to functions: can you imagine the fight you would have trying to pass hundreds of dynamically named variables to a function?

If you have a newer version of matlab you can also use a table , which stores the data together in one array but also allows key-name access to the columns. This might be a good alternative for your data.

In case you are interested, here are some pages explaining why dynamically assigning variable names is a really bad idea in MATLAB:

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

http://www.mathworks.com/matlabcentral/answers/56124-eval-is-evil-using-variables-created-dynamically-info-retrieval

http://www.mathworks.com/matlabcentral/answers/51946-systematic-do-not-use-global-don-t-use-eval

http://www.mathworks.com/matlabcentral/answers/79281-create-a-structure-name-based-on-the-string-of-a-variable-create-fieldnames-in-this-structure

http://www.mathworks.com/matlabcentral/answers/74778-loop-file-and-variablenames-when-importing

MATLAB: Hi Guys! I want to compute annual means of returns based on monthly returns. So I have 363 monthly returns vector and want to compute the following sequence mean(1:12), mean(13:24), mean(25:36)….and so on.

You seem to assume, that something is wrong with your code. Then please mention, why you think so. This is better than letting the readers guess it.

I assume, you do not want to store the output to MSCI1(31), but than MSCI1 should be a vector containing all results.
The last element of your loop counter i is 361. Then you cannot access MSCI(i+c+1), because this is out of bounds.

MSCI  = BB(1:363,4);
BOND  = BB(1:363,5);
c     = 12;
index = 1:c:(length(MSCI));
for k = 1:length(index) - 1
   v        = index(k);
   MSCI1(k) = mean(MSCI(v:v+c-1));
   Bond1(k) = mean(BOND(v:v+c-1));
end

A cheaper solution without a loop:

c     = 12;
n     = 363 - mod(363, c);
MSCI  = reshape(BB(1:n, 4), 12, []);
BOND  = reshape(BB(1:n, 5), 12, []);
MSCI1 = mean(MSCI, 1);
Bond1 = mean(BOND, 1);

By the way, path is an important Matlab function. Shadowing it by using its name for a variable can cause serious troubles.

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MATLAB: Dynamically assign structure names and loop them in fuction

MATLAB: Hi Guys! I want to compute annual means of returns based on monthly returns. So I have 363 monthly returns vector and want to compute the following sequence mean(1:12), mean(13:24), mean(25:36)….and so on.

Related Question