How do I pass the output of a command as argument to the other command?

I have created char array let say

a= sprintf('%s%d','elem_append_mod',6);

Now I already have an cell array of size 1 X 12 with the name "disp(a)".

When I execute the command

size(sprintf('%s%d','elem_append_mod',6),2)

I get the output as 16 (length of the string 'elem_append_mod6'). but what I require is the size of the cell array with the name 'elem_append_mod6' i.e. 12 !

[yashwant kolluru]

Thanks Guys! for yor help!

[Stephen23]

Do NOT create or access variable names dynamically

Ugh... this is a really bad idea!

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:

• the dimensions of numeric arrays: fully vectorized code is often the fastest MATLAB data processing!
• cell arrays
• structures, where you can include fields for each kind of data (e.g. Process type, Flow data, Temperature data, Notes, Units, etc). Structures can even be non-scalar, which is extremely useful! And yes, you can even define structure fieldnames dynamically.

There are many functions that support working on structures and cell arrays and can access their data easily, and many functions operate on complete numeric arrays all at once without any loops (i.e. vectorized code, which is something you need to learn about).

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://www.mathworks.com/matlabcentral/newsreader/view_thread/240867

http://blogs.mathworks.com/loren/2005/12/28/evading-eval/

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

http://ww.mathworks.com/matlabcentral/answers/143-how-do-i-make-a-series-of-variables-a1-a2-a3-a10

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-importin

http://stackoverflow.com/questions/10272522/use-and-implications-of-evalexpression-in-matlab-code

Here is a discussion of why it is a bad idea to include meta-data (such as an index) in a variable name:

http://www.mathworks.com/matlabcentral/answers/225435-save-variable-as-string-from-user-input

And for some other languages advising "DO NOT create dynamic variable names":

http://nedbatchelder.com/blog/201112/keep_data_out_of_your_variable_names.html

http://www.techrepublic.com/blog/it-security/never-use-dynamic-variable-names/

http://stackoverflow.com/questions/11118486/python-list-as-variable-name

http://www.reddit.com/r/learnpython/comments/2a9o4s/why_is_it_a_bad_idea_to_create_dynamic_variables/

http://stackoverflow.com/questions/25076883/creating-dynamically-named-variables-in-a-function-in-python-3-understanding-e

http://perl.plover.com/varvarname.html

https://24ways.org/2005/dont-be-eval

Answer 1

[Thorsten]

You have to use eval:

 eval(['size(' sprintf('%s%d','elem_append_mod',6) ', 2)' ])

BTW: If you use a fixed constant like 6, instead of

 a= sprintf('%s%d','elem_append_mod',6);

you can simply write

a = 'elem_append_mod6';

And it seems that you dynamically create variable names, which is not a good idea.

[yashwant kolluru]

yes! it worked! thanks so much!

[Jan]

And using eval is a really bad idea. See the hundreds of concerning threads in this forum.

[Image Analyst]

I'm lost. I get an error:

>> eval(['size(' sprintf('%s%d','elem_append_mod',6) ', 2)' ])
Error using eval
Undefined function or variable 'elem_append_mod6'.

So why was this accepted, and where is the 12? What line of code generates a 12? And why 12 anyway? What's 12?

[Thorsten]

The question was not 100% clear. I interpreted it such that yashwant has created a variable named 'elem_append_mod6' and now wanted to get the size of this variable (see the last line of the question).

Answer 2

[Image Analyst]

Cell arrays cannot be named disp(a) because variable names cannot have parentheses in the names. So I will assume your cell array is called "ca" rather than "disp(a)". You can, however, have a cell array with the name "elem_append_mod6". The cell array can have 12 cells in it if you want.

I suspect the difficulty lies in you not completely understanding cells and cell arrays. I think the FAQ will help tremendously, so please read it: http://matlab.wikia.com/wiki/FAQ#What_is_a_cell_array.3F

Then run this illustrative demo which kind of does what you say, but not what you want since I don't know what that is and don't know where the 12 comes from:

% Create a simple string variable.
a = sprintf('%s%d','elem_append_mod',6)
sa = size(a)
la = length(a)
whos a

% Create a cell array, called "ca", with a size of 1-by-12
ca = cell(1, 12); % Make 12 empty cells.
% Put a into cell #1
ca(1) = {a}
size(ca)
whos ca

% Now make another cell array with the name "elem_append_mod6"
elem_append_mod6 = cell(1, 12); % Make 12 empty cells.
% and put "a" into that cell array's first cell:
elem_append_mod6(1) = {a}
size(elem_append_mod6)
whos elem_append_mod6

In the command window:

a =
elem_append_mod6
sa =
     1    16
la =
    16
  Name      Size            Bytes  Class    Attributes

    a         1x16               32  char               

ca = 
    'elem_append_mod6'    []    []    []    []    []    []    []    []    []    []    []
ans =
     1    12
  Name      Size            Bytes  Class    Attributes

    ca        1x12              232  cell               

elem_append_mod6 = 
    'elem_append_mod6'
ans =
     1     1
  Name                  Size            Bytes  Class    Attributes

    elem_append_mod6      1x1               144  cell