I have made many functions where I do not always want to require input, so the first lines in many of them look like:
function something = thisIsMyFunction(OptionalStruct)
if(exist('OptionalStruct')
building on existing struct;
else
build a new struct;
end
end
According to the docs, the exist() search goes much faster when I can pass in a type along with the option, so it will only search for that type. I want to know if in this case (or ever) a struct is a variable and I can say:
if(exist('OptionalStruct', 'var')
You could use use isstruct to check if the variable is a structure.
if(exist('OptionalStruct')
if(isstruct(OptionalStruct))
building on existing struct;
else
build a new struct;
end
Whatever is assigned in a statement like
varName = ...;
is a variable and will make exist('varName', 'var') true.
It is totally irrelevant what type of value that variables holds or refers to.
And, seconding natan's comment:
It should take less time to test a thing like than this, than to post this question on SO.
Related
In Matlab, it is easy to generate "help" for a function, as follows.
function out = foo()
% helpful information about foo
end
When we execute help foo, we get "helpful information about foo".
However, suppose we would like to define help for a variable, probably as a definition. How could we do such a thing? It would be nice if we could do something like
x = 3; % m ... position
help x
and get "m ... position". However, I don't believe such functionality exists.
The only reasonable way I see around this is to define every variable as a struct with keys value and description.
x.value = 3;
x.description = 'm/s ... position';
This requires we define every variable as a struct, which is kind of annoying and, I worry (should I?), unperformant (it's simulation code and these variables are accessed repeatedly).
Is there another solution I'm not considering? Should I be worried about making every variable a struct?
Your code should be self-documenting. Instead of variable name x, use position.
Furthermore, all variables should be local, so you can easily look for its definition (with comment) within the function you are editing.
Variables declared further away (with larger scope within the function) should have longer, more self-explanatory names than variables with a smaller scope (e.g. use within a short loop.
There are only two three cases where variables are declared outside the function’s scope:
Class properties. You can actually document these.
In a script, you have access to variables that already existed before the script started. A good reason not to use scripts or depend on the base namespace in larger projects.
Global variables. You should never use global variables for many reasons. Just don’t.
Background -- I was reading up on accessing shadowed functions, and started playing with builtin . I wrote a little function:
function klear(x)
% go to parent environment...
evalin('base', builtin('clear','x')) ;
end
This throws the error:
Error using clear
Too many output arguments.
I think this happens because evalin demands an output from whatever it's being fed, but clear is one of the functions which has no return value.
So two questions: am I interpreting this correctly, and if so, is there an alternative function that allows me to execute a function in the parent environment (that doesn't require an output)?
Note: I'm fully aware of the arguments against trying to access shadowed funcs (or rather, to avoid naming functions in a way that overload base funcs, etc). This is primarily a question to help me learn what can and can't be done in MATLAB.
Note 2
My original goal was to write an overload function that would require an input argument, to avoid the malware-ish behavior of clear, which defaults to deleting everything. In Q&D pseudocode,
function clear(x)
if ~exist('x','var') return
execute_in_base_env(builtin(clear(x)))
end
There's a couple issues with your clear override:
It will always clear in the base workspace regardless of where it's called from.
It doesn't support multiple inputs, which is a common use case for clear.
Instead I'd have it check for whether it was called from the base workspace, and special-case that for your check for whether it's clearing everything. If some function is calling plain clear to clear all its variables, that's bad practice, but it's still how that function's logic works, and you don't want to break that. Otherwise it could error, or worse, return incorrect results.
So, something like this:
function clear(varargin)
stk = dbstack;
if numel(stk) == 1 && (nargin == 0 || ismember('all', varargin))
fprintf('clear: balking at clearing all vars in base workspace. Nothing cleared.\n');
return;
end
% Check for quoting problems
for i = 1:numel(varargin)
if any(varargin{i} == '''')
error('You have a quote in one of your args. That''s not valid.');
end
end
% Construct a clear() call that works with evalin()
arg_strs = strcat('''', varargin, '''');
arg_strs = [{'''clear'''} arg_strs];
expr = ['builtin(' strjoin(arg_strs, ', '), ')'];
% Do it
evalin('caller', expr);
end
I hope it goes without saying that this is an atrocious hack that I wouldn't recommend in practice. :)
What happens in your code:
evalin('base', builtin('clear','x'));
is that builtin is evaluated in the current context, and because it is used as an argument to evalin, it is expected to produce an output. It is exactly the same as:
ans = builtin('clear','x');
evalin('base',ans);
The error message you see occurs in the first of those two lines of code, not in the second. It is not because of evalin, which does support calling statements that don't produce an output argument.
evalin requires a string to evaluate. You need to build this string:
str = 'builtin(''clear'',''x'')';
evalin('base',ans);
(In MATLAB, the quote character is escaped by doubling it.)
You function thus would look like this:
function clear(var)
try
evalin('base',['builtin(''clear'',''',var,''')'])
catch
% ignore error
end
end
(Inserting a string into another string this way is rather awkward, one of the many reasons I don't like eval and friends).
It might be better to use evalin('caller',...) in this case, so that when you call the new clear from within a function, it deletes something in the function's workspace, not the base one. I think 'base' should only be used from within a GUI that is expected to control variables in the user's workspace, not from a function that could be called anywhere and is expected (by its name in this case) to do something local.
There are reasons why this might be genuinely useful, but in general you should try to avoid the use of clear just as much as the use of eval and friends. clear slows down program execution. It is much easier (both on the user and on the MATLAB JIT) to assign an empty array to a variable to remove its contents from memory (as suggested by rahnema1 in a comment. Your base workspace would not be cluttered with variables if you used function more: write functions, not scripts!
I have multiple variables var_1, var_2, var_3....var_9 (they are named like that) that I want to pass in a function. All of the variables are saved in the workspace. The function takes 2 variables, and spits out an output. I want to compare var_1 with all the variables, including itself, so I prefer to automate it in a loop.
So I want to execute
function(var_1,var_1)--> display answer, function(var_1,var_2)--> display answer...function(var_1,var_9)-->display answer all at once in a loop. I've tried the following, with no luck:
for i=1:7
functionname(var_1,var_'num2str(i)')
end
Where did I go wrong?
You cannot make a dynamic variable name directly. But you can use the eval-function to evaluate an expression as a string. The string can be generated with sprintf and replaces %d with your value.
for i=1:7
eval(sprintf('functionname(var_1,var_%d)', i));
end
But: Whenever you can, you should avoid using the eval function. A much better solution is to use a cell array for this purpose. In the documentation of Matlab there is a whole article about the why and possible alternatives. To make it short, here is the code that uses a cell array:
arr = {val_1, val_2, val_3, val_4, val_5, val_6, val_7, val_8, val_9};
for i = 1:length(arr)
functionname(arr{1},arr{i})
end
I have a function that returns two values, like so:
[a b] = myfunc(x)
Is there a way to get the second return value without using a temporary variable, and without altering the function?
What I'm looking for is something like this:
abs(secondreturnvalue(myfunc(x)))
not that i know of. subsref doesn't seem to work in this case, possibly because the second variable isn't even returned from the function.
since matlab 2009b it is possible to use the notation
[~, b] = function(x)
if you don't need the first argument, but this still uses a temporary variable for b.
Unless there is some pressing need to do this, I would probably advise against it. The clarity of your code will suffer. Storing the outputs in temporary variables and then passing these variables to another function will make your code cleaner, and the different ways you could do this are outlined here: How to elegantly ignore some return values of a MATLAB function?.
However, if you really want or need to do this, the only feasible way I can think of would be to create your own function secondreturnvalue. Here's a more general example called nth_output:
function value = nth_output(N,fcn,varargin)
[value{1:N}] = fcn(varargin{:});
value = value{N};
end
And you would call it by passing as inputs 1) the output argument number you want, 2) a function handle to myfunc, and 3) whatever input arguments you need to pass to myfunc:
abs(nth_output(2,#myfunc,x))
function y = myfunc(param)
C = param.C;
L = param.L;
Kp = param.Kp;
Ki = param.Ki;
...
Is there a way to generalize the above code? I know how to generalize the structure access using fieldnames() and getfield(), but not how to set variables without calling eval() (which is evil).
for n = fieldnames(param)'
name = n{1};
value = param.(name);
do_something_with(name,value); % ????
never mind, I figured it out; this helper function works:
function vars_pull(s)
for n = fieldnames(s)'
name = n{1};
value = s.(name);
assignin('caller',name,value);
end
The only way to create a variable whose name is determined at run-time is to use a function like eval, evalin, feval, or assignin. (assignin is the least evil choice BTW, at least you don't need to convert your value to a string and back.)
However, I question why you want to do that, why not just access the values through the input structure as you need them. If you want to save typing (speaking from experience, as I am extremely lazy), I usually name my input parameter structure something short, like p. The throughout my code I just access the fields directly, (e.g. p.Kp, and after a while I don't even see the p. anymore.) This also makes it easy to pass the structure into subfunctions as needed.
You can use the excellent submission at FileExchange:
V2STRUCT - Pack & Unpack variables to & from structures with enhanced functionality
Here's a workaround: save the structure to a .mat file using the '-struct' option, and then immediately reload it. Here's an example for struct variable X:
save('deleteme.mat','-struct','X');
load('deleteme.mat');
delete('deleteme.mat');
It's kludgey, but actually pretty fast, at least with an SSD.