I just discovered (to my surprise) that calling the following function
function foo()
if false
fprintf = 1;
else
% do nothing
end
fprintf('test')
gives and error Undefined function or variable "fprintf". My conclusion is that the scope of variables is determined before runtime (in my limited understanding how interpretation of computer languages and specifically Matlab works). Can anyone give me some background information on this?
Edit
Another interesting thing I forgot to mention above is that
function foo()
if false
fprintf = 1;
else
% do nothing
end
clear('fprintf')
fprintf('test')
produces Reference to a cleared variable fprintf.
MATLAB parses the function before it's ever run. It looks for variable names, for instance, regardless of the branching that activates (or doesn't activate) those variables. That is, scope is not determined at runtime.
ADDENDUM: I wouldn't recommend doing this, but I've seen a lot of people doing things with MATLAB that I wouldn't recommend. But... consider what would happen if someone were to define their own function called "false". The pre-runtime parser couldn't know what would happen if that function were called.
It seems that the first time the MATLAB JIT compiler parses the m-file, it identifies all variables declared in the function. It doesn't seem to care whether said variable is being declared in unreachable code. So your local fprintf variable immediately hides the builtin function fprintf. This means that, as far as this function is concerned, there is no builtin function named fprintf.
Of course, once that happens, every reference within the function to fprintf refers to the local variable, and since the variable never actually gets created, attempting to access it results in errors.
Clearing the variable simply clears the local variable, if it exists, it does not bring the builtin function back into scope.
To call a builtin function explicitly, you can use the builtin function.
builtin( 'fprintf', 'test' );
The line above will always print the text at the MATLAB command line, irrespective of local variables that may shadow the fprintf function.
Interesting situation. I doubt if there is detailed information available about how the MATLAB interpreter works in regard to this strange case, but there are a couple of things to note in the documentation...
The function precedence order used by MATLAB places variables first:
Before assuming that a name matches a function, MATLAB checks for a variable with that name in the current workspace.
Of course, in your example the variable fprintf doesn't actually exist in the workspace, since that branch of the conditional statement is never entered. However, the documentation on variable naming says this:
Avoid creating variables with the same name as a function (such as i, j, mode, char, size, and path). In general, variable names take precedence over function names. If you create a variable that uses the name of a function, you sometimes get unexpected results.
This must be one of those "unexpected results", especially when the variable isn't actually created. The conclusion is that there must be some mechanism in MATLAB that parses a file at runtime to determine what possible variables could exist within a given scope, the net result of which is functions can still get shadowed by variables that appear in the m-file even if they don't ultimately appear in the workspace.
EDIT: Even more baffling is that functions like exist and which aren't even aware of the fact that the function appears to be shadowed. Adding these lines before the call to fprintf:
exist('fprintf')
which('fprintf')
Gives this output before the error occurs:
ans =
5
built-in (C:\Program Files\MATLAB\R2012a\toolbox\matlab\iofun\fprintf)
Indicating that they still see the built-in fprintf.
These may provide insight:
https://www.mathworks.com/help/matlab/matlab_prog/base-and-function-workspaces.html
https://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html
This can give you some info about what is shadowed:
which -all
(Below was confirmed as a bug)
One gotcha is that Workspace structs, and classes on the path, have particular scoping and type precedence that (if you are me) may catch you out.
E.g. in 2017b:
% In C.m, saved in the current directory
classdef C
properties (Constant)
x = 100;
end
end
% In Command window
C.x = 1;
C.x % 100
C.x % 1 (Note the space)
C.x*C.x % 1
disp(C.x) % 1
Related
I have some parameters changing size on every loop iteration in main.m. I have placed the preallocations in another script called preallocation.m.
When the preallocations is placed in another script, I get a warning from Matlab for every parameter in the main script about concidering preallocating for improved speed.
Is there a way to remove these warnings? If I add another parameter that needs preallocating and forget to preallocate, I'd like to get a warning for that parameter.
I'd be tempted to make your preallocation script a function. Then you could write something like
[myvar1,myvar2,myvar3] = preallocate();
This would allow you to see all preallocated variables at a glance and would also keep mlint quiet.
In my MATLAB version (R2014b) this is done by right-clicking on the variable that has the squiggly red line onder it, and then selection the option Suppress "The variable myVar appears to...". You can then choose to suppress this single line, all warnings of this type in the entire file, or all warnings of this type in all files.
edit: if your MATLAB version does not have this context menu option, you can add %#ok<SAGROW> to the end of the offending line, like so:
someVar(ii+1) = someVar(ii) + someConstant; %#ok<SAGROW>
If you are hell-bent on not wanting these warnings and not using a preallocation function that returns many variables, you could also do the following:
function init = preallocate()
init.a = some stuff;
init.b = some stuff;
...
end
%% actual script
init = preallocate()
structvars(init); % using structvars from file exchange
structvars can be found here. Note that I haven't tried the package myself, so I am unsure as to how well it will work and how fast it is.
I have a rather bulky program that I've been running as a script from the MATLAB command line. I decided to clean it up a bit with some nested functions (I need to keep everything in one file), but in order for that to work it required me to also make the program itself a function. As a result, the program no longer runs in the base workspace like it did when it was a script. This means I no longer have access to the dozens of useful variables that used to remain after the program runs, which are important for extra calculations and information about the run.
The suggested workarounds I can find are to use assignin, evalin, define the variables as global, or set the output in the definition of the now function-ized program. None of these solutions appeal to me, however, and I would really like to find a way to force the workspace itself to base. Does any such workaround exist? Or is there any other way to do this that doesn't require me to manually define or label each specific variable I want to get out of the function?
Functions should define clearly input and output variables. Organizing the code differently will be much more difficult to understand and to modify later on. In the end, it will most likely cost you more time to work with an unorthodox style than investing in some restructuring.
If you have a huge number of output variables, I would suggest organizing them in structure arrays, which might be easy to handle as output variables.
The only untidy workaround I can imagine would use whos, assignin and eval:
function your_function()
x = 'hello' ;
y = 'world' ;
variables = whos ;
for k=1:length(variables)
assignin('base',variables(k).name,eval(variables(k).name))
end
end
But I doubt that this will help with the aim to clean up your program. As mentioned above I suggest ordering things manually in structures:
function out = your_function()
x = 'hello' ;
y = 'world' ;
out.x = x ;
out.y = y ;
end
If the function you would like to define are simple and have a single output, one option is to use anonymous functions.
Another option is to store all the variable you would like to use afterwards in a struct and have your big function return this struct as an output.
function AllVariables = GlobalFunction(varargin);
% bunch of stuff
AllVariables= struct('Variable1', Variable1, 'Variable2', Variable2, …);
end
Let's say you are some new programmer and you do something like...
%...la da da
%...programming away
if such && such
clear = 1;
else
clear = 0;
end
or in some other way, you assign the variable clear a value.
Is there some way to "clear" clear?
clearvars doesn't work. Clicking the workspace variable and manually clicking delete does work, but I think it's cheating.
This will do it:
builtin('clear','clear')
Note: Keep in mind to avoid such operations to keep code clarity. Only do overwrite when it is the exact action you want to take place. Otherwise it may cause future bugs if you forgot (or if another person uses your code and didn't realize it) that you have the clear (or any other) function overwritten. You could easily name this variable as doClear for example.
Any name, even builtin and feval can be overriden. In such case, you can use function handles instead to force MALTAB into interpreting a statement as a function call:
clear = str2func('clear');
clear('clear')
Obviously, str2func can also be overrriden! :) however, there exists a similar solution (inspired by Loren's article), which is creating a separate m-file that does the same thing:
function clearclear()
assignin('caller', 'clear', #clear);
Calling this function in the main workspace should allow you to do clear('clear') safely.
The second solution takes advantage of the fact that the m-file doesn't "see" the variable clear in the main workspace, and therefore can access the actual handle of the clear function properly.
A non intuitive way is
clear = rand(1000,500,700);
pack
This produces the following warning:
Warning: Variable 'clear' cannot be saved to a MAT-file whose version
is older than 7.3. To save this variable, use the -v7.3 switch.
Skipping...
It also suffers from the same issue that you can assign pack to be a variable.
Interesting problem! I found it surprisingly hard to find an ways to do this programatically (besides the one suggested by #TryHard)
Here is the I have come up with though it is a bit more powerfull than clear:
!matlab &
exit
Note that if you want to type this in the command line at once, you need to use a shift+enter in between.
I've written a piece of code which calls numerous functions, which in turn also call several sub-functions.
I am calling the main file from the command line and supplementing the call with certain arguments to initiate certain modes I have accounted for.
E.g. octave classify_file.m --debug <file> would run in my custom debug mode, which sets a constant debug to 1 and subsequently outputs all plots and relevant variables. Omitting the argument outputs only 1 variable.
Similarily I have a template and a histogram mode, which essentially all do the same thing, except output some more variables, matrices and plots depending on the mode.
As it is now, I have to include the debug, template and histogram constants as arguments to each and every function if I want them to be influenced by the respective modes.
This is cumbersome and confusing, there has to be a better way. I've never worked with global variables, but would this be a good place to use one? What's an elegant solution for this problem?
This is a situation in which global variables will come in handy, although as you may be aware they are sometimes frowned upon, and can also have certain performance implications in matlab. Personally I don't think passing the mode all the way down the call stack is too bad - although are you treating all 3 as separate arguments? The least you could do is put them in a struct in your highest level function so that you only have 1 argument:
mode.debug = [whatever]
mode.histogram = [whatever]
mode.template = [whatever]
myFunction(mode);
OR, if you can only have one mode at a time what about some integer constants?
mode = MODE_DEBUG
or
mode = MODE_NONE
I would define the constants by creating short functions, this is how the pi constant works for example.
Finally, there is an alternative to global variables which I rather like, which is functions that use persistent variables. For example:
function m = debugMode(newValue)
persistant isModeOn;
if nargin > 0
isModeOn = newValue
end
m = isModeOn;
end
This way you can call debugMode(1) to set it on, and you can call m = debugMode anywhere to get the value.
Suppose I'm using the MATLAB IDE and happen to have some very large objects in my workspace (e.g. arrays of 500k+ elements). Now, suppose that I stupidly and accidentally double click on one of these very large variables, which triggers a load to the array editor. Unfortunately, with arrays this big, MATLAB just hangs.
I've tried CTRL+C, CTRL+BREAK, CTRL+D, but none seem able to interrupt the behavior of the IDE. I know I can force matlab to quit, but reading all of those variables into the workspace in the first place takes a lot of time, and I may have unsaved changes in an editor window, etc.
The variable editor is launched using the command openvar. To solve your problem you can take advantage of a Matlab quirk that causes functions to be masked by variables with the same name. For example if you create a variable named plot the plot() function stops working.
The solution, although hackish, is to simply create an empty variable named openvar. Then anytime attempt to open the variable editor will fail because the function openvar is being hidden by the variable.
If you want to use the variable editor again simple call clear openvar to delete the variable and the function will be unmasked.
I found a way, but it's not the best, it requires a change of path and back once to get a handle to the original openvar
function openvar(name,array)
persistent org_openvar
if isempty(org_openvar)
curdir=pwd;
cd(fullfile(matlabroot,'toolbox/matlab/codetools'));
org_openvar = #openvar;
cd(curdir);
end
if numel(array)>1e5
if strcmp(questdlg(sprintf('Opening ''%s'' which has %d elements.\n\nAre you sure? This is gonna take a while!',name,numel(array)), ...
'Variable editor','Yes','Cancel','Cancel') , 'Yes')
org_openvar(name,array)
end
else
org_openvar(name,array)
end
end
getting that handle is the biggest problem, calling it is just fine. If openvar would be built in, you could use the function builtin:
builtin('openvar',name,array)
but this is unfortunately not the case :(
str2func in combination with the complete path also doesn't work, at least I don't get it to work...