Using 'assignin' to assign value to variable 'max'

I'm using the 'assignin' function to assign a value to the variable 'max' using the code below:

function get_data

assign_value;

display(max)

end


function assign_value

assignin('caller','max',5);

end

If I run 'get_data' from the command line, I get an error that I'm not passing enough input arguments to 'max,' since 'max' is a built-in function to MATLAB.

Interestingly, I can add the output 'max' to the function 'get_data' by writing

function max=get_data

and the function will run and display the value '5'.

My questions are:

1) Is this how 'assignin' is supposed to behave, i.e. does it not assign a value to a variable if the variable name is an instrinsic function in MATLAB?

2) Why is 'assignin' not able to assign value to 'max,' but I can execute 'max=5' in other contexts and MATLAB won't complain?

Answer 1

[Guillaume]

First and foremost, you shouldn't be using assignin and its friends (evalin, etc.). There's always better ways to do whatever you're trying to do. Also, you shouldn't call a variable max (or mean or sum or size or one of the oft used functions). The two together is really asking for trouble!

As to the behaviour you see, it is expected in recent versions of matlab. Matlab has a just in time compiler that optimises functions so they run faster. One decision the compiler must make before the code is run is whether something is a function call or a variable. In recent versions if it doesn't see an assignment to the something, it assumes it's a function. Anything that pops variables into existence break that assumption and it's documented (at least in the release notes) that it's no longer supported in recent versions. You'd have the same problem with a plain load or eval.

The workaround is to create the variable first:

function get_data
max = [];  %now the JIT compiler knows that max is a variable (a badly named variable!)
assign_value;
display(max);
end

The better work around of course is to define the variable as an output of assign_value:

function get_data
    my_max = assign_value;  %and don't use max as a variable name
    display(my_max)
end

function out = assign_value;
    out = 5;
end

It's simpler code as well. What's not to like?

[Cody Patterson]

Thank you for the clear explanation on this! I agree that assignin is not always the best tool to get the job done, and for this toy example, I would code it the same way as your second example. I would also not name a variable 'max.' My original problem showed up when naming a variable 'pe' which also happens to be a built-in function. However, I am automating data to be read in from fortran with the same variable names as fortran source code. I think assignin is necessary for this sort of task. If you know of otherways, I would like to hear them.