Question

Aside from parsing the function file, is there a way to get the names of the input and output arguments to a function in matlab?

For example, given the following function file:

divide.m

function [value, remain] = divide(left, right)
     value = floor(left / right);
     remain = left / right - value;
end

From outside the function, I want to get an array of output arguments, here: ['value', 'remain'], and similarly for the input arguments: ['left', 'right'].

Is there an easy way to do this in matlab? Matlab usually seems to support reflection pretty well.

EDIT Background:

The aim of this is to present the function parameters in a window for the user to enter. I’m writing a kind of signal processing program, and functions to perform operations on these signals are stored in a subfolder. I already have a list and the names of each function from which the user can select, but some functions require additional arguments (e.g. a smooth function might take window size as a parameter).

At the moment, I can add a new function to the subfolder which the program will find, and the user can select it to perform an operation. What I’m missing is for the user to specify the input and output parameters, and here I’ve hit the hurdle here in that I can’t find the names of the functions.

Answer 1

If your problem is limited to the simple case where you want to parse the function declaration line of a primary function in a file (i.e. you won’t be dealing with local functions, nested functions, or anonymous functions), then you can extract the input and output argument names as they appear in the file using some standard string operations and regular expressions. The function declaration line has a standard format, but you have to account for a few variations due to:

• Varying amounts of white space or blank lines,
• The presence of single-line or block comments, and
• Having the declaration broken up on more than one line.

(It turns out that accounting for a block comment was the trickiest part…)

I’ve put together a function get_arg_names that will handle all the above. If you give it a path to the function file, it will return two cell arrays containing your input and output parameter strings (or empty cell arrays if there are none). Note that functions with variable input or output lists will simply list 'varargin' or 'varargout', respectively, for the variable names. Here’s the function:

function [inputNames, outputNames] = get_arg_names(filePath)

    % Open the file:
    fid = fopen(filePath);

    % Skip leading comments and empty lines:
    defLine = '';
    while all(isspace(defLine))
        defLine = strip_comments(fgets(fid));
    end

    % Collect all lines if the definition is on multiple lines:
    index = strfind(defLine, '...');
    while ~isempty(index)
        defLine = [defLine(1:index-1) strip_comments(fgets(fid))];
        index = strfind(defLine, '...');
    end

    % Close the file:
    fclose(fid);

    % Create the regular expression to match:
    matchStr = '\s*function\s+';
    if any(defLine == '=')
        matchStr = strcat(matchStr, '\[?(?<outArgs>[\w, ]*)\]?\s*=\s*');
    end
    matchStr = strcat(matchStr, '\w+\s*\(?(?<inArgs>[\w, ]*)\)?');

    % Parse the definition line (case insensitive):
    argStruct = regexpi(defLine, matchStr, 'names');

    % Format the input argument names:
    if isfield(argStruct, 'inArgs') && ~isempty(argStruct.inArgs)
        inputNames = strtrim(textscan(argStruct.inArgs, '%s', ...
                                      'Delimiter', ','));
    else
        inputNames = {};
    end

    % Format the output argument names:
    if isfield(argStruct, 'outArgs') && ~isempty(argStruct.outArgs)
        outputNames = strtrim(textscan(argStruct.outArgs, '%s', ...
                                       'Delimiter', ','));
    else
        outputNames = {};
    end

% Nested functions:

    function str = strip_comments(str)
        if strcmp(strtrim(str), '%{')
            strip_comment_block;
            str = strip_comments(fgets(fid));
        else
            str = strtok([' ' str], '%');
        end
    end

    function strip_comment_block
        str = strtrim(fgets(fid));
        while ~strcmp(str, '%}')
            if strcmp(str, '%{')
                strip_comment_block;
            end
            str = strtrim(fgets(fid));
        end
    end

end