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Resolve Error: Undefined Function or Variable

Issue

You may encounter the following error message, or something similar, while working with functions or variables in MATLAB®:

Undefined function or variable 'x'.

These errors usually indicate that MATLAB cannot find a particular variable or MATLAB program file in the current directory or on the search path.

Possible Solutions

Verify Spelling of Function or Variable Name

One of the most common causes is misspelling the function or variable name. Especially with longer names or names containing similar characters (such as the letter l and numeral one), it is easy to make mistakes and hard to detect them.

Often, when you misspell a MATLAB function, a suggested function name appears in the Command Window. For example, this command fails because it includes an uppercase letter in the function name:

accumArray
Undefined function or variable 'accumArray'.
 
Did you mean:
>> accumarray

When this happens, press Enter to execute the suggested command or Esc to dismiss it.

Verify Inputs Correspond to the Function Syntax

Object methods are typically called using function syntax: for instance method(object,inputs). Alternatively, they can be called using dot notation: for instance object.method(inputs). One common error is to mix these syntaxes. For instance, you might call the method using function syntax, but to provide inputs following dot notation syntax and leave out the object as an input: for instance, method(inputs). To avoid this, when calling an object method, make sure you specify the object first, either through the first input of function syntax or through the first identifier of dot notation.

Make Sure Function Name Matches File Name

When you write a function, you establish its name when you write its function definition line. This name should always match the name of the file you save it to. For example, if you create a function named curveplot,

function curveplot(xVal, yVal)
     - program code -

then you should name the file containing that function curveplot.m. If you create a pcode file for the function, then name that file curveplot.p. In the case of conflicting function and file names, the file name overrides the name given to the function. In this example, if you save the curveplot function to a file named curveplotfunction.m, then attempts to invoke the function using the function name will fail:

curveplot
Undefined function or variable 'curveplot'.

If you encounter this problem, change either the function name or file name so that they are the same.

To Locate the file that defines this function, use the MATLAB Find Files utility as follows:

  1. On the Home tab, in the File section, click Find Files.

  2. Under Find files named, enter *.m

  3. Under Find files containing text, enter the function name.

  4. Click the Find button

Make Sure Necessary Toolbox Is Installed and Correct Version

If you are unable to use a built-in function from MATLAB or its toolboxes, make sure that the function is installed and is the correct version.

If you do not know which toolbox contains the function you need, search for the function documentation at https://www.mathworks.com/help. The toolbox name appears at the top of the function reference page. Alternatively, for steps to identify toolboxes that a function depends on, see Identify Program Dependencies.

Once you know which toolbox the function belongs to, use the ver function to see which toolboxes are installed on the system from which you run MATLAB. The ver function displays a list of all currently installed MathWorks® products. If you can locate the toolbox you need in the output displayed by ver, then the toolbox is installed. If you cannot, you need to install it in order to use it. If you cannot, you need to install it in order to use it. For help with installing MathWorks products, see Installation and Licensing.

Verify Path Used to Access Function Toolbox

Tip

If you have a custom file path, this step will delete it.

The MATLAB search path is a subset of all the folders in the file system. MATLAB uses the search path to locate files used with MathWorks products efficiently. For more information, see What Is the MATLAB Search Path?.

If the function you are attempting to use is part of a toolbox, then verify that the toolbox is available using ver.

Because MATLAB stores the toolbox information in a cache file, you need to first update this cache and then reset the path.

  1. On the Home tab, in the Environment section, click Preferences.

    The Preference dialog box appears.

  2. On the MATLAB > General page, select Update Toolbox Path Cache.

  3. On the Home tab, in the Environment section, select Set Path.

    The Set Path dialog box opens.

  4. Select Default.

    A small dialog box opens warning that you will lose your current path settings if you proceed. Select Yes if you decide to proceed.

Run ver to see if the toolbox is installed. If not, you may need to reinstall this toolbox to use this function. For more information about installing a toolbox, see How do I install additional toolboxes into an existing installation of MATLAB.

Once ver shows your toolbox, run the following command to see if you can find the function:

which -all <functionname>

replacing <functionname> with the name of the function. If MATLAB finds your function file, it presents you with the path to it. You can add that file to the path using the addpath function. If it does not, make sure the necessary toolbox is installed, and that it is the correct version.

Confirm The License Is Active

If you are unable to use a built-in function from a MATLAB toolbox and have confirmed that the toolbox is installed, make sure that you have an active license for that toolbox. Use license to display currently active licenses. For additional support for managing licenses, see Manage Your Licenses.