Validate Name-Value Arguments

Name-value arguments associate a name with a value that is passed to the function. Name-value arguments:

Can be passed to the function in any order
Are always optional
Must be declared after all positional and repeating arguments
Cannot appear in an arguments block that uses the Repeating attribute
Must use unique names even when using multiple name-value structures
Cannot use names that are also used for positional arguments

Declare name-value arguments in an arguments block using dot notation to define the fields of a structure. For example, the structure named NameValueArgs defines two name-value arguments, Name1 and Name2. You can use any valid MATLAB^® identifier as the structure name.

arguments
    NameValueArgs.Name1
    NameValueArgs.Name2
end

The structure name must appear in the function signature.

function myFunction(NameValueArgs)

Call the function using the field names in the name-value structure.

myFunction(Name1=value1,Name2=value2)

Before R2021a, pass names as strings or character vectors, and separate names and values with commas. Both syntaxes are valid in later releases.

The name of the structure used in the function signature is the name of the structure in the function workspace that contains the names and values passed to the function.

function result = myFunction(NameValueArgs)
    arguments
        NameValueArgs.Name1
        NameValueArgs.Name2
    end

    % Function code
    result = NameValueArgs.Name1 * NameValueArgs.Name2;
end

r = myFunction(Name1=3,Name2=7)

r =

    21

Name-value arguments support partial name matching when there is no ambiguity. For example, a function that defines LineWidth and LineStyle as its two name-value arguments accepts LineW and LineS, but using Line results in an error. In general, using full names is the recommended practice to improve code readability and avoid unexpected behavior.

The same name-value argument can be repeated in a function call without error, but the last version specified is the one MATLAB honors. For example, this call to plot specifies the value for Color twice. MATLAB displays the plot in red.

plot(x,y,Color="blue",LineStyle="--",Color="red")

Default Values for Name-Value Arguments

You can specify a default value for each name. If you do not specify a default value and the function is called without that name-value argument, then that field is not present in the name-value structure. If no name-value arguments are passed to the function, MATLAB creates the structure, but it has no fields.

To determine what name-value arguments have been passed in the function call, use the isfield function.

For example, the following function defines two required positional arguments (width and height) and two name-value arguments (LineStyle and LineWidth). In this example, the options structure has two fields (LineStyle and LineWidth) containing either the default values or values specified as name-value arguments when the function is called.

function myRectangle(width,height,options)
    arguments
        width double
        height double
        options.LineStyle (1,1) string = "-"
        options.LineWidth (1,1) {mustBeNumeric} = 1
    end

    % Function code
    ...
end

All of these syntaxes are valid ways to call this function.

myRectangle(4,5)
myRectangle(4,5,LineStyle=":",LineWidth=2)
myRectangle(4,5,LineWidth=2,LineStyle=":")
myRectangle(4,5,LineStyle=":")
myRectangle(4,5,LineWidth=2)

Before R2021a, pass names as strings or character vectors, and separate names and values with commas. For example:

myRectangle(4,5,"LineStyle",":","LineWidth",2)
myRectangle(4,5,"LineWidth",2,"LineStyle",":")

Using Repeating and Name-Value Arguments

If the function defines repeating arguments, then you must declare the name-value arguments in a separate arguments block that follows the repeating arguments block. For example, this function accepts two repeating arguments, x and y. After specifying all repeats of x and y, you can specify a name-value argument that assigns the value lin or log to the PlotType name.

To determine if the function call includes the PlotType argument, use the isfield function to check for the PlotType field in the scale structure.

function myLinLog(x,y,scale)
    arguments(Repeating)
        x (1,:) double
        y (1,:) double
    end
    arguments
        scale.PlotType (1,1) string
    end
    z = reshape([x;y],1,[]);
    if isfield(scale,"PlotType")
        if scale.PlotType == "lin"
            plot(z{:})
        elseif scale.PlotType =="log"
            loglog(z{:})
        end
    end
end

Call this function with or without the name-value argument.

myLinLog(1:5,1:5)
myLinLog(1:5,1:5,1:10,1:100:1000)
myLinLog(1:5,1:5,1:10,1:100:1000,PlotType="log")

Before R2021a, pass names as strings or character vectors, and separate names and values with commas. For example:

myLinLog(1:5,1:5,1:10,1:100:1000,"PlotType","log")

Multiple Name-Value Structures

Function argument blocks can contain multiple name-value structures. However, the field names must be unique among all structures. This function has two name-value structures: lineOptions and fillOptions. These structures cannot have the same field names.

The arguments in the myRectangle function are:

width and height are required positional arguments of type double.
lineOptions.LineStyle is a scalar string with a default value of "-".
lineOptions.LineWidth is a scalar numeric with a default value of 1.
fillOptions.Color is a string.
fillOptions.Pattern has no restrictions on its value.

function myRectangle(width,height,lineOptions,fillOptions)
    arguments
        width double
        height double
        lineOptions.LineStyle (1,1) string = "-"
        lineOptions.LineWidth (1,1) {mustBeNumeric} = 1
        fillOptions.Color string
        fillOptions.Pattern
    end

    % Function Code
    ...
end

Robust Handling of Name-Value Arguments

The best practice for implementing name-value arguments in your functions is by defining them in an arguments block. An arguments block eliminates the need to write your own code to parse the name-value arguments, and the block also helps implement robust argument parsing for both the "name",value syntax and the name=value syntax introduced in R2021a.

Enforcing Valid Names

Defining a name-value argument in an arguments block ensures that the name is a valid identifier. In turn, this helps ensure that your arguments work with both "name",value and name=value syntaxes. For example, a name-value argument that uses an invalid identifier can work with the comma-separated syntax:

myFunction(data,"allow-empty",true)

However, the same call using allow-empty=true throws an error. Defining the name-value argument in an arguments block ensures that the names you define are valid MATLAB variable names and compatible with the name=value syntax.

Avoiding Unexpected Results with Text Inputs

Functions that include both optional text inputs and name-value arguments run the risk of MATLAB interpreting text inputs as the names of the name-value arguments. This function includes two optional text inputs and a name-value argument.

function mySignal(tag,unit,opts)
    arguments
        tag = "0"
        unit = "ampere"
        opts.Magnifier {mustBeMember(opts.Magnifier,["small","medium","big"])}
    end
end

A user enters this function call intending to set the values of tag to "Mag" and unit to "coulomb":

mySignal("Mag","coulomb")

However, MATLAB, through partial matching, parses "Mag" as the name-value argument Magnifer. "coulomb" is not a valid value for that name, so the function errors.

One way to avoid this is to make tag a required argument by removing its default value:

function mySignal(tag,unit,opts)
    arguments
        tag
        unit = "ampere"
        opts.Magnifier {mustBeMember(opts.Magnifier,["small","medium","big"])}
    end
end

Because MATLAB does not parse required inputs as name-value arguments, the same function call now sets the value of tag to "Mag" and does not error.

Another option is to make all three inputs name-value arguments. This helps users avoid mistakes when specifying inputs because each value is associated with a name, and the order the user specifies the inputs does not affect the end results.

Name-Value Arguments from Class Properties

A useful function syntax in MATLAB uses the public properties of a class for the names of name-value arguments. To specify name-value arguments for all settable properties defined by a class (that is, all properties with public SetAccess), use this syntax in an arguments block:

structName.?ClassName

A function can use the "structName.? ClassName" syntax only once. Therefore, a function can define only one name-value structure that gets its field names from a class, even if using different classes and structure names.

If the class places restrictions on values that you can assign to the property by using property validation, then the function applies the validation to the individual name-value arguments. For information on property validation, see Validate Property Values.

For example, this function has two required arguments, x and y and accepts any public property name and value for the matlab.graphics.chart.primitive.Bar class.

function myBar(x,y,propArgs)
    arguments
        x (:,:) double
        y (:,:) double
        propArgs.?matlab.graphics.chart.primitive.Bar
    end
    propertyCell = namedargs2cell(propArgs);
    bar(x,y,propertyCell{:})
end

Call the function with the required inputs and any settable property name-value pairs.

x = [1,2,3;4,5,6];
y = x.^2;
myBar(x,y)
myBar(x,y,FaceColor="magenta",BarLayout="grouped")

Before R2021a, pass names as strings or character vectors, and separate names and values with commas. For example:

myBar(x,y,"FaceColor","magenta","BarLayout","grouped")

Override Specific Properties

You can override the class property validation by redefining the property name with a specific name-value argument in the arguments block.

structName.?ClassName
structName.PropertyName (dim1,dim2,...) ClassName {fcn1,fcn2,...}

The specific name-value argument validation overrides the validation defined by class for the individually specified property name.

For example, the following function defines name-value arguments as the properties of the matlab.graphics.chart.primitive.Bar class. The function also overrides the property name FaceColor to allow only these specific values: red or blue.

The matlab.graphics.chart.primitive.Bar class has a default value for FaceColor that is not one of the restricted values (red or blue). Therefore, the overriding declaration must assign a default value that satisfies the restriction placed by the mustBeMember validation function. That is, the default value must be red or blue.

This function converts the name-value structure to a cell array containing interleaved names and values using the namedargs2cell function.

function myBar(x,y,propArgs)
    arguments
        x (:,:) double
        y (:,:) double
        propArgs.?matlab.graphics.chart.primitive.Bar
        propArgs.FaceColor {mustBeMember(propArgs.FaceColor,{'red','blue'})} = "blue"
    end
    propertyCell = namedargs2cell(propArgs);
    bar(x,y,propertyCell{:})
end

Call the function using the two required arguments, x and y. Optionally pass any name-value pairs supported by the bar function and a value for FaceColor that can be either red or blue. Other values for FaceColor are not allowed.

x = [1,2,3;4,5,6];
y = x.^2;
myBar(x,y)
myBar(x,y,FaceColor="red",BarLayout="grouped")