Indexing into Function Call Results in MATLAB

MATLAB provides the option to return an array, vector, struct, etc. as the function return value. This means that the user can return an array/vector from a function call’s result. Naturally, there is a need to access those arrays/vectors and use them further in a MATLAB workspace. 

In this article, we shall see how to index function results in a MATLAB script with the help of various examples and methods.

The Dot (.) Indexing

This method of dot indexing is similar to indexing a function from a class definition. We first call a given function with required parameters and then, index its various local variables (properties) using the . indexing. See the syntax of the same below:

function out = square(var)

out = struct(“prop”, var.^2);

end
square(x).prop;

Here, we used dummy function that calculates the square of any given number or vector. When the function is called, we then use the (.) indexing and get the value of the square which is stored as a key : value pair in a struct.

This method can be used when you do not want to want to create unnecessary output variables. This dot indexing will directly extract the required value from the temporary variable prop, which was created by the function call and thus, is not stored in the MATLAB workspace. Now, let us see some examples of the same.

Example 1

We shall use the square function as given in the syntax above and illustrate it with a specific case.

Matlab

% Code
square([1,2,3,4]).val

%function
function k = square(var)

%creating a struct to access the value as a property
k = struct('val',var.^2);
end

Here, we create a square function which computes the square of a vector and instead of accessing it by the traditional storing in variable way, we use the function call indexing (dot indexing) to obtain the value.

Output:

Here, we obtained the square of passed vector without storing it into any variable.

The working of the above code can be understood from another approach where we use a variable to hold the value temporarily, let us see how.

Using a Temporary Variable Instead

What we did in the previous example, can be done with the help of a temporary variable. See the code below to understand.

Matlab

% Code
k = struct('val',[1 2 3 4].^2);
k.val
clear k

%verifying that k variable is deleted
disp(k)

The above code computes the square of a given vector and stores it as a property of a struct k. Now, we can access this property using the dot indexing of structures in MATLAB and then, we delete the struct k using the clear function of MATLAB. To verify that the variable is deleted, we display it using disp(), which should throw an error.

Output:

As it can be seen, we used k as a temporary variable and deleted it once its work was done. The dot (.) indexing in function call results does the same thing without the requirement of creating any extra variables.

Conclusion

This article discussed how to perform indexing into function call results in MATLAB using the dot (.) indexing method and explained the same with the help of an example. We also compared the same with the help of another example, illustrating the stepwise execution of the same dot(.) indexing method.