Does `union` in Lisp Alter the Lists Passed?
Lisp, known for its expressive power and flexibility, offers a wide range of functions to manipulate lists. One such function is `union`, which is used to combine two lists into a single list containing all the elements from both lists, without any duplicates. However, a common question that arises among Lisp users is whether the `union` function alters the lists passed to it. In this article, we will delve into this topic and provide a clear understanding of how the `union` function behaves in Lisp.
Understanding the `union` Function
The `union` function in Lisp takes two or more lists as arguments and returns a new list that contains all the elements from the input lists, with duplicates removed. The order of elements in the resulting list is not guaranteed to be the same as the order in the input lists. The syntax for the `union` function is as follows:
“`lisp
(union list1 list2 &rest more-lists)
“`
Here, `list1` and `list2` are the primary lists to be combined, and `&rest more-lists` allows for additional lists to be included in the union operation.
Does `union` Alter the Lists Passed?
The answer to the question of whether `union` alters the lists passed is a resounding no. The `union` function is designed to be non-destructive, meaning it does not modify the input lists. Instead, it creates a new list containing the union of the input lists. This behavior is evident from the fact that the `union` function returns a new list, while the original lists remain unchanged.
To illustrate this, consider the following example:
“`lisp
(let ((list1 ‘(a b c))
(list2 ‘(b c d)))
(union list1 list2))
“`
In this example, the `union` function is called with `list1` and `list2` as arguments. The resulting list is `(a b c d)`, which is the union of the two input lists. However, the original lists `list1` and `list2` remain unchanged:
“`lisp
(list1) ; Output: (a b c)
(list2) ; Output: (b c d)
“`
This demonstrates that the `union` function in Lisp does not alter the lists passed to it.
Conclusion
In conclusion, the `union` function in Lisp is a non-destructive operation that combines two or more lists into a new list containing all the elements, without duplicates. It does not modify the input lists, ensuring that the original lists remain unchanged. Understanding this behavior is crucial for effective list manipulation in Lisp programming.
