Mutation

1. Mutation • So far, our data abstractions consists of the following: • Constructors • Selectors/Accessors • Operations • Contract • Once a data object is created, it never changes. • Today, we’ll talk about how to alter the internal structure of data objects.

2. Why Mutation? • Saves space- can make small changes to existing object instead of creating new ones. • More freedom in creating data types- can freely manipulate pointers to alter structure of cons cells and lists.

3. Tools of Mutation: basic • (set! var x) • Evaluate x. Do not evaluate var. Instead, find its binding and change it to take on the value of x. • Difference between set! and define: • define always creates new binding. • set! alters existing binding.

4. Tools of Mutation: Data Structures • (set-car! pair x) • Changes car pointer in pair to point to value of x. • (set-cdr! pair x) • Changes cdr pointer in pair to point to value of x.

5. Example • (define x (list 1 2 3)) • We want to change x to the following: x 1 2 3 x 1 2 3

6. Example x 2 3 1 x 2 3 1 We want to change the cdr of the cddr of x to point to x: (set-cdr! (cddr x) x)

7. Side Effects • Mutation can cause unexpected side effects. • Example: (define a (list 1 2)) (define b a) (set-car! a 0) a => (0 2) b => (0 2) a b 1 2 a b 0 2

8. Equality • 2 tests for equality: • (eq? x y) • Tests if x and y point to exactly the same object. • If x and y are “eq”, then a change to one should be visible in the other as well • (equal? x y) • Tests whether x and y print to the same thing • Internal structure of x and y not necessarily the same.

9. Summary • Mutation allows us to change existing bindings and pointers. • Saves space and allows more programming freedom. • However, must be careful with side effects • New notions of equality: • Object equality: eq? • Looks the same: equal?