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CS61B Lecture #3: Containers
- Readers Available: from Vick Copy (corner of Euclid and Hearst) not Copy Central! Also online.
- Please read Chapter 2 of Assorted Materials on Java from the reader.
- Room change: Discussion 114 (3-4 Thurs.) is now in 289 Cory (used to be 3111 Etch.)
- Midterm is tentatively scheduled for the evening of 9 March (Thurs- day).
- Project 1 will be due the preceding week (1 March).
- Today. Simple classes. Scheme-like lists. Destructive vs. non- destructive operations. Models of memory.
Values and Containers
- Values are numbers, booleans, and pointers. Values never change.
3 ’a’ true
- Simple containers contain values:
x: 3 L: p:
Examples: variables, fields, individual array elements, parameters.
- Structured containers contain (0 or more) other containers:
h t
h: 3 t:
0 17
1 9
2
Class Object Array Object Empty Object
Alternative Notation
Containers in Java
- Containers may be named or anonymous.
- In Java, all simple containers are named, all structured contain- ers are anonymous, and pointers point only to structured containers. (Therefore, structured containers contain only simple containers).
p: 3
h t 7
h t
simple container (local variable)
structured containers (anonymous)
named simple containers (fields) within structured containers
- In Java, assignment copies values into simple containers.
- Exactly like Scheme!
Defining New Types of Object
- Class declarations introduce new types of objects.
- Example: list of integers:
public class IntList {
// Constructor function
// (used to initialize new object)
/** List cell containing (HEAD, TAIL). */
public IntList (int head, IntList tail) {
this.head = head; this.tail = tail;
// Names of simple containers ( fields)
public int head;
public IntList tail;
Destructive vs. Non-destructive
Problem: Given a (pointer to a) list of integers, L, and an integer in-
crement n, return a list created by incrementing all elements of the list
by n.
/** List of all items in P incremented by n. */
static IntList incrList (IntList P, int n) {
if (P == null)
return null;
else return new IntList (P.head+n, incrList(P.tail, n));
We say incrList is non-destructive, because it leaves the input objects
unchanged, as shown on the left. A destructive method may modify the input objects, so that the original data is no longer available, as shown on the right:
L:
Q:
After Q = incrList(L, 2):
L:
Q:
After Q = dincrList(L, 2) (destructive):
An Iterative Version
An iterative incrList is tricky, because it is not tail recursive.
Easier to build things first-to-last, unlike recursive version:
static IntList incrList (IntList P, int n) { if (P == null) return null; IntList result, last; result = last = new IntList (P.head+n, null); while (P.tail != null) { P = P.tail; last.tail = new IntList (P.head+n, null); last = last.tail; } return result; }
P: 3 43 56
An Iterative Version
An iterative incrList is tricky, because it is not tail recursive.
Easier to build things first-to-last, unlike recursive version:
static IntList incrList (IntList P, int n) { if (P == null) return null; IntList result, last; result = last = new IntList (P.head+n, null); while (P.tail != null) { P = P.tail; last.tail = new IntList (P.head+n, null); last = last.tail; } return result; }
P: 3 43 56
last:
result: 5
An Iterative Version
An iterative incrList is tricky, because it is not tail recursive.
Easier to build things first-to-last, unlike recursive version:
static IntList incrList (IntList P, int n) { if (P == null) return null; IntList result, last; result = last = new IntList (P.head+n, null); while (P.tail != null) { P = P.tail; last.tail = new IntList (P.head+n, null); last = last.tail; } return result; }
P: 3 43 56
last:
result: 5 45
An Iterative Version
An iterative incrList is tricky, because it is not tail recursive.
Easier to build things first-to-last, unlike recursive version:
static IntList incrList (IntList P, int n) { if (P == null) return null; IntList result, last; result = last = new IntList (P.head+n, null); while (P.tail != null) { P = P.tail; last.tail = new IntList (P.head+n, null); last = last.tail; } return result; }
P: 3 43 56
last:
result: 5 45
An Iterative Version
An iterative incrList is tricky, because it is not tail recursive.
Easier to build things first-to-last, unlike recursive version:
static IntList incrList (IntList P, int n) { if (P == null) return null; IntList result, last; result = last = new IntList (P.head+n, null); while (P.tail != null) { P = P.tail; last.tail = new IntList (P.head+n, null); last = last.tail; } return result; }
P: 3 43 56
last:
result: 5 45 58
