class 5 - lists

8/2/00 SEM107- © Kamin and Reddy Class 5 - Lists - 1

Class 5 - Lists

The list data typeRecursive methods on lists


Data structures

Contain large amounts of dataAllow for access to that dataMany different data structures, allowing

efficiency for various operations.


Lists

Lists are a simple data structure in which data are stored in a row.

We will talk first about lists of integers:x0, x1, ..., xn-1 (n >= 0)

Elements can be added and removed only at the beginning (x0) --- We don’t consider mutable lists.


Lists (cont.)

Terminology: First element (x0) is head of the list

List of remaining elements (x1, ..., xn-1) is tail of the list

Adding a new element to the front is called consing (for “constructing”)

The nil list is the list with no elements (n=0)


List operations

We assume a type IntList, for variables that contain such lists

We assume a class IL that defines the following class method: IntList cons (int i, IntList L) -

construct list containing i at front

And a class variable: IntList nil - the empty (zero-element) list


List operations (cont.)

IntList objects have the following instance methods: int hd () - return head of the list IntList tl () - return tail of the list boolean empty ()- is the list empty?


Examples of list operations

Suppose L is the list 2, 3, 5, 7

IL.cons(2,IL.cons(3,IL.cons(5,IL.cons(7,IL.nil)))) L.hd() returns 2L.tl() returns the list 3, 5, 7L.empty() returns falseIL.cons(13, L) returns the list 13, 2, 3, 5, 7IL.cons(13, L.tl()) returns the list

13, 3, 5, 7L.tl().tl() returns the list 5, 7


Example

We assume we have defined the classes IntList and IL as described above.

public static void main (String[] args) { IntList L = IL.nil; L = IL.cons(5, L); L = IL.cons(10, L); System.out.println(L.hd()); System.out.println(L.tl().hd());}


Recursion on lists

Writing recursive methods on lists follows the same principle as for integers: To compute f(L), assume f(L’) can be calculated

for lists L’ smaller than L, and use f(L’) to calculate f(L).

Some lists are small enough for f to be calculated directly


Example: printing lists

Print all the elements in a list, one per line: Assume you can print the tail of L (i.e. printList(L.tl())), so how do you print L?


Example: printing lists (cont.)

static void printList (IntList L) { if (!L.empty()) { System.out.println(L.hd()); printList(L.tl()); }}



Print all the elements in a list, all on one line, separated by a comma and a space: Assume you can print the tail of L (i.e. printList(L.tl())), so how do you print L?

Answer: print L.hd(), followed by a comma and space, but only if the tail of L is non-empty.



static void printList (IntList L) { if (L.empty()) return; else if (L.tl().empty())) System.out.print(L.hd()); else { System.out.print(L.hd()); System.out.print(“, “); printList(L.tl()); }}


Example: addtoend

The method

can be defined recursively. It adds the integer i at the end of the list L. For example, if L is the list 3, 5, 7, then addtoend(L, 2) returns the list 3, 5, 7, 2.

We will see how to define addtoend in the next class, but we can use it for ...IntList addtoend (IntList L, int i)


Example: reading integers

Given integer n, read n integers from the user and place them in a list. Assume you can read n-1 integers and place them in

a list; how can you read n integers? Answer: read the n-1 integers, creating a list L, then

read one more integer and place it at the end of L.


Example: reading integers (cont.)

static IntList readList (int n) { if (n == 0) return IL.nil; else { IntList L = readList(n-1); int i = Keyboard.readInt(); return addtoend(L, i); }}

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