Course A201: Introduction to Programming

Course A201:Introduction to Programming 10/28/2010

Outline for this week • List • Lists are mutable!!; List indexing and slicing; List functions • Access Items in nested sequences • String function split() and join() • Shared references and list COPY • Dictionary • Structure; How to use get(); other dictionary functions

Lists are Mutable • Change Item value using index in list • Strings and Tuples are NOT mutable! >>> b = [“Python”, “C++”, “Java”, “HTML”] >>> b[0] = “PHP” >>> print(b) >>> b[1:3] = [“PHP”] >>> print(b) >>> b[1:2] = “PHP” • What will happen if you forgot to put “PHP” in []?

Lists are Mutable • Change Item value using index in list • Strings and Tuples are NOT mutable! >>> b = [“Python”, “C++”, “Java”, “HTML”] >>> b[0] = “PHP” >>> print(b) >>> b[1:3] = [“PHP”] >>> print(b) >>> b[1:3] = “PHP” • b will become: • [“PHP”, “P”, “H”, “P”] • So, DO NOT forget [].

Lists are Mutable • Delete items by using keyword del >>> b = [“Python”, “C++”, “Java”, “HTML”] >>> del b[1] >>> print(b) >>> del b[:] >>> print(b)

List Indexing and Slicing • Just like String/Tuple indexing and slicing >>> b = [“Python”, “C++”, “Java”, “HTML”] >>> b[0:2] >>> b[-2] >>> b[0:1000] >>> b[4]

List Methods • Refer to Lecture Slides: List Methods

Recall Function&Method [functions] • input1, input2, input3, … • You give me some inputs I am a function. I will perform a certain job. • I give you some outputs back, explicitly or implicitly • output1, output2, output3, …

List Methods • What are the jobs of these list methods perform and what are the return values? • So, how about append(), remove(), sort(), index(), insert()?

List Methods • The return values of append(), remove(), sort() and insert() are None! • Try this: >>> b = [“Python”, “C++”, “Java”, “HTML”] >>> b = b.append(“Perl”) >>> print(b) None • So, NEVER NEVER write code in this manner!

Access Items in nested sequences • Refer to Lecture Slides: Access Items in nested sequences

Access Items in nested sequences • Try this: for i in range(len(scores)): print(i, “:”, scores[i][0], “scored”, scores[i][1]) vs. for i in range(len(scores)): name, score = scores[i] print(i, “:”, name, “scored”, score) • A special way of assignment two variables at the same time using tuple: name, score = (‘Joe’, 2000)

An example with Lists • Refer to Lecture Slides: An example with Lists

More math functions • Refer to Lecture Slides: More functions (related to your assignment this week)

String method: split() • Split one string into a list, you can specify the “split-symbol” >>> string = “1 2 3 4” >>> string.split() [‘1’, ‘2’, ‘3’, ‘4’] >>> string = “1,2,3,4” >>> string.split(“,”) [‘1’, ‘2’, ‘3’, ‘4’] The value in variable string stays the same!

String method: join() • Join all the elements in one list into one string, you can also specify the “join-symbol” >>> string = “#” >>> list1 = [‘1’, ‘2’, ‘3’, ‘4’] >>> string.join(list1) ‘1#2#3#4’ >>> string = “%” >>> string.join(list1) ‘1%2%3%4’ The value in variable string, and list1 stays the same!

Shared reference and COPY • Refer to Lecture Slides: Shared reference >>> list1 = [1, 2, 3, 4] >>> list2 = list1 >>> list3 = list1[:] >>> list1[0] = ‘hello’ >>> print(list1) [‘hello’, 2, 3, 4] >>> print(list2) [‘hello’, 2, 3, 4] >>> print(list3) [1, 2, 3, 4] • This is shared reference • This is making a COPY

Dictionaries • Refer to Lecture Slides: Dictionaries

Have a nice evening! • See you tomorrow~

Course A201: Introduction to Programming