Python Programming: An Introduction to Computer Science

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Mark Watts
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1 Python Programming: An Introduction to Computer Science Sequences: Strings and Lists Python Programming, 2/e 1

2 Objectives To understand the string data type and how strings are represented in the computer. To be familiar with various operations that can be performed on strings through built-in functions and the string library. To understand the basic idea of sequences and indexing as they apply to Python strings and lists. Python Programming, 2/e 2

To be familiar with various operations that can be performed on strings through

3 Outline String List Examples Python Programming, 2/e 3

4 String What s string? How to obtain substring? Build-in functions in String library Python Programming, 2/e 4

5 String What s string? How to obtain substring? Build-in functions in String library Python Programming, 2/e 5

6 String: Data Type The most common use of personal computers is word processing. Text is represented in programs by the string data type. A string is a sequence of characters enclosed within quotation marks (") or apostrophes ('). Python Programming, 2/e 6

7 Examples Print two strings Check string types: type Get string from keyboard input Python Programming, 2/e 7

8 String: Indexing We can access the individual characters in a string through indexing. The positions: Numbered from the left, starting with 0. Index from the right: negative indexes. <string>[<expr>] value of expr determines which character is selected from the string. Python Programming, 2/e 8

9 String: Indexing Notes: In a string of n characters, the last character is at position n-1 since we start counting with 0. Indexing returns a string containing a single character from a larger string. Python Programming, 2/e 9

10 String: Slicing We can also access a contiguous sequence of characters, called a substring, through a process called slicing. Slicing: <string>[<start>:<end>] start and end should both be ints The slice contains the substring beginning at position start and runs up to but doesn t include the position end. Python Programming, 2/e 10

Slicing: <string>[<start>:<end>] start and end should both be ints The slice

11 String operators Operator Meaning + Concatenation * Repetition <string>[] Indexing <string>[:] Slicing len(<string>) Length for <var> in <string> Iteration through characters Python Programming, 2/e 11

12 Examples: Given a string hello world! What is the character at the 6 th position? What is the character at the -3 rd position? Please print out the characters at the 1 st, 4 th, and 8 th position. Please print out world in the given string. What s the length of this string? Python Programming, 2/e 12

13 Simple String Processing Usernames on a computer system The scheme for generating usernames is to use the user s first initial followed by up tp seven letters of the user s last name. Example: Elmer Thudpucker: ethudpuc John Smith: jsmith Python Programming, 2/e 13

followed by up tp seven letters of the user s last name.

14 Simple String Processing Converting an int that stands for the month into the three letter abbreviation for that month. Python Programming, 2/e 14

15 Simple String Processing One weakness this method only works where the potential outputs all have the same length. How could you handle spelling out the months? Python Programming, 2/e 15

16 Lists Strings are always sequences of characters, but lists can be sequences of arbitrary values. Lists can have numbers, strings, or both! mylist = [1, "Spam ", 4, "U"] Python Programming, 2/e 16

17 Lists We can use the idea of a list to make our previous month program even simpler! Python Programming, 2/e 17

18 Lists Notes: The months line overlaps a line. Python knows that the expression isn t complete until the closing ] is encountered. Lists are mutable, meaning they can be changed. However, strings can not be changed. Python Programming, 2/e 18

19 Strings Representation Inside the computer, strings are represented as sequences of 1 s and 0 s, just like numbers. A string is stored as a sequence of binary numbers, one number per character. It doesn t matter what value is assigned as long as it s done consistently. Python Programming, 2/e 19

A string is stored as a sequence of binary numbers, one number per

20 String Representation How to program an encoder? Using python programs to automate the process of turning messages into sequence of numbers. How to program a decoder? Using python programs to turn a sequence of numbers back into a readable message. Python Programming, 2/e 20

into sequence of numbers. How to program a decoder?

21 Encoder program A for loop iterates over a sequence of objects, so the for loop looks like: for ch in <string> The ord function returns the numeric (ordinal) code of a single character. Python Programming, 2/e 21

22 Decode program How do we get the sequence of numbers to decode? Read the input as a single string split it apart into substrings, each of which represents one number. (split method) The chr function converts a numeric code to the corresponding character. Python Programming, 2/e 22

Other String/List Methods There are a number of other string methods. Try them all! http://docs.python.org/library/string.

23 Other String/List Methods There are a number of other string methods. Try them all! There are a number of other list methods. html Python Programming, 2/e 23

24 Example Input/Output as String Manipulation Often we will need to do some string operations to prepare our string data for output ( pretty it up ) Let s say we want to enter a date in the format 05/24/2003 and output May 24, How could we do that? Python Programming, 2/e 24

Example-solution Input the date in mm/dd/yyyy format (datestr) Split datestr into month, day, and year strings Convert the month string into a month

25 Example-solution Input the date in mm/dd/yyyy format (datestr) Split datestr into month, day, and year strings Convert the month string into a month number Use the month number to lookup the month name Create a new date string in the form Month Day, Year Output the new date string Python Programming, 2/e 25

Python Loops and String Manipulation

WEEK TWO Python Loops and String Manipulation Last week, we showed you some basic Python programming and gave you some intriguing problems to solve. But it is hard to do anything really exciting until