Introduction to Programming with Python – Class 3SYED FARJAD ZIA ZAIDI

Class Objectives

Class Objective

Understanding Strings and some useful string operations.

Write few simple programs that use string functions and lists.

Class Material

• Chapter 6, 8 - Python for Informatics: Exploring Information

Reading

Strings

Strings are amongst the most popular types in Python. We can create them simply by enclosing characters in quotes. Python treats single quotes the same as double quotes.

Creating strings is as simple as assigning a value to a variable. For example:

message = “Hello World”

Introduction to Indexing Strings can be divided into substrings using indexing.

To access substrings, use the square brackets for slicing along with the index or indices to obtain your substring, this is called Indexing. Following is a simple example:

message = 'Hello World!'

messageTwo = "Python Programming"

print "var1[0]: ", message[0]

print "var2[1:5]: ", messageTwo[1:5]

String Concatenation

String concatenation is a process in which two strings are merged together. Concatenation is very simple and uses the operator ‘+’ to concatenate.

For Example:

message = “Hello”

messageTwo = “ “

messageThree = “World”

print message + messageTwo + messageThree

Escape Characters

Following table is a list of commonly used escape or non-printable characters that can be represented with backslash notation.

An escape character gets interpreted; in a single quoted as well as double quoted strings.Backslash Notation Description

\n New Line

\t Space

\s Tab

String Special Operators

+

• Concatenation - Adds values on either side of the operator

*• Repetition -

Creates new strings, concatenating multiple copies of the same string

Python Comparison Operators

• Slice - Gives the character from the given index

[]

• Range Slice - Gives the characters from the given range

[ : ]

Python Comparison Operators

• Membership - Returns true if a character exists in the given string

in

• Membership - Returns true if a character does not exist in the given string

not in

Python Triple Quotes

Python's triple quotes comes to the rescue by allowing strings to span multiple lines, including verbatim NEWLINEs, TABs, and any other special characters.

String Functions:Method Description

capitalize() Capitalizes first letter of string

count(str, beg=0, end=len(string)) Counts how many times str occurs in string or in a substring of string if starting index beg and ending index end are given

find(str, beg=0, end=len(string)) Determine if str occurs in string or in a substring of string if starting index beg and ending index end are given returns index if found and -1 otherwise

index(str, beg=0, end=len(string)) Same as find(), but raises an error if str not found

len(string) Returns the length of the string

String Functions:

Go to the following link for more string functions:

http://www.tutorialspoint.com/python/python_strings.htm

You can also find string functions by writing the following in the python IDLE

help(str)

Exercise 1

Write code using find() and string slicing to extract the number at the end of the line below. Convert the extracted value to a floating point number and print it out.

text = "X-DSPAM-Confidence: 0.8475";

Exercise 2 Given a string and a non-negative int n, return a larger string that is n

copies of the original string. 

string_times('Hi', 2) → 'HiHi'string_times('Hi', 3) → 'HiHiHi'string_times('Hi', 1) → 'Hi'

Exercise 3 Given a string, return a string where for every char in the original,

there are two chars.

double_char('The') → 'TThhee'

double_char('AAbb') → 'AAAAbbbb'

double_char('Hi-There') → 'HHii--TThheerree'

Any Queries?

Link to Facebook Group:

https://www.facebook.com/groups/introtopython.iccbs/

Email:

farjad_bullet@rocketmail.com