Python Lecture 6

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.
BS GIS Instructor: Inzamam Baig
Lecture 6
Fundamentals of Programming

Strings
A string is a sequence of characters. You can access the characters
one at a time with the bracket operator
>>> fruit = 'apple'
>>> letter = fruit[1]
The second statement extracts the character at index position 1 from
the fruit variable and assigns it to the letter variable
The expression in brackets is called an index

>>> fruit = 'apple'
>>> print(letter)
p
in Python, the index is an offset from the beginning of the string, and the offset of the first
letter is zero
>>> fruit = 'apple'
>>> print(letter)
a

>>> letter = fruit[1.5]
TypeError: string indices must be integers
a p p l e
0 1 2 3 4
Index Number

len
len is a built-in function that returns the number of characters in a
string
>>> fruit = 'apple'
>>>len(fruit)
5

>>> length = len(fruit)
>>> last = fruit[length]
IndexError: string index out of range

>>> length = len(fruit)
>>> last = fruit[length-1]
e

Negative Indexing
>>> fruit[-1]
e
a p p l e
-5 -4 -3 -2 -1
Index Number

Traversal through a string with a loop
fruit = 'apple'
index = 0
while index < len(fruit):
letter = fruit[index]
print(letter)
index = index + 1

Traversal through a string with a loop
fruit = 'apple'
for char in fruit:
print(char)

String slices
A segment of a string is called a slice. Selecting a slice is similar
to selecting a character
s = 'Pythonista'
print(s[0:6])
Python
print(s[6:12])
ista

>>> fruit = 'apple'
>>> fruit[:3]
app
>>> fruit[3:]
le

>>> fruit = 'apple'
>>> fruit[3:3]
If the first index is greater than or equal to the second the result
is an empty string

Strings
Strings are immutable
It is tempting to use the operator on the left side of an assignment,
with the intention of changing a character in a string
>>> greeting = 'Hello, world!'
>>> greeting[0] = 'J'
TypeError: 'str' object does not support item assignment

>>> greeting = 'Hello, world!'
>>> new_greeting = 'J' + greeting[1:]
>>> print(new_greeting)
Jello, world!

Looping and Counting
The following program counts the number of times the letter "a"
appears in a string
word = 'apple'
count = 0
for letter in word:
if letter == 'p':
count = count + 1
print(count)

The in operator
The word in is a boolean operator that takes two strings and
returns True if the first appears as a substring in the second
>>> 'a' in 'apple'
True
>>> 'seed' in 'apple'
False

String comparison
if word == 'apple':
print('Word Found.')

if word < 'apple':
print('Your word,' + word + ', comes before
apple.')
elif word > 'apple':
print('Your word,' + word + ', comes after
apple.')
else:
print('All right, apples.')

String Methods
Strings are an example of Python objects.
An object contains both data (the actual string itself) and methods,
which are effectively functions that are built into the object and are
available to any instance of the object
Python has a function called dir which lists the methods available for
an object
The type function shows the type of an object and the dir function
shows the available methods

>>> stuff = 'Hello world'
>>> type(stuff)

>>> dir(stuff)
['capitalize', 'casefold', 'center', 'count',
'encode', 'endswith' , 'expandtabs', 'find',
'format', 'format_map', 'index' , 'isalnum',
'isalpha', 'isdecimal', 'isdigit', 'isidentifier' ,
'islower', 'isnumeric', 'isprintable', 'isspace' ,
'istitle', 'isupper', 'join', 'ljust', 'lower',
'lstrip' , 'maketrans', 'partition', 'replace',
'rfind', 'rindex' , 'rjust', 'rpartition', 'rsplit',
'rstrip', 'split' , 'splitlines', 'startswith',
'strip', 'swapcase', 'title' , 'translate', 'upper',
'zfill']

>>> help(str.capitalize)
Help on method_descriptor: capitalize(...)
S.capitalize() -> str Return a capitalized version
of S, i.e. make the first character have upper case
and the rest lower case.
>>>

>>> word = 'apple'
>>> new_word = word.upper()
>>> print(new_word)
APPLE

A method call is called an invocompute scienceion; in this case,
we would say that we are invoking upper on the word

Find
>>> word = 'apple'
>>> index = word.find('a')
>>> print(index)
0

The find method can find substrings as well as characters:
>>> word.find('le')
3
It can take as a second argument the index where it should start:
>>> word.find('p', 2)
2

White Space Removal
One common task is to remove white space (spaces, tabs, or
newlines) from the beginning and end of a string using the strip
method
>>> line = ' Here we go '
>>> line.strip()

Startwiths
>>> line = 'Have a nice day'
>>> line.startswith('Have')
True
>>> line.startswith('h')
False

Lower and Upper
>>> line.lower()
'have a nice day'
>>> line.upper()
'HAVE A NICE DAY'

Parsing Strings
Often, we want to look into a string and find a substring
From adnan@kiu.edu.pk Sat Jan 5 09:14:16 2008
and we wanted to pull out only the second half of the address
(i.e., kiu.edu.pk) from each line
we can do this by using the find method and string slicing

>>> data = 'From adnan@kiu.edu.pk Sat Jan 5 09:14:16 2008'
>>> at_position = data.find('@')
>>> print(at_position)
10
>>> space_position = data.find(' ',at_position)
>>> print(space_position)
21
>>> host = data[at_position+1:space_position]
>>> print(host)
kiu.edu.pk
>>>

First, we will find the position of the at-sign in the string
Then we will find the position of the first space after the at-sign
And then we will use string slicing to extract the portion of the
string which we are looking for

Format Operator
The format operator, % allows us to construct strings, replacing
parts of the strings with the data stored in variables
>>> number = 42
>>> '%d' % number
42
>>> dollars = 42
>>> 'I have spotted %d dollars.' % dollars
'I have spotted 42 dollars.'

>>> 'In %d years I have spotted %g %s.' % (3, 0.1, 'dollars')

>>> '%d %d %d' % (1, 2)
TypeError: not enough arguments for format string
>>> '%d' % 'dollars'
TypeError: %d format: a number is required, not str

String format()
The format() reads the type of arguments passed to it and
formats it according to the format codes defined in the string
# default arguments
print("Department {}, Field {}.".format("Computer
Science", "GIS"))

# positional arguments
print("Field {1}, Department {0}.".format("Computer
Science", "GIS"))

# keyword arguments
print("Department {depart}, Field
{field}.".format(depart = "Computer Science", field
= "GIS"))

# mixed arguments
print("Department {depart}, Field
{0}.".format("GIS", depart = "Computer Science"))

Number Formatting
Number Formatting Types
Type Meaning
d Decimal integer
c Corresponding Unicode character
b Binary format
o Octal format
x Hexadecimal format (lower case)
X Hexadecimal format (upper case)
n Same as 'd'. Except it uses current locale setting for number separator
e Exponential notation. (lowercase e)
E Exponential notation (uppercase E)
f Displays fixed point number (Default: 6)
F Same as 'f'. Except displays 'inf' as 'INF' and 'nan' as 'NAN'
g General format. Rounds number to p significant digits. (Default precision: 6)
G Same as 'g'. Except switches to 'E' if the number is large.
% Percentage. Multiples by 100 and puts % at the end.

# integer arguments
print("Decimal number:{:d}".format(123))
# float arguments
print("Floating point number:{:.2f}".format(123.4567898))
# octal, binary and hexadecimal format
print("Binary: {0:b}, Octal: {0:o}, Hexadecimal:
{0:x}".format(12))

Number Formatting
# integer numbers with minimum width
print("{:5d}".format(12))
# width doesn't work for numbers longer than padding
# padding for float numbers
print("{:8.3f}".format(12.2346))

# integer numbers with minimum width filled with
zeros
# padding for float numbers filled with zeros
print("{:08.3f}".format(12.2346))

Number formatting with alignment
Type Meaning
< Left aligned to the remaining space
^ Center aligned to the remaining space
> Right aligned to the remaining space
= Forces the signed (+) (-) to the leftmost position

# integer numbers with right alignment
print("{:>5d}".format(12))
# float numbers with center alignment
print("{:^10.3f}".format(12.2346))
# integer left alignment filled with zeros
print("{:<05d}".format(12))
# float numbers with center alignment
print("{:=8.3f}".format(-12.2346))

Python Lecture 6

More Related Content

What's hot (19)

Similar to Python Lecture 6 (20)

More from Inzamam Baig (13)

Recently uploaded (20)

Python Lecture 6

Editor's Notes