Lecture 8 strings and characters

Introduction to
computational thinking
Module 8 : Strings and Characters
Module 8 : Strings and Characters 1
Asst Prof Michael Lees
Office: N4‐02c‐76
email: mhlees[at]ntu.edu.sg

Contents
1. What is a string (defining)
2. Accessing strings
3. String operations
4. Functions and Methods for
Strings
5. Formatting Strings
Chapter 4

What is a string?
• A string is a sequence of characters
• Notionally, a character is a letter or a symbol
• But, we’ll find out about characters later.

Defining a string
• A string is indicated using:
– ‘ ‘ or “ “
• The sequence of characters is important,
and is maintained.
Character
Sequence

Defining a string (more)
• Then there is “““ ””” (triple quotes)
• Preserves vertical and horizontal formatting
• E.g.,
“““ This is
a vertically
and horizontally formatted string. ”””
• Compare with:
“This is not vertically
ordered’”

More definitions
• Using single or double is fine:
– S = “a string”
– S = ‘a string’
• Don’t use both!
– S = “a string’ WRONG!
• How to put in a apostrophe?
– S = “Mike’s string” #Use different quotes
– S = ‘Mike’s string’ #Use an escape character

Index
• Elements (or characters) in a string are in a
sequence, we can identify each element with
an index ( a position in the sequence )
• Can start index from either end of sequence.
– Positive values indicate counting from left, starting
at 0
– Negative values indicate counting from right,
starting with ‐1
Module 8 : Strings and Characters 7 of 53

Hello World
• String = “Hello World”
• Index 8 and ‐3 point to same location: r

Accessing one element
• We can use [ ] to access particular characters
in a string:
myStr = “Hello World”
x = myStr[1]
print x #will print e
print mystr =>?
print myStr[-2] => ?
print myStr[11] => ?

Slicing: parts of a String
• You can also select a part of the string, slice of
characters
• Similar syntax, [ start : finish ]
– start : index of start of subsequence
– finish: index of position after we want the
subsequence to end (i.e., the sequence will not
contain character myStr[finish])
• If start or finish are not specified they default to the
beginning and end of the string respectively.

Slicing: parts of a String
• Slicing uses half‐open ranges, common in
python. (Different from fully closed ranges)
• The first index is included, the last index is the
one after what is included.
myStr[1:4] => ?

Slicing: Examples
print(myStr[1:6])
print(myStr[1:2])
print(myStr[-7:-1])
print(myStr[-3:-5])
print(myStr[:6])
print(myStr[5:])
ello
e
o Worl
‘ ‘ (empty)
Hello
World
Output

One more example
myStr[3:-2]

Slicing and skipping
• We can specify a third argument:
– [ start : finish : countBy ]
• countBy specifies a ‘skip’
• Defaults:
– start = beginning
– finish = end
– countBy = 1
myStr[1:11:2] =>’el ol’

Slicing and skipping
myStr[::2] =>’HloWrd’

Python Idioms
• Idiom : “A form of expression natural to a language, person, or
group of people”
• These are things particularly unusual/unique to python
• Copying a string:
aString = “String to copy”
newStr = aString[:]
newStr = ‘’.join(aString)
• To reverse a string:
aString = “Madam I’m Adam”
revString = aString[::-1] => ?

STRING OPERATIONS
Module 8: Strings and Characters

Basic operations
opStr = “Basic”
• length of a string : len()
len(opStr) => 4
• Concatenate strings : +
opStr + “ operations”
=> “Basic operations”
• Repeat String : *
opStr * 3 => “BasicBasicBasic”

More operations
• Operations + and * will make new strings,
input arguments not changed
• Order? important for +, irrelevant for *
• Arguments to operators must be of certain
type:
– + : two strings
– * : string and integer
• Notice the meaning of + depends on argument type (works
with integers also)

The type function
• Easy to check type of variable:
myStr = ‘hello world’
type(myStr) => <type ‘str’>
myStr2 = 245
type(myStr2) => <type ‘int’>
myStr + myStr2 => ?
myStr * myStr2 => ?

Back to Characters
• There are two common systems for
representing characters in machines now (to
store a character in memory it must be binary,
so we give it a code)
• ASCII (older) and Unicode (more modern,
more characters)
• Tables indicate mapping of characters to ASCII
or Unicode

ASCII vs Unicode
• ASCII stores everything in 8 bits, so can only
manage 256 different characters.
• Unicode, was an extension of ASCII to include
all characters of all languages.
• The Unicode code space is divided into 17
planes. Each plane contains 65,536 code
points (16‐bit) and consists of several charts.
• Total of: 1,114,112 possible, uses 96,000.

Getting the code
• ord() takes a string of length 1 as input and
will return the unicode of the character (for
standard symbols this is same as ASCII)
• chr() takes an ASCII code and returns a string
on length 1 containing the corresponding
character
ord(‘a’) => 97
chr(97) => ‘a’
Encrpyt (see later):
code = ord(‘a’)
chr(code+1) => ‘b’

Comparing characters
• When comparing a single character the code
is used to compare.
‘a’ == ‘a’ => true
‘a’ < ‘b’ => true
‘A’ < ‘B’ => true
‘1’ < ‘9’ => true
‘a’ < ‘B’ => false!

Comparing strings
• Checks first element in strings, if equal move
to each in turn (left to right)
• If they are not equal, the relationship of the
string is the same as those two characters
• If one string is shorter (but equal to a point),
the shorter string is smaller.

Examples
• ‘aa’ < ‘ab’
– First element is same, move to second. a < b so
‘aa’ < ‘ab’ => true
• ‘aaab’ > ‘aaac’
– First three elements same, b<c so ‘aaab’ > ‘aaac’
=> false.
• ‘aa’ < ‘aaz’
– The first string is the same but shorter. Thus it is
“smaller” => true.

Membership operations
• Does one string contain another.
– a in b: returns true is string b contains string a
myStr = “abcdefg”
‘c’ in myStr => true
‘cde’ in myStr => true
‘cef’ in myStr => false
myStr in myStr => true

Strings are Immutable
– aStr = ‘spam’
– aStr[1] = ‘l’  ERROR
• However, you can use it to make another
string (copy it, slice it, etc).
– newStr = aStr[:1] + ‘l’ + aStr[2:]
– aStr  ‘spam’
– newStr => ‘slam’
Module 8 : Strings and Characters
• Strings are immutable, that is
you cannot change one once
you make it:
29

FUNCTIONS & METHODS FOR
STRINGS

Functions reminder
• A function is a piece of code that performs
some operation. Details are hidden
(encapsulated), only it’s interface exposed.
• Way to arrange a program to make it easier to
understand.
• Functions have arguments as input and may
return some output.

A string fuction: len()
• We have already seen one string
function:
–len(‘test string’) => 11(not 10)
• Input: a string
• Output : an integer indicating length of
string.

String method
• A method is a variation on a function
– like a function, it represents a program
– like a function, it has input arguments and an
output
• Unlike a function, it is applied in the context of
a particular object.
• This is indicated by the ‘dot notation’
invocation
• Each string is itself and object.

Example
• upper() is a string method.
• It will output a new string which is the same as
the string on which it was called, except all letters
will now be upper case.
myStr = “shouting!”
mystr.upper() => “SHOUTING!”
• object : myStr
• method : upper()
• method call: myStr.upper()

Methods in general
• We’ll touch on this more later.
• But in general the form is:
– object.method()
• We say object is calling the method method.
• Different objects have different methods available,
defined by the type (class).
• How to find out all methods available on strings? Use a
reference! : Python online…
– http://docs.python.org/lib/string‐methods.html
• Iintegrated Development Environment (IDE) such as
IDLE will help.

Find
myStr = “Find in a string”
myStr.find(‘d’) => 3
• find is another string method, called using
same object.method() notation.
• It takes a single character string as input and
returns the index of the position where the
character is first seen (from left to right).
• ‘d’ is called an argument of the method.

Chaining methods together
Methods can be chained together.
• Perform first operation, yielding an object
• Use the resulting object for the next method
myStr = ‘Python Rules!’
myStr.upper()  ‘PYTHON RULES!’
myStr.upper().find(‘H’)
=> 3

CHALLENGE 8.1 – SECRET MESSAGE
Create a simple program to do simple encrypting and decrypting of a
string. Nothing too complicated: just so a human cannot read the
encrypted message.

Thought process
• Character replacement.
• Replace each character in the string with a
different one
• Need to be able to translate back
• A lookup table?
• Or simple way to shift the ASCII code?

CHALLENGE 8.2 – REMOVE WHITE SPACE
Remove all white space characters from a string

Though process
• Similar solution? iterate and re‐build string
without space?
• Think about the task, is it common?
• If it seems common, there is probably a built‐
in method!
• Look in the python reference (online or book)

CHALLENGE 8.3 – MESSING WITH VOWELS
make all vowels in the second half of a string capitalized

Thought process
• Common? No
• What do we have available to us?
• A method…
• Iterate through string, check if its in second
half of string (using len()) and re‐build,
changing case where necessary
• Alternatively, split the string in half first.

FORMATTING STRINGS

Pretty printing
• The default print function is fine.
• More is possible
• format allows us to perform low‐
level type‐setting (prettier strings)

Basic form
• Look at an example:
n = 500
description = “a lot!”
print(“In this class we have {}
students, which is {}”.format(n,
description))
• This will show:
In this class we have 500 students, which is a lot!

Basic form
• format creates a new string!
“In this class we have {} students,
which is {}”.format(n, description)
• This outputs a new string where {} will be
replaced by the data inside the format
method (this case n and description)

Example
print(“In this class we have {} students, which is {}”.format(n, description))
String in quotes
format
• Objects are matched in order with format
descriptors. The substitution is made and resulting
string created (then printed)
In this class we have 500 students, which is a lot!

Format String
• We can put ‘stuff’ inside the {} which specifies
how the data should be printed (i.e., make it
pretty)
• Overall:
{:[align] [min_width] [.precision][descriptor]}
[ ] means optional

Descriptors (codes)
• s string
• d decimal
• e floating point exponent
• f floating point decimal
• u unsigned integer

width and alignment
• < left
• > right
• ^ center
• Integers indicate width
print(‘{:>10s} is {:<10d} years old.’.format(‘Bill’, 25))
• ‘Bill’ is printed right aligned as a string, with at least
10 spaces.

Example of space
print(“MIKE, OFFICE: {:3d} TEL: {:<10d} ”.format(76, 67906277))
MIKE, OFFICE: 76 TEL:67906277
3 spaces wide,
right justified
(by default)
10 spaces wide,
left justified

Precision
• print(math.pi)
3.14159265359 #default is 11
• print('{:.4f}'.format(math.pi))
3.1416 (4 decimal places of precision,
with rounding)
• print('{:10.2f}'.format(math.pi))
• ‘ 3.14’ (10 spaces for
characteristic and the decimal point – not
including mantissa)

CHALLENGE 8.4 – ASCII table
Create a formatted table of all the ASCII characters and codes.
Code should be first item in table (right aligned) and character second
item in table (left aligned). Only do printable characters 33 to 127

Thought process
• Iterate through the characters
• Iterate how? Use code from 30 to 127.
• Use chr() and ord() functions to display
• For each character we should print a
formatted string…

A Solution
for x in range(33, 127):
print('|{:3d}|{:<3s}|'.format(x, chr(x)))

Take home lessons
• Understand various operations on strings,
remember the basic ones.
• Understand slicing and indexing
• ASCII and Unicode: what they are and how to
use them
• Method vs. function
• The use of triple quotes (formatting)
• Strings are useful… especially for files.

Further reading/exercises
• http://code.google.com/edu/languages/google‐python‐class/strings.html
• http://diveintopython3.org/strings.html
• http://docs.python.org/tutorial/introduction.html#strings
• (I always assume you will read the book)

Lecture 8 strings and characters

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