Lecture 8 strings and characters

Introduction to computational thinking

Module 8 : Strings and Characters

Module 8 : Strings and Characters 1

Asst Prof Michael LeesOffice: N4‐02c‐76

email: mhlees[at]ntu.edu.sg

Contents

1. What is a string (defining)2. Accessing strings3. String operations4. Functions and Methods for

Strings5. 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.

CharacterSequence


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 verticallyordered’”


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 = “Hello World”

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 OPERATIONSModule 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’Module 8 : Strings and Characters 24 of 53

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

Module 8: Strings and Characters


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 methodmyStr = ‘Python Rules!’

myStr.upper() ‘PYTHON RULES!’

myStr.upper().find(‘H’)

=> 3


CHALLENGE 8.1 – SECRET MESSAGECreate 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 SPACERemove 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 VOWELSmake 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 STRINGSModule 8: Strings and Characters


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 quotesformat

• 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!Module 8 : Strings and Characters 48

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 widthprint(‘{:>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 tableCreate 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)


Technology

Lecture 8 strings and characters