Digression: Scoring Matrices
Lecture 11
Strings processing and functions
L11 Strings Processing – 2
Objectives
• Revision of string data type
• To get familiar with processing of strings
• To get familiar with various operations that can
be performed on strings through built-in
functions
• Take examples related to string processing
L11 Strings Processing – 3
Revision: String Data Type
A string is a sequence of characters enclosed within
quotation marks (“) or apostrophes (‘)
>>> str1=”Hello”
>>> str2=’spam’
>>> print(str1, str2)
Hello spam
>>> type(str1)
>>> type(str2)
L11 Strings Processing – 4
Revision: String indexing
H e l l o B o b
0 1 2 3 4 5 6 7 8
In a string of n characters, the last character is
at position n-1 since we start counting with 0.
>>> greet[1]
‘e’
>>> greet[6]
‘B’
>>> greet[-1]
‘b’
>>> greet[-3]
‘B’
L11 Strings Processing – 5
Revision: String slicing / substring
H e l l o B o b
0 1 2 3 4 5 6 7 8
>>> greet[0:3]
‘Hel’
>>> greet[5:9]
‘ Bob’
>>> greet[:5]
‘Hello’
>>> greet[5:]
‘ Bob’
>>> greet[:]
‘Hello Bob’
This is same as greet
L11 Strings Processing – 6
Revision: String operations
:
L11 Strings Processing – 7
Simple String Processing
• Abbreviations of species names.
– In a particular bioinformatics database, names of
species are abbreviated to the first 3 letters of the first
part (genus) and first 2 letters of the second part
(species)
– For example
• Canis familiaris (dog) ⇒ CANFA
• Pan troglodytes (chimp) ⇒ PANTR
L11 Strings Processing – 8
Simple String Processing
# get genus and species names
genus = input(“Please enter the genus: “)
species = input(“Please enter the species: “)
SPECIES_CODE = genus[:3] + species[:2]
# Convert to upper case
SPECIES_CODE = SPECIES_CODE.upper()
• We’ll be looking at string functions, including upper(),
later
• This form of functions are called methods. Notice
variable name followed by dot followed by function call.
L11 Strings Processing – 9
String 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.
• The mapping of characters to binary codes is
arbitrary
– as long as everyone uses the same
mapping.
L11 Strings Processing – 10
String Representation
• In the early days of computers, each manufacturer
used their own encoding of numbers for characters.
• ASCII system (American Standard Code for
Information Interchange) uses 127 characters using
8-bit (1 byte) codes
• Python also supports Unicode which maps 100,000+
characters using variable number of bytes
• http://www.asciitable.com/
http://www.asciitable.com/
L11 Strings Processing – 11
String Representation
• The ord function returns the numeric (ordinal) code of
a single character.
• The chr function converts a numeric code to the
corresponding character.
>>> ord(“A”)
65
>>> ord(“a”)
97
>>> chr(97)
‘a’
>>> chr(65)
‘A’
Note that ‘A’ < ‘a’
L11 Strings Processing - 12
Programming an Encoder
• Using ord and chr we can convert a string into and out of
numeric form.
• The encoding algorithm is simple:
get the message to encode
for each character in the message:
print the letter number of the character
• A for loop iterates over a sequence of objects, so the for
loop over a string looks like:
for
L11 Strings Processing – 13
Programming an Encoder
# text2numbers.py
# A program to convert a textual message into a sequence of
# numbers, utlilizing the underlying Unicode encoding.
def main():
print(“This program converts a textual message into a sequence”)
print (“of numbers representing the Unicode encoding of the message.\n”)
# Get the message to encode
message = input(“Please enter the message to encode: “)
print(“\nHere are the Unicode codes:”)
# Loop through the message and print out the Unicode values
for ch in message:
print(ord(ch), end=“,”)
print() # Go to new line at the end of code sequence
Replace new line by comma
L11 Strings Processing – 14
Programming an Encoder
L11 Strings Processing – 15
Programming an Encoder
# A program to convert a textual message into a sequence of
# numbers, utilizing the underlying Unicode encoding.
Improved
def main():
print(“This program converts a textual message into a sequence”)
print(“of numbers representing the Unicode encoding of the message.\n”)
message = input(“Please enter the message to encode: “)
print(“\nHere are the Unicode codes:”)
for ch in message[:-1]:
print(ord(ch), end=”,”)
print(ord(message[-1]))
L11 Strings Processing – 16
Programming a Decoder
• We now have a program to convert messages into a
type of “code”, but it would be nice to have a
program that could decode the message!
• The outline for a decoder:
get the sequence of numbers to decode
message = “”
for each number in the input:
convert the number to the appropriate character
add the character to the end of the message
print the message
L11 Strings Processing – 17
Programming a Decoder
• The variable message is an accumulator variable,
initially set to the empty string, the string with no
characters (”” or ‘’).
• Each time through the loop, a number from the
input is converted to the appropriate character and
appended to the end of the accumulator.
• How do we get the sequence of numbers to decode?
• Read the input as a single string, then split it apart
into substrings, each of which represents one
number.
L11 Strings Processing – 18
Programming a Decoder
• The new algorithm:
get the sequence of numbers as a string, inString
split inString into a sequence of small strings (of digits)
message = “” # Empty string
for each of the smaller strings:
change the digits into the number they represent
append the ASCII character for that number to message
print message
string → list of digit strings → list of numbers → string
“102,114” → [“102”,”114”] → [102,114] → “fr”
L11 Strings Processing – 19
Programming a Decoder
This is a list.
More about
them in the
next lecture
• Strings have useful methods associated with them
• One of these methods is split. This will split a string into
substrings based on a separator, e.g space.
>>> “Hello string methods!”.split()
[‘Hello’, ‘string’, ‘methods!’]
• This is the same as:
>>> a = “Hello string methods!”
>>> a.split()
L11 Strings Processing – 20
Programming a Decoder
• Split can use other separator characters other than
space, by supplying the character as a parameter.
>>> “32,24,25,57”.split(“,”)
[’32’, ’24’, ’25’, ’57’]
L11 Strings Processing – 21
Programming a Decoder
# numbers2text.py
# A program to convert a sequence of Unicode numbers into
# a string of text.
def main():
print (“This program converts a sequence of Unicode numbers into”)
print (“the string of text that it represents.\n”)
# Get the message to encode
inString = input(“Please enter the Unicode-encoded message: “)
# Loop through each substring and build Unicode message
message = “”
for numStr in inString.split(“,”):
codeNum = int(numStr) # convert the (sub)string to a number
# append character to message
message = message + chr(codeNum)
print(“\nThe decoded message is:”, message)
L11 Strings Processing – 22
Programming a Decoder
• The split function produces a sequence of strings.
• Each time through the loop, the next substring is:
– Assigned to the variable numStr
– Converted to the appropriate Unicode character
– Appended to the end of message.
L11 Strings Processing – 23
Programming a Decoder
L11 Strings Processing – 24
From Encoding to Encryption
• The process of encoding information for the purpose
of keeping it secret or transmitting it privately is
called encryption.
• Cryptography is the study of encryption methods.
• Encryption is used when transmitting credit card
and other personal information through an insecure
medium e.g. Internet.
• The Unicode mapping between character and
number is an industry standard, so it’s not “secret”.
L11 Strings Processing – 25
Encryption : Substitution Cipher
• In a simplistic way, if we replace the Unicode with some other
code (that is only known to the sender and receiver) we will
achieve encryption.
• This is called substitution cipher, where each character of the
original message, known as the plaintext, is replaced by a
corresponding symbol in the cipher alphabet.
• The resulting code is known as the ciphertext.
• This type of code is relatively easy to break.
• Each letter is always encoded with the same symbol, so using
statistical analysis on the frequency of the letters and trial and
error, the original message can be determined.
L11 Strings Processing – 26
More String Methods
• There are a number of other string methods. Try
them all! https://docs.python.org/3/library/stdtypes.html#string-methods
– str() – Return a string representation
– s.capitalize() – Copy of s with only the first
character capitalized
– s.title() – Copy of s; first character of each
word capitalized
– s.center(width) – Center s in a field of given
width
https://docs.python.org/3/library/stdtypes.html#string-methods
L11 Strings Processing – 27
More String Methods
– s.count(sub) – Count the number of
occurrences of sub in s
– s.find(sub) – Find the first position where sub
occurs in s
– s.join(list) – Concatenate list of strings into
one large string using s as separator.
– s.ljust(width) – Like center, but s is left-
justified
– s.rjust(width) – Like ljust, but s is right-
justified
L11 Strings Processing – 28
More String Methods
– s.lower() – Copy of s in all lowercase letters
– s.lstrip() – Copy of s with leading whitespace
removed
– s.replace(oldsub, newsub) – Replace
occurrences of oldsub in s with newsub
– s.rfind(sub) – Like find, but returns the right-
most position
L11 Strings Processing – 29
More String Methods
– s.rstrip() – Copy of s with trailing whitespace
removed
– s.split() – Split s into a list of substrings
– s.upper() – Copy of s; all characters converted
to uppercase
L11 Strings Processing – 30
Summary
• We learned how strings are represented in a
computer.
• We learned about functions which can be used with
strings.
• We took examples where various operations can be
performed on strings.
• We learned fundamental concept of Cryptography.
Lecture 11�Strings processing and functions
Objectives
Revision: String Data Type
Revision: String indexing
Revision: String slicing / substring
Revision: String operations
Simple String Processing
Simple String Processing
String Representation
String Representation
String Representation
Programming an Encoder
Programming an Encoder
Programming an Encoder
Programming an Encoder
Programming a Decoder
Programming a Decoder
Programming a Decoder
Programming a Decoder
Programming a Decoder
Programming a Decoder
Programming a Decoder
Programming a Decoder
From Encoding to Encryption
Encryption : Substitution Cipher
More String Methods
More String Methods
More String Methods
More String Methods
Summary