1. CMPT 120 Lecture 10 – Unit 2 – Cryptography and Encryption –
The realm of secret codes
Python – Range function

2. Last Lecture, we did …

3. Let’s implement our program now!
Problem Statement:
Write a program that encrypts messages using a transposition algorithm called “odd&even”
odd&even transposition algorithm:
Create a cypher that is made of 2 strings
String 1 contains the characters in odd positions
String 2 contains the characters in even positions

4. Let’s decrypt our message!
Problem Statement:
Write a program that decrypts messages that have been encrypted using the transposition algorithm odd&even

5. Our first decryption algorithm
In order to implement our first decryption algorithm, we need to know …
String concatenation
Arithmetic operator
Indexing
Slicing
len( ) function
range( ) function

6. Reading Review String Indexing: "Hello World!"[6] ->
message = "slicing"
message[ ] ->
String Slicing:
message[ : ] ->
message[ : : ] ->
" "[2:8:3] ->

7. Reading Review range( ) function
What will we see printed on the Python Interpreter shell window if :
range(10)
range(1, 11)
range(0, 30, 5)
range(0, 10, -3)
range(0, -10, -1)
range(0)
To see the sequence produced by the function range( ) on the Python Interpreter shell window :
list(range(...))

8. Review - Syntax of a for loop
Can be a string
<statement outside (before) the loop> for <iterating variable> in <sequence> : <first statement to be repeated> <second statement to be repeated> ... <last statement to be repeated> <statement outside (after) the loop>
Can be a list
Can be produced using range(…)

9. Review - Syntax of a for loop
<statement outside (before) the loop>
for <iterating variable> in <sequence> :
<first statement to be repeated>
<second statement to be repeated>
...
<last statement to be repeated>
<statement outside (after) the loop>
Important – About Indentation
Statements inside the loop (i.e., statements executed at each iteration of the loop) are the statements indented with respect to the for keyword
Statements outside the loop (before and after the loop) are the statements that are not indented with respect to the for keyword – these statements are considered to be at the same level of indentation as the for loop

10. Review - range( ) function
Very useful in for loop
Syntax: range([start,] stop [,step]))
Produces a list of integers
How does it work?
If theLength = 5
Then range(theLength)
produces [0, 1, 2, 3, 4]

11. Next Lecture Let’s practice what we have learnt so far
Practice Exam 2 -> paper-based