1. String operations; More on function definitions; Conditional execution
Intro2CS – week 2

2. Essence of Programming
Take basic operations
Combine them to do something more complex
print(“some string”)
print(“*******”)
print(“* HI! *”)

3. Abstraction
It is often useful to take something complex and encapsulate it.
Then we can use it as a new basic operation
def print_boxed_hi():
print(“*******”)
print(“* HI! *”)
print(“And now, a greeting:”)
print_boxed_hi()

4. Basic operations seen so far
Output
Assignment, Arithmetic operations
Turtle stuff
print(“Hello World”)
r1 = (-b + math.sqrt(b*b – 4*a*c)) / (2*a)
turtle.forward(150)

5. Combination methods seen so far
Sequential execution
Conditional Execution
Repeated Execution (loops)
do_something1()
do_something2()
Combination methods to come
if condition:
do_something()
for x in some_range_of_values:
do_something(x)

6. This Lecture More basic operations on Strings
More flexibility in defining new “operations” (functions)
Conditional execution
Later in the week: loops

7. What are all the basic operations?
“Old school” programming: minimal set of basic operations; come with the language; need to know all of them
“New school” programming: many basic operations; can get more libraries; need to know how to find what you need
Python philosophy: “batteries included”

8. Why spend time on String Operations?
Example of quickly learning new basic operations
No need to remember everything
Useful
Maybe you should remember…
Example of a complex data type
More to come later (e.g. list=[3, 5, 8] )

9. The Authoritative Reference

10. The Python Standard Library

11. Strings in The Python Standard Library
1. Introduction
2. Built-in Functions
3. Built-in Constants
3.1. Constants added by the site module
4. Built-in Types
4.1. Truth Value Testing
4.2. Boolean Operations — and, or, not
...
4.7. Text Sequence Type – str …
4.13. …
37. …

12. Google it

13. Stack Overflow

14. Strings are sequences of characters

15. Negative indices; Slices

16. Operations

17. Unicode

18. Unicode

19. Special character encoding

20. Input

21. More operations

22. Defining functions Parameters Return Values
Abstraction and Encapsulation

23. Defining and Using

24. Comments

25. Parameters
def print_twice(message): “““print the given message twice“““ print(message)
s = “Hello”
print_twice(s)

26. Parameters
def print_twice(message): “““print the given message twice“““ print(message)
Hello
s = “Hello”
print_twice(s) s

27. Parameters
def print_twice(message): “““print the given message twice“““ print(message)
Hello
s = “Hello”
print_twice(s) s
message

28. Parameters
def print_twice(message): “““print the given message twice“““ message = message + “\n” + message print(message)
s = “Hello”
print_twice(s)

29. Parameters
def print_twice(message): “““print the given message twice“““ message = message + “\n” + message print(message)
Hello
s = “Hello”
print_twice(s) s
message

30. Parameters
def print_twice(message): “““print the given message twice“““ message = message + “\n” + message print(message)
Hello
Hello\nHello
s = “Hello”
print_twice(s) s
message

31. Parameters
def print_twice(message): “““print the given message twice“““ message = message + “\n” + message print(message)
Hello
Hello\nHello
s = “Hello”
print_twice(s) s
message

32. Scope

33. Name conflict

34. Name conflict

35. Multiple parameters
def print_k_times(message, k): “““print k times the given message“““ message = k *(message + “\n”) print(message)
s = “Hello”
print_k_times(s, 3)

36. Multiple parameters
def print_k_times(message, k): “““print k times the given message“““ message = k *(message + “\n”) print(message)
s = “Hello”
print_k_times(message=s, k=3)

37. Multiple parameters
def print_k_times(message, k): “““print k times the given message“““ message = k *(message + “\n”) print(message)
s = “Hello”
print_k_times(k=3, message=s)

38. Default values

39. Returning a value
def cubic_root(x): “”” returns the cubic root of x “”” return x**(1/3)
y = cubic_root(8) # 2.0
print(cubic_root(y / 2) + 1)

40. Returning a value
import math def fourth_root(x): “”” returns the fourth root of x “”” y = math.sqrt(x) return math.sqrt(y)

41. Assigning multiple values

42. Returning multiple values
def my_divmod(x, y): “”” returns the quotient and remainder of the integer division x/y“”” return x//y, x%y
q, r = my_divmod(23, 4)

43. Returning multiple values
def split_date(s): “”” split a date give in the format dd/mm/yyyy into its components. returns year, date, month (as integers) “”” y = int(s[-4:]) m = int(s[3:5]) d = int(s[:2]) return y, m, d

44. Conditional Execution
If statement
More on Boolean values

45. Combining Operations Do this and then do that do_this() do_that()
In case 1 do this; otherwise do that
Case 1?
yes no
do_this()
do_this()
do_that()
do_that()
do_this()
do_that()
if case1():
do_this()
else
do_that()

46. If - else

47. Multiple statements in If-else

48. Elif and Nested Ifs
if age > 18: if country == “Germany”: drink = “beer” elif country == “France”: drink = “wine” elif country == “Russia”: drink = “vodka” else drink = “water” elif age > 5: drink = “soda” else: drink = “milk”

49. Boolealn Expressions

50. Boolean Functions

51. Another Implementation

52. If as an expression