1. Intro to Programming using Python Lecture #2
Yosef Mendelsohn
With many thanks to Dolores Kalayta for the original version of these notes.

2. Strings
In addition to number and Boolean values, Python supports string values ( i.e. non-numeric values)

3. Strings
A string value is represented as a sequence of characters enclosed within quotes:
'Hello, how are you?'
A string value can be assigned to a variable.
greeting = 'Hello, how are you?'
word1 = "rock"
word2 = "climbing"
String values can be manipulated using string operators and functions
word1 + word2
"rockclimbing"
word1 + ' ' + word2
rock climbing
+ is the string concatenation operator
String and generally enclosed in single quotes, but double quotes are acceptable.
Double quotes are necessary if you have a single quote within the string
For example print (“That’s all folks”)

4. Strings: indexing and slicing
greeting = 'Hello, how are you?' word1 = "rock" word2 = "climbing" word1[0] 'r' --> this is known as 'indexing' word1[0:2] 'ro' --> this is known as 'slicing' Note that the second number is exclusive word1[-1] 'k'
+ is the string concatenation operator
String and generally enclosed in single quotes, but double quotes are acceptable.
Double quotes are necessary if you have a single quote within the string
For example print (“That’s all folks”)

5. Concatenation
Adding two or more strings together to make a longer string is known as concatenation
This is used a lot in programming
Only string data can be concatenated
Example:
>>> age = 16 >>> print('Adam is ' + age + ' years of age') TypeError: must be str, not int
Key point: Since the data type of 'age' is int, we get an error. Recall that we can only concatenate strings.
>>> age = 16
>>> print ('Adam is ' + age + ' years of age')
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
print ('Adam is ' + age + ' years of age')
TypeError: must be str, not int
>>> print ('Adam is ' + str(age) + ' years of age') Ad
Adam is 16 years of age
There are many, many more operations and functions for strings,

6. Explicit conversion of int to string
One very common solution when dealing with conflicting data types, is to attempt to convert one data type to another.
The works for some (but only some!) values.
Converting "10" to 10 --> sure!
Converting "ten" to 10 --> not possible!
To change an int data type to a string use str()
>>> age = > 'age' holds an integer
>>> age = str(age) --> 'age' now holds an string!
>>> age = 16
>>> print ('Adam is ' + age + ' years of age')
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
print ('Adam is ' + age + ' years of age')
TypeError: must be str, not int
>>> print ('Adam is ' + str(age) + ' years of age') Ad
Adam is 16 years of age
There are many, many more operations and functions for strings,

7. Explicit conversion of int to string
One very common solution when dealing with conflicting data types, is to attempt to convert one data type to another.
The works for some (but only some!) values.
Converting "10" to 10 --> sure!
Converting "ten" to 10 --> not possible!
To change an int data type to a string use str()
>>> age = 16 >>> print('Adam is ' + age + ' years of age') TypeError: must be str, not int
>>> print ('Adam is ' + str(age) + ' years of age') Adam is 16 years of age
>>> age = 16
>>> print ('Adam is ' + age + ' years of age')
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
print ('Adam is ' + age + ' years of age')
TypeError: must be str, not int
>>> print ('Adam is ' + str(age) + ' years of age') Ad
Adam is 16 years of age
There are many, many more operations and functions for strings,

8. Casting
Casting refers to converting between data types. It is very important in programming.
As mentioned a few moments ago, sometimes casting is possible (e.g. "10" to 10), and sometimes it isn't (e.g. "ten" to 10).
Numeric types / constructors
Converting something to an integer: int(value)
x = "322"
y = int(x) --> y holds the integer 322
Converting a value to a float: float(value)
x = "3.14" --> x holds the string "3.14"
y = float(x) --> y holds the float 3.14
x = 3 --> x holds the integer 3
y = float(x) --> y holds the float 3.0

9. Checking the data type type()
Because data types are so important, you might want to keep in mind the function
type()
which will tell you the data type of an object.
x = "322"
y = int(x) --> y holds the integer 322
type(y) --> will output 'int'
x = 3 --> x holds the integer 3
y = float(x) --> y holds the float 3.0
type(x) --> outputs 'int'
type(y) --> outputs 'float'

10. Boolean expressions and operators

11. Boolean Expressions
How does Python interpret an expression like the following?
>>> 5 <= 18
Python can evaluate Boolean expressions, i.e. expression whose value is TRUE or FALSE
Python interprets 5 <= 18 as a statement whose truth is being questioned
True

12. bool Boolean data types, i.e. True or False, are called bool in Python
Boolean expressions often contain COMPARISON operators
Operator
Meaning
<
Less than
>
Greater than ==
Equal to (TWO equal signs) !=
Not equal to
<=
Less than or equal to
>=
Greater than or equal to
See notes:
Examples in IDLE
3 < 2
False
1 = 1
Syntax: can’t assign to literal # = is the assignment operator
# == is the Equal operator
1 == 1
True
1 != 1
== 12
n = 17
n/2 # this evaluates immediately to 8.5
8.5
>>> n// 2 8
>>> n == (n//2) * 2 do you know what is the value of n? Is this T or F?
Answer is FALSE

13. Boolean operators and, or, not and, or are binary operator
You have seen (or will see) these in discrete math classes
and, or are binary operator
expression1 and expression2
expression1 or expression2
not is a unary operator
ie: applies to a single value
not an expression
Boolean expressions can be combined into fairly complex statements using various combinations of boolean operators

14. Boolean operators expression1 and expression2
True if both expressions are true
False if any of the two expressions is false
expression1 or expression2
True if any of the two expressions are false
False if both expression are false
not expression
True if the expression is false
False if the expression is true

15. Operator Precedence ** * / // % + - <, >, ==, <=, >=, !=
Arithmetic then comparison then boolean **
* / // %
+ -
<, >, ==, <=, >=, !=
not
and or
Extra In IDLE
>>> 3 < 8 and 4 >= 12 False
>>> 3 < 8 or 7 >=15 True
>>> 3 < 8 and 7 >=15 False
>>> not (3 < 2) False
>>> 5 % 2 == 2 or 24 // 7 == 3 True

16. Try in IDLE 2 < 3 and 3 < 4 4 == 5 and 3 < 4 False and True
True and True
not(3 < 4)
not(True)
not(False)
4 + 1 == 5 or 4 – 1 < 4
4 + 1 == 5 and < 4

17. First interactive Python program
Allowing the user to interact with our programs

18. Variables: assignment statements
Formula for calculating compound interest
A is the value after t periods
P is the principal amount (initial investment)
r is the annual nominal interest rate ( not reflecting the compounding)
n is the number of times the interest is compounded per year
t is the number of years the money is invested
Recall our compound interest program
Given: n = 12 months, t = 10 years, p = 5000, r = 1/100

19. compound_interest_v1.py RESULTS:
======= RESTART: C:\Users\ymendels\Dropbox\401\compound_interest_v1.py =======
The investment balance is
The interest amount is
>>>

20. Problem
Calculate the new balance and the interest earned based on the values specified by the user
Need to get from the user:
Initial investment, P
Annual interest rate, r (expressed as float)
Number of times the interest is compounded per year, n
The number of years the money is invested, t
Return to user
New balance
Amount of interest earned
There are various ways of getting input from the user. We will start with input() and eval()

21. input() function / Casting
To have the user input data:
input('Enter name: ')
# input() function returns a string value
To save the data input, assign it to a variable
firstName = input('Enter name: ')
x = input('Enter number: ')
Important: Note that input() function returns a string
User types 4 – this is saved as the string '4'
However, we can not do mathematical calculations on text (string) data!
To convert a string to a number use the eval() function
num = int(num)  converts to an int
num = float(num) converts to a float
This is known as “casting”. See compound_interest_interactive_casting.py
Do in IDLE
#See compound_interest_interactive.py
# ask user for input values
P = eval(input('Enter initial investment: '))
r = eval(input('Enter annual interest rate: '))
n = eval(input('Enter number times interest is compounded per year: '))
t = eval(input('Enter the number of years: '))
A = P * ( 1 + (r/n)) ** (n * t)
balance = round(A, 2)
interest = round((balance - P),2)
print ('The investment balance is ', balance)
print ('The interest amount is ', interest)
Now – what if I got a 5% interest rate
Balance is
Interest is
input() and eval() are built-in functions

22. Practice Problem You scored x/100, y/100, z/100 on homework
What is your total homework percentage?
Modify the previous program to get the input from the user.
Calculate the area of a circle; let the user specify the radius of the circle.
See notes:
Try this practice problem asking the user to enter their 3 scores. Assume that all the homeworks are worth
100 points each.
( x + y + z)/300
(x + y + z) - Min (x, y z) /200 leave this for a homework exercise:
The instructor drops the lowest homework score; what is your homework percentage?
Using variable for radius of a circle, calculate the area of a circle – saved as practice_area_circle
# example using math library
# Area = pi * radius ** 2
import math
radius = eval(input('Input circle radius: '))
area = math.pi * (radius ** 2)
print ('Area of the circle is ',area)

23. Lists Another Python object Another type of object Float Integer
String
Boolean
Now we look at Lists

24. List A sequence of objects
Numbers
Strings
Combination of numbers and string
Other lists
Represented as a comma-separated sequence of objects enclosed within square-brackets [ ]
Example:
List of numbers [1,4,30, 16]
List of strings on next slide

25. Lists
Lists are created by placing a series of values (or variables) inside square brackets
Each item in the list is separate form the following item by a comma
Lists are ordered
names = ['Betty', 'Veronica', 'Jughead']
exam_scores = [87, 96, 72, 48, 93]
letter_grades = ['A', 'C', 'A', 'D']
random_stuff = ['Hello there!', 3.14, names]
Note some things about the last list, random_stuff:
It has different data types in it. This is fine, but not very common.
One of the items in the list is another list! Also perfectly fine. More common than lists of varying data types, but still not as common. Most lists – though by no means all – tend to contain a single data type.

26. Difference between strings and lists
The built-in Python function: type()
is a good one to know. It comes in handy surprisingly often when debugging.
Notation:
String – sequence of characters enclosed in quotes alphaString = ‘abcdefg’
List – is a sequence of objects
alphaList – [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’]
Strings are immutable objects
Lists are mutable objects – I can change the items in an existing list, but, I cannot change the characters in an existing string

27. Operators
Strings and Lists

28. + operator Concatenation of lists >>> s = ['he']
Concatenation of strings
>>> 'he' + 'llo'
'hello'
>>> s = 'Hello'
>>> t = 'World'
>>> s + t
'HelloWorld'
How do you get the space between Hello and World?
Concatenation of lists
>>> s = ['he']
>>> t = ['llo']
>>> s + t
['he', 'llo']
>>> s = [1, 2, 3]
>>> t = [4, 5, 6]
[1, 2, 3, 4, 5, 6]
Joins the 2 strings or joins 2 lists
Can only join a list to a list and a string to a string
>>> s = ‘Hello’
>>> t = ‘World’
Get space
>>> s + ‘ ‘ + t
#result of
>>> ‘3’ + ‘5” # is not 8 35
List1 + List2 = List
>>> '3' + '5'

29. Indexing Operator []
Individual characters of a string and items in a list
can be accessed using the indexing operator [ ]
s =['Apple','Orange', 'Peach']
s =
' A p p l e '
0 1 2
>>> s[0]
'A'
>>> s[4]
'e'
>>> s[-1]
What is returned from
s[1:4]? s[1:]? s[:-1]? s[::-1]?
3 4
>>> s[0]
'Apple'
>>> s[2]
'Peach'
>>> s[-1]
>>> 'Peach'
1 2
S[1:4] returns ‘ppl’ start at index 1 and go up to but do not include index 4 = substring or slicing
S[:5] returns ‘Apple’ all characters before index 5
5 is the length of the string
S[:len(5)] returns ‘Apple’
S[1:] returns ‘pple starts at index 1 to the end
S[:-1] returns ‘Appl’ starts at index 0 up to but not containing index[-1]
S[::-1] returns ‘elppA’ start at index -1 and returns the rest of the string – reverse order
List question
S[1][4] returns g in Orange item at index 1 and character at index [4]
s[: 2] returns [‘Apple’, ‘Orange’] starts at index 0 and does not include item at index 2
S[1:] returns [‘Orange’, ‘Peach’] includes items beginning at 1 and to the end of the list
What is returned from s[1][4]?

30. String and List Operators
Usage
Explanation
x in s
x is a substring of s
x not in s
x is not a substring of s
s + t
Concatenation of s and t
s * n, n * s
Concatenation of n copies of s
s[i]
Character at index i of s
String Operators
>>> help(str)
Usage
Explanation
x in lst
x is an item of lst
x not in lst
x is not an item of lst
lst + lstB
Concatenation of lst and lstB
lst*n, n*lst
Concatenation of n copies of lst
lst[i]
Item at index i of lst
S = ‘Orange’
‘r’ in S
‘a’ in S
‘b’ in S
‘b’ not in S
S * 3
lst = [‘a’, ‘b’, ‘c’]
‘a’ in lst
‘x’ not in lst
Lst * 3 [‘a’, ‘b’, ‘c’, ‘a’, ‘b’, ‘c’, ‘a’, ‘b’, ‘c’]
List Operators
>>> help(list)

31. Commonly used String and List functions
Usage
Explanation
len(s)
(function) Length of string s
Usage
Explanation
len(lst)
Number of items in lst
min(lst)
Minimum item in lst
max(lst)
Maximum item in lst
sum(lst)
Sum of items in lst
S = ‘Orange’
Len(s) returns 6
S= [‘Apple’, ‘Orange’, ‘Peach’]
Len(s) return 3 # items
Max(s) returns Peach – the last item in the list
Min(s) returns Apple – the first item in the list
Sum(s) unsupported operand types(s) – cannot sum strings
N = [3,2,4,1] list of integers
Sum(n) return 10
Min(n) returns 1
Max (n) returns 4
You can use min and max with a string, but not sum – get unsupported error
Method is the object-oriented word for function. That’s pretty much all there is to it (i.e. no real difference)
Functions are defined outside of a classes, while methods are defined inside or part of classes.

32. Count Method Method lst.count(item)
Returns the number of occurrences of item in list lst
s.count(target)
The number of occurrences of substring target in string s
>>> ages = [25, 21, 35, 34, 24, 23, 24, 21,21,35]
Ages.count(25) returns 1
Ages.count(21) returns 3
Rhyme = ‘Mary has a little lamb its fleece was white as snow. Everywhere that Mary went the lamb was sure to go’
Rhyme.count(‘the’) returns 1
Rhyme.count(‘Mary’) return 2
Rhyme.count(‘was) returns 2
Num = [1,2,3,4,5,1,2,3,4,4,]
Num.count(1) returns 2
Num.count(4) returns 3
S = ‘abcabcabcde’
s.count(‘ab’) returns 3

33. Variables and assignments
Mutable and immutable types
We need to understand how values are created in memory (RAM) – Python’s memory management

34. How variables are stored in memory
A value will have only one copy in memory and all the variables having this value will refer to this memory location. Example: Suppose you have the variables x, y and z having a value 10. This does not mean that there will be 3 copies of 10s in memory. In fact, there will be only one 10 and all three variables x,y, z will point (reference) this value. Once a variable is updated, say x = x + 1 a new value will be allocated in memory and x will point(reference) this value. A useful function when trying to understand this concept is: id() The function returns a form of the address where the value is stored in memory. >>> a = 10 >>> b = 10 >>> c = 10 >>> id(a), id(b), id(c) ( , , ) >>> a += 1 >>> id(a) id() will return an objects memory address (object’s identity). As you have noticed, when you assign the same integer value to the Variables, we see the same ids.

35. Mutable vs Immutable List Contents of a list can be changed - mutable
>>> s = ['Apple', 'Orange', 'Peach']
>>> print (s)
['Apple', 'Orange', 'Peach']
>>> t = s
>>> print (t)
>>> s[1] = 'Pear'
>>> print(s)
['Apple', 'Pear', 'Peach']
String
Contents of a string cannot be changed - immutable
>>> s = 'Apple , Orange, Peach'
>>> print (s)
Apple, Orange, Peach
>>> s[1] = 'Pear'
TypeError: 'str' object does not support item assignment
>> > s[1] = 'P'
What do we need to do to replace the first p in Apple with an uppercase P?
Draw the object diagram
>>> s = [‘Apple’, ‘Orange’, ‘Peach’]
>>> print(s)
t = s
>>> print (s)
[‘Apple’, ‘Orange’, ‘Peach’]
>>> print(t)
STRING
>>> s = ‘Apple, Orange, Peach’
To replace p with P
>>> s= ‘APple, Orange, Peach’
>>>Print (s)
APple, Orange, Peach
>>>Print(t)
Apple, Orange, Peach
S = ‘APple, Orange, Peach’
T = ‘Apple, Orange, Peach’
Now have two lists in memory

36. Assignment 3 4.5 'hello' [1, 2, 3] >>> a = 3
>>> type(a)
It is not the identifier (variable) that has a type. Rather it is the object assigned to that variable that has the type.
<class ‘int’>
>>> b = 4.5
>>> c = 'hello'
>>> d = [1, 2, 3] a b c d
If the object is mutable, replace value in the current object (list)
If the object is immutable, create a new object for the variable (string)
type(a) returns <class ‘int’> int is immutable
Assignment statement creates the object in memory and binds the name (variable) to the object
int
float
str
list 3
4.5
'hello'
[1, 2, 3]
Object a is of type int = object a belongs to class int

37. Example – mutable object
b is a list – mutable
At step 2 a and b refer to the same object
At step 4 change number in index 2 to 4
b is now [1,2,4]
a is now [1,2,4]
Because a list is mutable you can replace one number with another.
To variable pointing to the same mutable object changes the value for both variable objects
>>> a
>>> a = 10
>>> a
>>> b

38. Example – immutable object
>>> y = x
>>> y = 100
>>> x
>>> y
23 is an integer and is immutable
At step 2 x and y refer to the same object
At step 3 for y to 100 – cannot make the change, need to create a new object
x is now 23
y is now 100]
Because a an integer is immutable, you can not change the object value. Python will create a new object for the new value.

39. Swapping: The Traditional Approach
>>> x = 3
>>> y=4
>>> temp = x
>>> y = temp
Objective:
We want to swap the values of x and y
i.e. assign the value of y to x,
and assign the value of x to y
Traditional approach required 5 steps
In python: multiple assignment statement
x,y = y, x
Because a an integer is immutable, you can not change the object value. Python will create a new object for the new value.

40. Swapping: A Python Approach
>>> x=3
>>> y=4
>>> x,y = y,x
Objective:
We want to swap the values of x and y
Python gives us a really nice shortcut:
Traditional approach required 5 steps
In python: multiple assignment statement
x,y = y, x

41. Change the flow of control
Decision Structures
Change the flow of control
A Python program is a sequence of statements that are executed in succession; in order starting from the statement in line 1 and the next statement in succession. That is not what we usually experience when using an application on a computer. The IF statement can control which statements are executed based on some input.

42. Decisions if condition: Comparisons using relational operators
• ==, !=
• <, <=
• >, >=
Condition - comparison expression that evaluates to a Boolean value (True or False)
Comparisons using boolean operators
and or
not
Boolean variables
Python: Comparisons using in operator
x in s (x substring of s)
x not in s( x substring of s)
List
x in lst (x object in list)
x not in lst ( x object in list)
What objects may be in a list?
String
Decisions are based on a condition evaluating to True or False
You can compare numbers, strings , lists, Booleans.
Do the following in IDLE
If x < y and y < z uses relational operators and boolean operator – compound comparison
X = 3, y = 9, z = 10 print (‘yes’) returns yes
X = 3, y = 9, z = 6 print (‘yes’) returns empty
words = [‘hello’, ‘the’, ‘their’, ‘world’, ‘bye’, ‘good’]
S = ‘hello the their world bye good’
‘the’ in words
True
‘the’ in s
s.count(‘the’) returns 2
Words.count(‘the’) returns 1
Objects in a list may be strings, numbers (int, float)
Operand
Operations A B
A and B
A or B
Not A
True
False

43. <indented code block>
One-Way Decisions
if <condition>:
<indented code block>
<non-indented statement>
Condition described on previous slide
block of statements when condition tests true
must be indented.
Examples 1,2,3,4
Indented code block is one or more python statements; may be another if statement

44. Two-Way Decisions if <condition>:
<indented code block 1> else:
<indented code block 2>
<non-indented statement>
if <condition 1>:
<indented code block 1> elif <condition 2>:
<indented code block 2>
<non-indented statement>
Examples 5 and 6

45. Series of IFs if <condition_1>: <indented code block 1>
<non-indented statement>
Month = eval(input(‘enter month number: ‘)
If month == 1:
print (“January”)
If month ==2:
print(“February”)
If month
Print (“month is an invalid number”)

46. Nested IFs if <condition_1>: if <condition_A>:
<indented code block A>
else:
<indented code block not A>
<indented code block 1>
<non-indented statement>
temperature = eval(input(‘temperature: ‘)
If temperature > = 50:
if temperature >= 80:
print (‘Good day for swimming’)
else:
print (“Good day for golfing’)
Else:
(‘Good day to play tennis’)

47. Practice
Examples 1 - 4

48. Algorithms
IMPORTANT!!!!!
Watch out for the "yeah yeahs"…

49. Example 1
Problem: If age is greater than 62, print ‘You are eligible for social security benefits
Describe the algorithm
Get a number from the user
Determine if number is > 62
If number < Print message
Key Knowledge:
Do a numeric comparison
input() function returns a string; need to convert to integer
def example1():
' test for social security eligibility '
age = eval(input('Enter your age ')) # do first without eval
if age > 62:
print('You are eligible for social security benefits')

50. Example 2
Problem: If the value of variable name is in the list [‘Mozart’, ‘Puccini’, ‘Rossini’, ‘Wagner’, ‘Verdi’, ‘Strauss’], print ‘One of the 6 most popular opera composers’
Describe the algorithm
Define a list of composer names
Get a composer name (string) from the user
Determine if composer name is in the list Print the message
Key Knowledge:
Input from user is stored in the variable name
def example2():
' opera composers'
composers = ['Mozart', 'Puccini', 'Rossini', 'Wagner', 'Verdi', 'Strauss']
name = input('Enter your favorite opera composer ')
if name in composers: # list operator - in operator
print(name, 'is one of the 6 most popular opera composers')

51. Example 3
Problem: If a substring of characters appears more than 2 times in the string values associated with the variable report, then print ‘Yes’
Define the algorithm
Create and store a sequence of meaningful characters
Get sequence of characters from the user i.e. substring
Determine number of times substring occurs in the sequence
If number of occurrences is > Print the message
Key knowledge:
Store sequence of meaningful characters as a string in the variable report
String type has a count method
def example3():
' test for substring '
report = 'the rabbit jumped over the fence and there is no stopping her. \nif she does it again, then we will have to bring her into the barn.'
print(report)
letters = input('Enter string to search for ')
ct = report.count(letters)
#if report.count(letters) > 2:
#print('Yes', letters, 'occurs ' + str(report.count(letters)) + ' times')
if ct > 2:
print('Yes', letters, 'occurs ' + str(ct) + ' times')

52. Example 4
Problem: if at least one of the Boolean variables north, south, east, west is True, print ‘I can escape’
Describe the algorithm
Initialize Boolean variables to True or False
Determine Boolean operator that returns True if at least one value in the Boolean variables is True
If True Print the message
Key knowledge:
Boolean operator that returns True if at least one of the Boolean variables is True in ‘or’. Review truth tables.
Test case 1: all variables are False
Test case 2: all variables are True
Test case 3: two variable are False
Test case 4: three variables are False
Use:
def example4():
north = False south = False east = False west = False
If north or south or east or west:
print(‘I can escape’)
def example4a(): # output directions that you can escape from if there is time.
north = False
south = True
east = False
west = True
if north == True:
print ('I can escape north')
else:
print ('I cannot escape north')
if south == True:
print ('I can escape south')
print('I cannot escape south')
if east == True:
print ('I can escape east')
print ('I cannot escape east')
if west == True:
print ('I can escape west')
print ('I cannot escape west')