Midterm #1 Review

About These Slides

The first slides on formatting summarize the syntax of the format specifier and using print to format.

The remainder of the slides are just parts of all of the previous slides stitched together.

Formatting With Print

• use keyword arguments to define the separator between arguments and the character that gets printed at the end

print('foo', 'bar', 'baz', sep=",", end="x")

Format Function

The format function takes 2 arguments:

• the value to be formatted
• the format specifier (a string)

The format function returns a string

Format Function contintued

The syntax for the second argument, the format specifier is as follows:

[flags][width][.precision]type 

• some flags we used were > and < to specify where padding would be placed… and , to specify that a comma should be included
• width is the total width (number of characters returned)
• . and precision for number of decimal places
• type… which is the type of the value that we're expecting to format

Types for Formatting

These are some of the possible types that we can use in our format specifier:

• d - integer
• f - float
• s - string
• % - percent

If the type you specify does not match the type of the value of the first argument, you'll get an error.

format("2", ',d')
#   ValueError: Unknown format code 'd' for object of type 'str'

Format Examples

>>> format("2", '>10s')
'         2'
>>> format("hello", '>10s')
'     hello'
>>> format("hello", '<10s')
'hello     '
>>> format(2000.213, ',.1f')
'2,000.2'
>>> format(2000.213, '>20,.1f')
'             2,000.2'
>>> format(.2199, '.2%')
'21.99%'

Intro to Programming

Introductory Material

• mostly for short answer questions
• read the slides from the first class
• …as well as the first chapter
• know the vocabulary (high level vs low level languages, etc.)
• binary to decimal and back

Programs and Languages

Define: program →

Binary, Bits, and Bytes

What are the possible values that a bit can hold? →

More About Programming Languages!

What does a compiler do?

Tools

What's the name of the language we're using?

Tools Continued

What extension is used for Python source code files?

Values and Types

What are Values?

• values are just data
  • it can be stored in a variable
  • it can be computed in an expression
  • it can't be evaluated further (2 + 3 is not a value, because it can be evaluated further)
• some examples of values:
  • -123456
  • "a string is a value"
  • 1.00000001
  • True, False

Literals

The representation of a bare value in code is sometimes called a literal.

• "a string " a string literal
• 254 - an integer literal
• False - a bool literal

Values can be Classified by Data Type

A data type is a set of values. The type of a value determines how it can be used in expressions. Sometimes data type is also referred to as just type… or class. Name as many types as you can. →

Strings, Integers and Floating Point Numbers

Strings

• a string is a sequence of characters (any characters).
• including spaces and punctuation
  • and by spaces, I mean spaces, tabs, newlines, etc
  • for example " " is a string!
  • so is "" (this, however, is an empty string)
• are delimited by quotation marks - must start and end with quotes!
• (btw, a delimiter is just a character or characters that marks a boundary… or distinguishes a value from other values)
• for example "foo", 'bar', and """baz""" are all strings

Unbalanced Quotes

I'm sure you know by now what happens if you have an extra quote or a missing quote. →

>>> "oops " I quoted again"
SyntaxError: invalid syntax

There Are Three Ways to Quote Strings

What are the three ways to quote strings?

"double quoted string"
'single quoted string'
"""more than
one line.  omg!"""

When Would You Use One Over the Other?

• single or double quotes allow you to put in single or double quotes into a string without needing to escape them
  • "I'm"
  • 'use a " to delimit a string'
• triple double quotes allow you to span multiple lines (you can't span multiple lines with single or double quotes)

Constructing Strings / String Operators

So far, we've seen a couple of methods of putting strings together. What are they? →

What's String Concatenation?

String concatenation works intuitively. Describe what it is. →

#  syntax error
"lucky number " + 9

#  will work fine
"lucky number " + str(9)

String Formatting

What is string formatting? →

greeting = "Hi.  My name is %s and I have %s %s"
name = "Joe"
number_of_pets = 314
pet = "aardvarks"
result = greeting % (name, number_of_pets, pet)
print(result)

String Multiplication

You can also use the * operator to multiply a string by an int (note that this will give an error if you use a float instead of an int!)

s = "aardvark " * 3
print(s)
#  prints out: aardvark aardvark aardvark  

String Operators at a Glance

These are the string operators that we learned (note - they all return strings, but the types of arguments may vary depending on operator). What were they again? →

One Last Thing Regarding Strings (Escape!)

What do we do if we need a character that has special meaning in a string, such as a single or double quote? →

print("escaping using \"backslash\"")
print("single  quotes ''''") 
print("""some "double quotes"""")

BTW, Does Anyone Remember Comments?

What are the two ways that we can write in a comment?

counter = 5     # this is a comment that's on the same line as code
"""
this one
is a comment too
"""

ints and floats

Integers and floating point numbers are both numeric types.

• ints and floats are just numbers
• (no quotes needed… though a decimal point will automatically make a value into a float)
• however - Don't use commas! They don't mean what you expect. →
• sometimes ints and floats can be expressed with scientific notation (though we won't cover this in the exam)

Integers

int - integers

• whole number - negative or positive
• examples: 1751, -95
• "unlimited" (arbitrarily high)

Floating Point Numbers

float - floating point numbers

• real number - contains decimal point
• 10.00, 491.545532111
• may overflow
• small and large numbers expressed in scientific notation (though we won't cover this in the exam)

Operators

• addition, multiplication and subtraction: +, *, and -
• division: / (results in a float even if two integers - ex 10 / 2 → 5.0)
• integer division: // (next integer to the left - ex -23 // 3 → -8)
• modulo: % (ex 10 % 7 → 3)
• exponentiation: ** (ex 2 ** 3 → 8)

Numeric Operator Precedence

What order are these numeric operators evaluated in? →

>>> (5 + 15) * 2 ** 2 + 20
100

Numeric Operators and Type Compatibility

• most of these numeric operators will only work with numeric types (float, int)
• if you try to add, subtract, divide with an numeric type and a string, you will get an error
• always use int() or float() to convert your string to a numeric type if you're going to do calculations

Results of the Following Operations

What are the resulting values and types after evaluating the following expressions? →

• 28 / 7
• 28 // 10
• 28 % 10
• 28 / "7"

Implementing a Formula

An obvious use case for these numeric operations is implementing a formula.

For example: find the hypotenuse of a right triangle using this formula… h = √(x² + y²), where x is 8 and y is 6 →

import math
x, y = 8, 6
#  you can also use (some value) ** 0.5
h = math.sqrt(x ** 2 + y ** 2)

A Shortcut

There's a shortcut to add a value to an existing variable (the same exists for subtraction as well). What is the syntactic sugar that we use to increment and decrement variables? →

a = 0

#  increment a by some value, just set the variable, a, equal to a + 5
a = a + 5

#  or... alternatively, use a shortcut +=:
a += 5

Bools and Range Objects

The bool Type

Python has a bool type to represent Boolean values. Tell me everything you know about the bool type. →

Range Objects

One last type / class / kind of data that we've encountered was a range object. Describe what a range object is. →

A Guessing Game

1. 1
2. 1.0
3. "1"
4. """1.0"""
5. 1.111
6. '1,111'
7. True
8. range(5)
9. 1 + 5
10. "hello" + " my friends!!!"

A Guessing Game (Answers)

1. 1 - int
2. 1.0 - float
3. "1" - str
4. """1.0""" - str
5. 1.111 - float
6. '1,111' - str
7. True - bool
8. range(5) - range object
9. 1 + 5 - int
10. "hello" + " my friends!!!"

Built-in Functions

Conversion Functions

For each type that we learned, there's a function with the same name that will create a value of that type. They always return their associated type!

• int()
• str()
• float()
• bool()

range() is special in that it can take 1, 2 or 3 parameters to create a range object.

Conversion Functions Continued

• note that int() and float() will only accept int and float "like" values… "5" will be converted (even with spaces), but "five" will cause an error
• these can be used to avoid errors when using numeric or string operators

num = 99
print(str(num) + " red balloons")

• remember - you don't have to convert if your value is already the type you're converting to

num = "99" # no need to convert
print(num + " red balloons")

Other Built-In Functions

Name some built-in functions! →

A Word About input

• it takes one argument… which is what gets displayed to the user (the prompt)
• the program will wait until the user provides input
• input returns a string that represents the user's input
• even if the input looks numeric (say 234)
• it still returns a string (in the previous case "234")

Variables and Variable Naming

Variables

• variable - name that refers to a value
• this terminology is important; very specific… name and value
• we can now use that name instead of the explicit value
• the equals sign, = is the assignment token or assignment operator that we use to bind a name to a value
  • name on left
  • value on right

some_variable_name = "a value"

Assignment vs Equality

Don't confuse the assignment operator with the equality operator! What's the difference between the two? →

#  one equal sign is assignment
a = 1  

#  two equal signs mean test for equality
a == 1

Reassignment / Rebinding

• you can reassign or rebind
• let's see that in action →

>>> a = 23
>>> a
23
>>> a = "foo"

While We're on the Subject… Multiple Assignment

You can assign multiple variables in one line.

>>> a, b = 50, 60
>>> a
50
>>> b
60

Naming Variables

• you can make them as long as you want… though I suppose it could crash your computer
• names can only consist of numbers, letters and/or underscores
• the first character has to be a letter or an underscore
• case sensitive - case matters
• the name can't be a keyword or reserved word

Am I a Valid Name?

Are these valid variable names? →

• _foo
• 1_foo
• foo
• 1foo
• $foo
• foo$

Am I a Valid Name (Answers)?

Are these valid variable names? →

• _foo - yes
• 1_foo - no
• foo - yes
• 1foo - no
• $foo - no
• foo$ - no

Comparison Operators, Boolean Operators

Comparison Operators

Name six comparison operators?→

Comparison Operators Continued

• again - these operators always return a bool
• these operators do what you would expect
  • == - returns True if both sides are equal →
  • != - returns True if both sides are not equal →
  • >, <, >=, <= - returns True if value on the left is greater than, less than, greater than / equal, or less than equal to the value on the right →
• remember that the equal sign is always second for these operators >=, <=

Comparison Operators and Different Types

• objects of different types, except different numeric types, are never equal
  • equals (==) will always return False for different types →
  • not equals (!=) will always return True for different types →
• the <, <=, > and >= operators…
  • will raise a TypeError if the objects are of different types that cannot be compared →
  • will happily compare numeric types (for example comparing floats and ints works as you'd expect)! →

What are Logical Operators?

Logical Operators are operators that combine Boolean values.

• always return another Boolean value.
• can be used to create more complex Boolean expressions.

Three Logical Operators:

Name 3 logical operators, how many operands they take, and what operands they need to return True. →

Logical Operators in Action

>>> True and False
False
>>> True and True
True
>>> True or False
True
>>> not True
False
>>> not not True
True

Truth Table - AND

and takes two operands. Each operand can be True or False (or will evaluate to True or False!).

Can you guess how many possible combinations ther are for these two operands? What will the Truth Table look like? →

"""
(using p and q to represent the operands
...and t and f for true and false)
 p | q | p and q
----------------
 f | f | f
 f | t | f
 t | f | f
 t | t | t
"""

Truth Table - OR

Let's fill out a truth table for or! →

"""
(using p and q to represent the operands
...and t and f for true and false)
 p | q | p or q
----------------
 f | f | f
 f | t | t
 t | f | t
 t | t | t
"""

How About Something More Complicated?

Let's fill out a truth table for p and not q and r! →

"""
(using p and q to represent the operands
...and t and f for true and false)
 p | q | r | p and not q and r
-----------------------------
 t | t | t | f
 t | t | f | f
 t | f | t | t
 t | f | f | f
 f | t | t | f
 f | t | f | f
 f | f | t | f
 f | f | f | f
"""

Let's Evaluate Some Simple Boolean Expressions

• True and False →
• True and not False →
• True or False →
• True or not False →
• False or False →
• not False and not False →

Chaining Boolean, Comparison, and Other Operators

You can chain together operators to make complex Boolean expressions!

• Boolean expressions can involve Boolean, comparison, and other operators
• they can be arbitrarily complex! (don't do that)

What do you think this evaluates to? →

-10 ** 2 <= -100 or "foo" == "bar" and 100 >= 100

True

Order of Operations / Operator Precedence

A summary can be found in the official documentation for Python 3, but here's the short version:

• parentheses are evaluated first (obv)
• numeric / string operations (in turn, are also ordered)
• comparison operations (==, !=, >, <, >=, >=)
• not
• and
• or

Conditionals

Anatomy of an If Statement

Write an if statement testing if a and b are not equal. If they're not equal, print the value of a and b twice. →

a, b = "foo", "bar"

Let's See That Again

a, b = "foo", "bar"
if a != b:
	# totally ok?  yes!
	# but why?
	# perhaps when done more reasonably, readability
	print a
	print a


	print b

	print b

Oh Yeah, Else What?

We can use else to execute code if the original condition was not met

• go back one level of indentation to mark that the previous code block has ended
• keyword else
• body - indented, body ends when indentation goes back one level
• not required, obvs

What About Multiple Chained Conditions?

What if else is not fine-grained enough? For example, how about a program that asks for cake and handles a yes, no, or anything other than…

"""
Do you want cake?
> yes
Here, have some cake.

Do you want cake?
> no
No cake for you.

Do you want cake?
> blearghhh
I do not understand.
"""

Consecutive Ifs

One way to do it is consecutive if statements… →

answer = input("Do you want cake?\n> ")
if answer == 'yes':
        print("Here, have some cake.")
if answer == 'no':
        print("No cake for you.")
if answer != 'yes' and answer != 'no':
        print("I do not understand.")

Else If (elif)

We can use elif to chain a series of conditions, where only one path of code will be executed

• if statement like usual
• go back one level of indentation to mark that the previous code block has ended
• keyword elif
• condition
• colon
• body - indented, body ends when indentation goes back one level
• not required obv
• even if more than one true, only the first true executes!
• can add an else at the end

elif Example

How would we redo the cake exercise with elif? →

answer = input("Do you want cake?\n> ")
if answer == 'yes':
        print("Here, have some cake.")
elif answer == 'no':
        print("No cake for you.")
else:
        print("I do not understand.")

How to Order Conditions

• if more than one condition in a series of elif's is true
  • only the first true condition is executed!
  • other are skipped, including else
• ordering conditions so that the broadest ones are evaluated last may help avoid this issue
• be careful of conditions that never get evaluated
  • an above condition may already account for it
  • here's an example…

How to Order Conditions Continued!

Here's a contrived exercise:

• determine whether a number is 101 or if it's greater than 100
• if it's 101, it should only print out that it's "exactly 101" (it shouldn't print out greater than 100)

Here's an implementation. What gets printed if n = 200? What gets printed if n = 101? →

if n > 100:
	print("more than 100")
elif n == 101:
	print("exactly 101")

Ordering and Fizz Buzz

• fizz buzz
• print out 1 to 100 …with the following exceptions:
• for multiples of three, print out "Fizz" instead of the number
• for multiples of five, print out "Buzz" instead of the number
• for multiples of both three and five print “FizzBuzz”
• example output, next slide, plz!

FizzBuzz Output

1
2
Fizz
4
Buzz
Fizz
.
.
14
FizzBuzz
16
.
.
98
Fizz
Buzz

FizzBuzz Solution

for i in range(1, 101):
	if i % 3 == 0 and i % 5 == 0:
		print("FizzBuzz")
	elif i % 3 == 0:
		print("Fizz")
	elif i % 5 == 0:
		print("Buzz")
	else:
		print(i)

Another Gotcha!

Will this code work?? →

answer = input("Do you want cake?\n> ")
if answer == 'yes':
        print("Here, have some cake.")

if answer == 'no':
        print("No cake for you.")
else:
        print("I do not understand.")

Nesting If Statements

• it behaves as you'd expect
• remember to get indentation right
• if there are multiple elif's or else's, you can use indentation to see which initial if statement they belong to
• this works for any combination of if, elif and else
• note that sometimes nested if statements are equivalent to and
• best to simplify - that is, less nesting, better

Nested If Statements and And

How would you simplify this? →

a = 100
if a > 25:
	if a < 200:
		print("I'm in here")
print("We're out there")

a = 100
if a > 25 and a < 200:
	print("I'm in here")
print("We're out there")

Nested If Statements Example

We could have impemented the cake exercise using nested if statements. →

answer = input("Do you want cake?\n> ")
if answer == 'yes':
    print("Here, have some cake.")
else:
    if answer == 'no':
        print("No cake for you.")
    else:
        print("I do not understand.")

Modules

We Looked at 3 Modules

What are some modules we've used, and what do they do? →

So… What Can These Modules Do?

• math
  • pi - a constant that contains the value of pi→
  • floor(x) - returns the smallest integer less than or equal to x→
  • ceil(x) - returns the smallest integer greater than or equal to x→
  • sqrt(x) - returns the square root of x→
  • cos(x) - returns the cosine of x radians →

So… What Can These Modules Do (Continued)?

• random
  • random() - return a random float that's between 0 and 1→
  • randint(a, b) - returns a random int that's a <= n <= b→
• sys
  • exit([arg])_ - exits form python→
  • version - a constant that contains the version of Python→

Iteration

Sample While Loop

Write a program that continually asks the user for numbers, and asks them if they'd like to keep going. In the end, it should output the average of all of the numbers entered→

I'll calculate the average for you!
Current total: 0
Numbers summed: 0
Please enter a number to add
> 10
Do you want to continue adding numbers (yes/no)?
> yes
Current total: 10
Numbers summed: 1
Please enter a number to add
> 12
Do you want to continue adding numbers (yes/no)?
> no
The average is 11.0

A Potential Solution

total = 0
count = 0
answer = 'yes'
print("I'll calculate the average for you!")
while answer == 'yes':
        print("Current total: " + str(total))
        print("Numbers summed: " + str(count))
        total = total + int(input("Please enter a number to add\n> "))
        count = count + 1
        answer = input("Do you want to continue adding numbers (yes/no)?\n> ")
print("The average is "+ str(total / count))

Other While Loop Exercises

• simulate a blackjack AI by continuing to hit until some threshold that's close to 21
• a number guessing game
• rock paper scissors until # of wins is greater than 0
• simulate password protection

Counting Dice (For Loop Example)

Roll a die 1000 times; count how many times a one is rolled! Print out the result. Use a for loop.→

import random
ones = 0
for count in range(1, 1000):
	roll = random.randint(1, 6)
	if roll == 1:
		ones = ones + 1
print(str(ones) + " of 1000 rolls were ones")

Other For Loop Exercises

• sing 99 bottles of beer!
• number guessing game with limited number of guesses (use break for win!)
• input that affects range… say hello x number of times…

For Loops…

When should you use them? →

While Loops

When should you use them? →

Let's Try Using Both…

• count to 0 to 25 by 5's
  • implement using while
  • implement using for
• print out a series of numbers, each randomly chosen from (1 through 10) that add up to 50 (you can go over)
  • implement using while
  • implement using for