Review

• Arithmetic and variables
• Data types
• Text editor, command line, and python shell

What we will cover today

• Boolean Expressions and Conditionals
• Loops
• Functions

Boolean Expressions

We can tell the computer to compare values and return True or False. These are called Boolean expressions

• Test for equality by using ==. We can't use = because that is used for assignment
• Test for greater than and less than using > and <

a = 5
b = 5
print(a == b)
# Combine comparison and assignment
c = a == b
print(c)

print(3 < 5)
            

Boolean Expressions continued

The following chart shows the various Boolean operators

a = 3
b = 4
print(a != b)
print(a <= 3)
print(a >= 4)
            

Remember: Equals does not equal "equals equals"

Conditionals

When we want different code to execute dependending on certain criteria, we use a conditional

if x == 5:
    print('x is equal to 5')
            

if x == 5:
    print('x is equal to 5')
else:
    print('x is not equal to 5')
            

Indentation

In Python, blocks begin when text is indented and
ends when it returns to the previous indentation

if x == 5:
    print('x is equal to 5')
    x_is_5 = True
    print('Still in the x == 5 block')
else:
    print('x is not equal to 5')
    x_is_5 = False
    print('Still in the else block of x == 5')
print('Outside of the if or else blocks.')
print('x_is_5:')
print(x_is_5)
            

Chained conditionals

Conditionals can also be chained

if x > 5:
    print('x is greater than 5')
elif x < 5:
    print('x is less than 5')
else:
    print('x is equal to 5')
            

Nested conditionals

Conditionals can also be nested

            if x > 5:
                print('x is greater than 5')
                if x > 10:
                    print('...it is also greater than 10')
                print('Done evaluating the x > 10 block')
            print('Done evaluating the x > 5 block')
                        

Let's Develop It

Write a program that uses if statements to determine
what to do given some user input

The code below is an example:

health = 100
print("A vicious warg is chasing you.")
print("Options:")
print("1 - Hide in the cave.")
print("2 - Climb a tree.")
input_value = input("Enter choice:")
if input_value == '1':
    print('You hide in a cave.')
    print('The warg finds you and injures your leg with its claws')
    health = health - 10
elif input_value == '2':
    print('You climb a tree.')
    print('The warg eventually looses interest and wanders off')
            

Iteration

It is often useful to perform a task and to repeat the process until a certain point is reached.

The repeated execution of a set of statements is called iteration

x = 10
while x > 0:
    print(x)
    x = x - 1
print('Done')
          

The while statement takes a predicate, and as long as it evaluates to True, the code block beneath it is repeated.

This creates a loop. Without the x = x - 1 statement,
this would be an infinite loop

While loops

Consider the following example that uses iteration to derive a factorial

A factorial of a number is equal to that number * every positive integer less than that number. E.g. The factorial of 4 is 4 * 3 * 2 * 1, which equals 24

input_value = input('Enter a positive integer:') # input('')
n = int(input_value)
result = 1
while n > 1:
    result = result * n
    n = n - 1
print("The factorial of " + input_value + " is:")
print(result)
                                        

This implementation does not work for negative numbers. Why?

For loops

It is also useful to loop through a collection of elements, visiting each one to do some work, then stopping once all elements are processed.

This can be accomplished with a for loop

numbers = [1, 3, 8]
for number in numbers:
    print("The current number is:")
    print(number)
                    

For loops continued

Let's examine the example carefully

numbers = [1, 3, 8]
for number in numbers:
    print("The current number is:")
    print(number)
                    

The for loop has three parts:

• The collection to loop over - numbers
• The name to give each element when the loop begins again - number
• The block of statements to execute with the element - The two print statements

Let's Develop It

• Write a program that obtains user input like the last program
• However, this program should not exit until the user types "quit".
• Hint: A loop should help you

Functions

A named section of code that performs a specific task

When one uses a function, one makes a function call

a = '3'
print(type(a))
a = float(a)
                    

Function calls

a = 3
print(type(a))
                    

A function can take arguments

In the example above, the variable a is passed
as an argument to the function type

Arguments can also be called parameters

# Some more function call examples

int('32')
str(32)
                    

Function definition

The following example is a function definition.
This allows us to create our own functions

def print_plus_5(x):
    print(x + 5)
                    

The function definition has the following parts

• The def keyword signifies we are defining a function
• The name of the function being defined - print_plus_5
• The arguments in parentheses - x
• The function body, which is a block of indented code that executes when the function is called. - print x + 5

Function returns

A function can also return a value

def plus_5(x):
    return x + 5

y = plus_5(4)
                    

• This allows us to call a function to obtain a value for later use.
  (Not the same as printing the value)
• In this example, the function call plus_5(4) evaluates to 9, and y is set to this value
• To determine what a function will return, use the substitution method.
• If return is not used, the function returns None

Functions with no arguments

A function does not have to take arguments,
as in the following example:

def newline():
    print('')

newline()
# prints an empty line. Nothing is returned
                    

This is useful when the function does some work but doesn't need any parameters.
i.e. The function is intended to always do the same thing

Functions with more than one argument

A function can also take more than one argument separated by commas. For example:

def find_rectangle_area(width, height):
    return width * height

area = find_rectangle_area(3, 4)
# area is set to the value 12
                    

Scope

The scope of a variable is the area of code in which a variable is still valid and can be used.

Variables defined within a function can not be used elsewhere.

def get_triangle_area(base, height):
    rect_area = base * height
    return rect_area / 2.0

triangle_area = get_triangle_area(10, 20)

print(height)
# NameError
print(rect_area)
# NameError
                    

Import statements

The import statement allows us to use Python code that is defined in other files

Import statements can be used to import various specialized modules - such as scikit or nympy - into our code

module: A published collection of useful functions for doing a specific type of coding. There are math, statistics, data science, and graphics modules, to name a few types.

The from keyword allows us to only import parts of a Python file

from math import sqrt
sqrt(2)
                    

Let's Develop It

• Write a program with a function called pigLatin that takes in a word, and prints that word in Pig Latin (that is, move the first letter to the end of the word and add "-ay"). Call the function 3 times with 3 different words.
• Write a program that adds 5 to a number, and returns the result. Call the function at least three times with various numbers, and print the result of each function call.

Let's Develop It

• Write a program that asks the user to guess a number between a given range, such as 1 to 10
• The program should give the user hints such as "too high" or "too low". Alternatively, the hints might be "warm" or "cold" depending on how close they are to the number
• The computer will need to have a random number for the user to guess:

#At the top of the file
from random import randint

# Use this line where you need to have a random number.
# (Hint, this is probably used before the user input loop)
random_number = randint(1, 10)
                        

Questions?

Homework

• Write a function that a bar could use to check to see if a customer is of drinking age.

  isOfDrinkingAge(age)

• If it is a saturday in spring and it is above 65 degrees, Kermit plants flowers. Write a function to check whether he will plant flowers today.

  willKermitPlantFlowers(dayOfWeek, season, currentTemperature)

• Zach knows something is wrong with his basement if the temperature is below 50 or the humidity is above 70%. Write a function that can check whether his basement has a problem.

  basementIsOk(temperature, humidity)

Homework

Given any list of any sort of mixed types,write a function that prints the type each item in the list.

You can try this list:

[1, 'hello', 12.33, 'data', [1,2]]

typeIdentifier(list)

#should output something like this:
1 is a <class int>
hello is a <class string>
12.33 is a <class float>
          

Homework

Write a function that asks the user to guess a number between 1 and 100

• If their guess is within 20, tell them they are 'warm'

• If their guess is within 10, tell them they are 'hot'

• If their guess is within 5, tell them they are 'scalding'

• if their guess is within 50, tell them they are 'cool'

• otherwise tell them they are 'cold'

• If they guess the number, congratulate them and ask if they'd like to play again.

Optional Extra Challenge: Let them know if they are 'warmer' or 'colder' than their last guess.

Optional Function Practice

# write a function to...
# ...add 10 to any number
# ...divide any number by 100
# ...add any 2 numbers
# ...subtract any 2 numbers
# ...convert celsius to farenheight
# ...convert farenheight to celsius
# ...convert inches to centimeters
# ...convert centimeters to inches
# ...calculate sales tax for any given total amount
# ...calculate the area of a triangle (1/2 base * height)
# ...calculate the volume of a rectangular prism (length * width * height)
          

Boolean Practice

Do this excercise from Learn Python The Hard Way

http://learnpythonthehardway.org/book/ex28.html

Want help?