03_python_variables_and_control_flow.md

Python Variables and Logic

• Declaring Variables
  • Variable Naming Syntax
• Logic Operators
• Control Flow
• Homework

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Variables

• You can think of variable assignment as giving a name to something so that it can be accessed later by different parts of your program.
  • You can use a variable to store any data type
  • Important for reusability

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Declaring Variables

• Variables are assigned using the = operator.
  • variable = value
• Need to print to see the result of assigning a variable.
• Can assign two or more variables on the same line.
  • x, y = 1, 2

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Give it a good name

• You should strive to give your variables well-defined, succinct names
• Variable naming conventions are covered in PEP 8.
• Use snake case to name variables:
  • Replace spaces with _
  • this_is_a_variable = 8
• There are some reserved words that can’t be used as variable names.
  • Python reserved words list

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Boolean Operators

Operator	Meaning
>	Greater Than
>=	Greater Than or Equal
<	Less Than
<=	Less Than or Equal
==	Equals or Is Equivalent
!=	Does not Equal

print(5 > 4)   # --> True
print(5 >= 6)  # --> False
print(5 < 4)   # --> False
print(5 <= 6)  # --> True
print(5 == 6)  # --> False
print(5 != 6)  #--> True

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Logical Operators

• Operator Precedence: not, and, or

print(not True)       #--> False
print(not False)      #--> True
print(True and True)  #--> True
print(True and False) #--> False
print(True or False)  #--> True

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BREAKOUT (2 mins)

What is the result of the following boolean expression?

(not (not (True or False) and (True and True))) and True or True

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BREAKOUT Solution

What is the result of the following boolean expression?

• note that the or True on the end will evaluate the whole thing as True. We'll simplify the whole thing anyway

(not (not (True or False) and (True and True))) and True or True
#          --simplify---
(not (not True and (True and True))) and True or True
#     --------
(not (False and (True and True))) and True or True
#                -------------
(not (False and True)) and True or True
#     --------------
(not False and True or True
#---------
True and True or True
#------------
True or True
#----------
True

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Bitwise Operators: ~, &, |

• ~, &, | are the matching bitwise operators for not, and, and or
• Don’t use these or worry about them for now
• They may not behave exactly as you expect

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Exclusive or (XOR)

• ^ is the bitwise operator for exclusive or
• For now, think of this as asking the question “are these two things different?”

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Order of Logical Operators

Order of Operations
~
&
^
not
and
or

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BREAKOUT (3 minutes)

What do each of these return?

• not 7 > 2
• 3 >= 2 or 5 < 1
• not 8
• bool(‘’) and 5 != 3
• bool(‘’) and 5 != 5

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BREAKOUT Solution

What do each of these return?

• not 7 > 2
  • False
• 3 >= 2 or 5 < 1
  • True
• not 8
  • False
• bool(‘’) and 5 != 3
  • False
• bool(‘’) and 5 != 5
  • False

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Control Flow: if/else

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if

• We use if statements if we want to evaluate code only under a certain condition
• Only when the statement following the if keyword evaluates to True will Python run the code in the body of the if block
• Syntax:

if some_condition:
    # execute this code
else:
    # execute this code

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if/elif/else

• Often, we will want to execute some other code if the condition in the if statement is not met.
  • Use an if-else statement
• What if we want to check multiple conditions?
  • Use if-elif-else
  • You can use multiple elif statements
  • Order matters!

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if/elif/else syntax

if some_condition:
    # execute this code
elif another_condition:
    # execute other code
elif yet_another_condition:
    # execute this code
else:
    execute some other code

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if/elif/else

• We can incorporate comparison operators and logical operators into our if statements

x = 7
y = False

if x < 10 and not y:
    print('all is well')
elif x > 3 and y:
    print('danger')
else:
    print('nothing matters')

will output

# --> all is well

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BREAKOUT (5 minutes)

• Write a code snippet that checks two numbers, x and y:
  • Check if the sum of two numbers is greater than both numbers
    • If it is, print "Both numbers are positive"
  • Check if the sum is equal to either number
    • If it is, print "At least one number is zero"
  • Otherwise, print "At least one number is negative"

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BREAKOUT solution

if x + y == x or x + y == y:
    print('At least one number is zero')
elif x + y > x and x + y > y:
    print('Both numbers are positive')
else:
    print('At least one number is negative')

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Homework

• Complete the Day 1 Homework if not completed
• Complete all Challenges in these Learn Sections
  • Variables
  • Scalar Types
  • Basic Operators
  • Control Flow