Introduction to Python II

Review of Basic Programming Concepts

  • Syntax
  • Data types
  • Operators
  • Variables 
  • Lists

Review Exercises

Do these exercises in the interpreter

  • If you have 3 boxes containing 25 chocolates each, and 10 bags containing 32 sweets each, how many sweets and chocolates do you have in total? (Hint: You can use one equation)
  • Create variables for your first and last name. Now create a string and use placeholders (%s) to add your name.
  • Make a list of 3 of your favorite drinks and name it drinks. Make a list of 3 of your favorite foods and name it foods. Join these two lists and name the result favorites. Finally print the variable favorites

If-Statements

  • 'if' is followed by a 'condition' and a colon (:), and if the condition is met, the following block of code executes 
    • ex if age > 17:

print('you are too old!')

  • Conditional operators
    • ==, !=, >, <, >=, <=
  • What is the difference between = and ==

What Else is There?

  • If-statements can be extended with an if-then-else-statement
  • So, if the condition isn't met, it would execute a different block of code (if something is true, then do this, otherwise do that); ex:

>>> age = 18

>>> if age == 12:

. . .      print('Hello')

. . .  else:

. . .      print('Goodbye')

 

Else-if-statements

  • If-statements can be further extended using elif (else-if)
  • If the previous if-statement's condition isn't met, then it check the else-if's condition, and so forth.

elif Example

>>> age = 12

>>> if age == 11:

. . .      print("You are 11")

. . .  elif age == 12:

. . .      print("You are 12")

. . .  elif age < 13:

. . .      print("You are younger than 13")

. . .  else:

. . .      print("You are older than 13")

 

What will be printed?

Combining Conditions

  • You can have multiple conditions in any conditional statement, by using and and or

>>> if age == 10 or age == 11 or age == 12:

. . .      print("You are %s" % age)

  • If any of the conditions are met, then the code runs, hence the or

>>> if age >= 10 and age <= 12:

. . .      print("You are %s" % age)

  • In this case, both of the conditions have to be true for the code to run, hence the and

None

  • Another value that can be assigned to a variable is None 
  • This is an empty value, with nothing in it (also known as null or NIL in other languages)
  • None can be used to reset variables and make them unused
  • An example is if we don't want to print a message when a variable is empty
    • >>> money = None
    • >>> if money == None:
    • . . .      print("You have no money!")

Loops

  • Repeatedly executing code written in the loop until/while a condition is met
  • for loop & while loop
  • How can this be useful?

For Loops

  • For loops iterate a limited number of times, with a variable taking on                        the values in the given list
  • for variable in list:​
    • ​The variable iterates through the list and is assigned the value of the current iteration
  • range(x, y) function returns [x, x+1, x+2, ..., y-1]
    • ex range(5, 9) == [5, 6, 7, 8]
      • range(4) == [0, 1, 2, 3]
  • Like if statements, the for loop is defined and then followed by a block of code
    • >>> for x in range(5):                                             >>> for s in shoppingList:
    • . . .       y = x + 1                                                          . . .      print(s)
    • . . .       print(x+y)
  • Indentations matter!

Code Blocks & Nested Loops

  • A block of code is a group of statements
  • White space (tab or space) separate blocks of code, like in the loop example above.
    • Consistent spacing!
  • The 3 dots (. . . instead of >>>) in the interpreter means you're typing in a block of code
  • Blocks placed inside each other require the extra spacing as shown to the right
  • Loops placed inside each other are nested loops, and each block's loops fully complete before returning to the outer loop

this is block 1
this is block 1
      this is block 2
      this is block 2
      this is block 2
            this is block 3
            this is block 3
      this is block 2
this is block 1
this is block 1
 

Example of Nested Loops

  • Try typing this into your interpreter:
    • >>> myList = [ 'a', 'b', 'c' ]
    • >>> for u in myList:
    • . . .        print(u)
    • . . .        for v in myList:
    • . . .               print(v)
  • Remember how you'd earn $1820 a year ($35 a week) through your chores and paper route? We can calculate the money earned throughout the weeks!
    • >>> savings = 0
    • >>> for week in range(1, 53):
    • . . .        savings = savings + 35
    • . . .        print("Week %s = $%s" % (week, savings))

More for loops

  • Physical exercise

While we're on Loops

  • While loops iterate through the block of code while a condition is met
    • Different from for loops, because it's while, not until
  • while condition:
  • >>> x = 1
  • >>> while x < 200
  • . . .        x = 2*x
  • . . .        print(x)

Break

  • You can break out of a loop before the condition is met
    • >>> while True:
    • . . .        # tons of code
    • . . .        if someCondition == True:
    • . . .                break
  • Games use while True to keep the game running and refreshing the screen!

Input()

  • You can take in user-inputted text in the code, and store it in a variable
  • input("text")
    • The user types something in and presses Enter
    • Whatever they typed in is returned by input() as a string
  • >>> name = input("Please enter your name: ")
  • >>> print("Hello, %s!" % name)
  • If they input a number, it is still returned as a string
    • use the int() function to change it to an int
    • >>> age = input("What is your age? ")
    • >>> age = int(age)
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