Programming in Python

BEES 2021

Agenda

  • Review: Reading User Input
  • Conditional Control Flow
    • If Statement
    • Else
    • Elif
    • Pass
  • Looping Control Flow
    • While Loop
  • Inputs are always String
  • Example:

 

 

  • We can use type conversion to read different types
  • Example:

 

 

 

name = input("What's your name? ")

print(type(name))

print("Hello " + name)
print("Hello ", name, 5)
value1 = float(input("Enter a number: ")) # Read a first number from the user

value2 = float(input("Enter another number: ")) # Read a second number from the user

print("The sum of your two numbers is: " + str(value1 + value2))

USER input

  • Just like reading a book, recall Python reads each line of code in turn, progressively updating it's "state" (aka memory) with variables and definitions.

  • Like a choose-your-own-adventure book, the program can skip lines and jump around the book using "conditionals".

  • Imagine Python has a cursor and these conditionals cause it to skip or include lines depending on their logical value

CONDITIONAL CONTROL FLOW

if ... else ...

 

if boolean expresion:

      stmt 

else:

      stmt

 

if "dogs" > "cats": # Strings are compared lexicographically (dictionary ordering)
  print("Dogs really are the best")
else:
  print("But Narges is a cat lover")

If example

Else and ElIf example

Nested conditionals

# Consider the following if/elif/else block
x = float(input("Give me a number? : "))

if not x > 2:
    print("x is less than or equal to 2")
elif x < 5:
    print("x is greater than 2 but less than 5")
elif x <= 10:   
    print("x is greater than or equal to 5 but less than or equal to 10")
else:
    print("x is greater than 10")
# You can achieve the same with nested if statements:
x = float(input("Give me a number? : "))

if x < 5:
  if not x > 2: # This if is nested in the outer if statement, 
    # it is only tested if the outer conditional evaluates to True
    print("x is less than or equal to 2")
  else:
    print("x is greater than 2 but less than 5")
else:
  if x <= 10: 
    print("x is greater than or equal to 5 but less than or equal to 10")
  else:
      print("x is greater than 10")

Inline conditionals

  • We can write inline if/else statement
  • Sometimes it makes the code more readable

pass

  • Sometimes when you're writing code, you want a placeholder statement - use pass 
x = 4

if x > 5:
  pass # Pass acts as a placeholder for the code you will write

print(x)

while ... 

 

while boolean expression:

      stmt ​

 

i = 0

while i < 10:
  print(" i is:", i)
  i = i + 1

print("we're done!")
  
# You read this program as:

# Set i to 0

# check if i < 0, if it is:
  # print "i is: ", i
  # add one to i
  # go back to the start of the while loop

# print "we're done"

While example

putting it all together

Lecture 3 challenge

Questions?