if statements (slides)

CSc 110 If statements

Expression vs Statement

• Expression evaluates to value
• Statement does something

3 + 4 # expression
print(3 + 4) # statement

a = 8 # statement
a != 9 # expression
print(a) # statement

The if statement

• If statements can be used to run code conditionally

  • Before if-statements: Code has pretty much just run in a straight line

  • With ifs: Can run code optionally, depending on the value of a condition

This means our code can branch in different directions

The if statement

if condition:
    statement 1
    statement 2
    . . .
    statement N

The condition is an expression that is evaluated to a bool.

Example:

def greeting(name):
  if name == "Bond":
    return "Welcome on board 007."
  else:
    return "Hello, " + name

def main():
  user_name = input("Enter your name:\n")
  print( greeting(user_name) )

main()

Improve the function

Instead of checking if the name entered is “Bond”, also check whether the name is “James Bond”.

Conditional execution

The computer program branches out, or makes decisions

def greater_than_zero(n):
  if n > 0:
    return "Greater than zero"
  else:
    return "Not greater than zero"
  
def main():
  result = greater_than_zero(4)
  print(result)
  
  result = greater_than_zero(0)
  print(result)
  
  result = greater_than_zero(-3)
  print(result)
  
main()

Greater than zero
Not greater than zero
Not greater than zero

Write a function

Write a Python function called absolute that takes one numeric argument (integer or float) n and returns the absolute value of n: if n is positive, it results n, if n is negative, it returns n * -1

Test cases:

print( absolute(4) ) # 4
print( absolute(-4) ) # 4
print( absolute(0) ) # 0

Write a function – solution 1

def absolute(n):
  if n > 0:
    return n
  else:
    return -n
  
def main():
  print( absolute(4) )
  print( absolute(-4) )
  print( absolute(0) )
  
main()

4
4
0

Write a function – solution 2

def absolute(n):
  if n > 0:
    return n
  return -n
  
def main():
  print( absolute(4) )
  print( absolute(-4) )
  
main()

4
4

Which lines run in the function absolute?

• When expression n > 0 is True, line 3 runs (return is used to exit a function and returns a value)
• When expression n > 0 is False, line 4 runs (skip line 3)

Write a function

1. Function age_milestones takes one integer age
2. It returns:
   • ‘You may apply to join the military.’ if age is greater or equal to 18
   • ‘You may drink.’ if age greater or equal to 21
   • ‘You may run for president.’ if age greater or equal to 35
3. The latter conditions include former so concatenate them

print( age_milestones(18) ) # You may apply to join the military.
print( age_milestones(30) ) # You may apply to join the military. You may drink.
print( age_milestones(0) ) # 

Write a function – solution

def age_milestones(age):
  '''
  This function prints an informative message based on,
  a person's age.
  Args:
    age: integer representing a person's age
  Returns:
    A string with a message to the user
  '''
  message = ""
  if age >= 18:
      message += 'You may apply to join the military.'
     
  if age >= 21:
      message += ' You may drink.'
     
  if age >= 35:
      message += ' You may run for president.'
      
  return message
     
def main():
  print( age_milestones(18) ) # You may apply to join the military.
  print( age_milestones(30) ) # You may apply to join the military. You may drink.
  print( age_milestones(0) ) # 
 
main()

You may apply to join the military.
You may apply to join the military. You may drink.

Quiz 04

current time

You have 10 minutes to complete the quiz.

No need to write main() function.

Input validation

String methods

In addition to having built-in functions (len(), print(), int(), float(), etc.), Python also has a number of methods we will be using in this class.

Check the documentation for string methods and read what .isnumeric() does.

Validating numbers

• The input() function always returns a string
• We can use the string built-in method .isnumeric() to determine if a string represents an integer
• .isnumeric() returns True if all the characters are numeric (0-9)

Try these out:

name = "Jimmy42"
name.isnumeric()

False

age = 37
age.isnumeric() # this throws an error

Write a validation function

Write a Python function validate_age that takes a string argument age. It returns True if age contains only 0-9 digit characters, and False otherwise.

In your previous file for age milestone, in the main function:

• call input() to type an age
• call validate_age to validate your input
• if it is True, call age_milestones to print messages
• if it is False, print ‘Invalid age entered’

Age milestones

def age_milestones(age):
  '''
  This function prints an informative message based on,
  a person's age.
  Args:
    age: integer representing a person's age
  Returns:
    A string with a message to the user
  '''
  message = ""
  if age >= 18:
      message += 'You may apply to join the military'
     
  if age >= 21:
      message += 'You may drink'
     
  if age >= 35:
      message += 'You may run for president'
      
  return message

def validate_age(age):
  return age.isnumeric()
     
def main():
  '''
  This functions takes input from the user and calls the
  check_age() functiont to print a message
  '''
  age = input('How old are you?\n')
  if validate_age(age):
    print(age_milestones(int(age)))
  else:
    print("Invalid age entered")
 
main()