while loop intro (class slides)

CSc 110 - Intro to While Loops
Adriana Picoral

While loops

• A while loop allows a programmer to repeat code
• You can think of it as an if-statement with the potential to repeat

statements . . .

while conditionA:
    statementA
    statementB
    . . .
    statementN

statements . . .

What will happen?

number = 15
while number < 50:
    print('number is less than 50')

While loops

• What if the condition never evaluates to False?
  • Infinite loop!
• There are two ways around this:
  • Break (do not use in this class!)
  • Designing the code such that the condition will eventually become False

What will happen?

To ensure our condition (number < 50) will eventually be evaluated as False, we need to updated number inside our loop:

number = 15
while number < 50:
    print('number is less than 50')
    number += 1

While loops – visualization

Go to Python Tutor to visualize how the while loop runs.

While loop – example

Factorial: 5! = 1 * 2 * 3. * 4 * 5 = 120

def factorial(n):
  result = 1
  current_n = 1
  while current_n <= n:
    result = result * current_n
    current_n += 1
  return result

def main():
  print( factorial(5) )
  print( factorial(6) )
  
main()

120
720

Write a function

Write a function called add_up_to that takes an numeric argument n. The function should add all numbers from 1 to n in a while loop, and then (outside the loop) return the sum

print( add_up_to(5) ) # 15
print( add_up_to(10) ) # 55

Name your file sum_up.py (your filename is different than your function name) and submit it to attendance on gradescope

Write a function – solution

def add_up_to(n):
  sum = 0
  current_number = 0
  while current_number <= n:
    sum += current_number
    current_number += 1
  return sum

def main():
  print( add_up_to(5) )
  print( add_up_to(10) )
  
main()

15
55

Age milestones

Modify the code below to use a while loop to request a valid input from the user.

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):
    age = int(age)
    print(age_milestones(age))
  else:
    print("Invalid age entered")
 
main()

Age milestones – solution

Modify the code below to use a while loop to request a valid input from the user.

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):
  if age.isnumeric() and int(age) < 150:
    return True
  return False
     
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')
  while validate_age(age) == False:
    print("Invalid age entered. Please enter a valid age.")
    age = input('How old are you?\n')
  age = int(age)
  print(age_milestones(age))
  
main()