Overview of for and while loops

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For loops

For loops iterate through a finite sequence of values.

  • finite loops
  • lists [0,1,2,3,4,5]
  • lists ['a', 'b', 'c', 'd']
  • range(x)
  • range(x,y)
  • range(x,y,z)
  • range(x,y,-z)
  • for i in <list>:
  • for i in <range>:

For loops in Python flow chart.png

While loops

While loops repeat a potentially unfinite number of iterations.

  • indefinite loops
    • Since while loop is indefinite loops, when for loop is definite, it is better to use while loop to set up the conditions that have to be satisfied to prevent the program from crashing.
  • condition-based loops
  • accumulator pattern
    • running totals

While loop flow chart.png

General loop controls

  • break
  • continue

Input validation

  • repeating until valid input given


  • keeping track of state


  • converting Strings to uppercase or lowercase in order to make validation case-insensitive

Example programs in Python

Number validation example

Note the use of the while loops to indefinitely iterate until an acceptable number has been entered.

msg = "What's your lucky integer"

num = ""

while not num.isnumeric():
    num = input(msg)
num = int(num)

while num != 50:
    print("Sorry, that's not right.")
    num = int(input("What's your favorite number?"))

Name validation example

Note the use of the while loop to indefinitely iterate until an acceptable name has been entered.

name = input("What's your name").lower()

while not name == "bob":
   name = input("What's your name?")

print("Hi Bob!")

Running total example

Note the use of a flag, the accumulator pattern, and the while loop

hoursTotal = 0

keepLooping = True #start the flag as true

#keep looping as long as the flag is true
while keepLooping:

    #set the starting point of our variable to make the loop kick in
    num = "foo"
    while not num.isnumeric():
        num = input("Please enter a number of hours:")
        if num.lower() == "stop":
            keepLooping = False #set the flag to false
            break #break out of immediate while loop

    #if the user entered stop, break out of the main while loop
    if not keepLooping:

    #this only executes if the user entered a number
    num = int(num)

    hoursTotal = hoursTotal + num

print("You worked " + str(hoursTotal) + " hours total")

Examples from Fall 2013

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