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Repetition
Examples
• When is repetition necessary/useful?
Types of Loops
- Counting loop
- Know how many times to loop
- Sentinel-controlled loop
- Expect specific input value to end loop
- Endfile-controlled loop
- End of data file is end of loop
- Input validation loop
- General conditional loop
- Repeat until condition is met
while
while condition: statements x= while x < 10: print x x = x + 1
while
x=1 #initialization of control variable while x < 10: #condition print x #task to be repeated x = x + 1 #update - VERY VERY IMPORTANT
Sentinel-controlled
num = input("Enter number - 0 to quit: ") while num != 0: print “You entered “, num num = input("Enter number - 0 to quit: ")
• Which is the control variable?
Input Validation
num = input("Enter number between 0 and 100: ") while num < 0 or num > 100: #a more complex condition print "Invalid input" num = input("Enter number between 0 and 100: ")
for
for x in range(10): print x mystring = "CS is cool!" for c in mystring: print c
• Loop iterates over a list
• Initialization and update happen automatically
Infinite Loops
• If your program “hangs” – you probably forgot to
update your control variable
x= while x==1: print “x is 1”
• Why is this bad?
x= end_value= while x != end_value: #do something
Infinite Loops
- Why is this bad? x= end_value= while x != end_value: #do something x *= 2 x= end_value= while x < end_value: #better #do something
Alternative
while 1: num = input(“Enter a number - 0 to quit: “) if num == 0: break #combines intialization and update
Exercises
1. Write a while loop that prints all of the even
numbers between 1 and 100.
- Create two versions of this loop, one that uses an if
statement and one that does not
2. Write a program that uses the module random
to select a random number between 1 and 10
(example below) and asks the user to
repeatedly enter a number until he/she has
guessed the random number.
#import the module random import random #call the randint function passing in the range num = random.randint(1, 10)
Exercise
2. Design a program which prompts the
user for a number of rows between 0
and 10 and prints the following pattern:
