Computer Science 15-110, Spring 2011
Class Notes:  Loops

1. "for" loops
1. "for" loop over a range
1. Range from 0 to y:  use range(y+1)
2. Range from x to y:  use range(x, y+1)
3. Range from x to y with step z:  use range(x, y+1, z)
4. Range in reverse (use a negative step)
2. "for" loop over a String
1. With an index
2. Without an index
2. "while" loops
1. Basic Syntax and Use
2. Read Until a Sentinel
3. Read Until a Condition
4. Incorrect Usage:  Iterate Over a Range of Integers
3. Nested loops

Loops

1. "for" loops

1. "for" loop over a range
   
   
1. Range from 0 to y:  use range(y+1)
   
   # print the numbers from 0 to 4 (not 5)
   for x in range(5):
       print x
   
   
2. Range from x to y:  use range(x, y+1)
   
   # print the numbers from 10 to 14 (not 15)
   for x in range(10, 15):
       print x
   
   
3. Range from x to y with step z:  use range(x, y+1, z)
   
   # print the even numbers from 10 to 14 (not 15)
   for x in range(10, 15, 2):
       print x
   
   
4. Range in reverse (use a negative step):
   
   # print the even numbers from 14 to 10 (not 9)
   for x in range(14, 9, -2):
       print x
   
   
2. "for" loop over a String
   
   
1. With an index
   
   # print each character in a String
   s = "Amazing!"
   for i in range(len(s)):
     print s[i]
   
   
2. Without an index
   
   # print each character in a String
   for ch in "Wow!":
       print ch
   
   
2. "while" loops
   
   
1. Basic Syntax and Use
   
   while (test):
       body
   
   For example:
   
   def beHappy():
       happy = False
       while (not happy):
           response = raw_input("Are you happy yet? [yes or no] ")
           happy = (response == "yes")
       print "Well, now I'm happy, too!"
   
   Another example:
   
   def sumOfDigits(n):
       sum = 0
       n = abs(n)
       while (n != 0):
           sum += (n % 10)
           n /= 10
       return sum
   
   
2. Read Until a Sentinel
   
   # Keep reading names until the user enters "done"
   count = 0
   sentinel = "done"
   input = None
   print "Keep entering names.  To quit, enter", sentinel
   while (input != sentinel):
       input = raw_input("Enter name #" + str(count+1) + ": ")
       if (input != sentinel):
           count += 1
   print "You entered", count, "names!"
   
   
3. Read Until a Condition
   
   # keep reading numbers until their sum exceeds 10
   sum = 0.0
   while (sum < 10):
       print "Sum = " + str(sum) + ". ",
       input = float(raw_input("Next number --> "))
       sum += input
   print "Final sum = ", sum
   
   
4. Incorrect Usage:  Iterate Over a Range of Integers
   
   # sum numbers from 1 to 10
   # note: you should use a "for" loop here, not a "while" loop
   sum = 0
   counter = 1
   while (counter <= 10):
       sum += counter
       counter += 1
   print "The sum from 1 to 10 is", sum
   
   Once again, but with a bug!:
   
   # sum numbers from 1 to 10
   # warning: buggy version!
   sum = 0
   counter = 0
   while (counter <= 10):
       counter += 1
       sum += counter
   print "The sum from 1 to 10 is", sum
   
   And once more, using a "for" loop:
   
   # sum numbers from 1 to 10
   sum = 0
   for counter in range(1,11):
       sum += counter
   print "The sum from 1 to 10 is", sum
   
   
3. Nested loops
   
   n = int(raw_input("How many rows? "))
   for row in range(n):
       for col in range(n):
           print "*",
       print
   
   Another example:
   
   n = int(raw_input("How many rows? "))
   for row in range(n):
       for col in range(row+1):
           print "*",
       print
   
   Again, with a useful helper function:
   
   def printStars(starCount):
       for col in range(starCount):
           print "*",
       print
   
   n = int(raw_input("How many rows? "))
   for row in range(n):
       printStars(row+1)
   
   

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