Computer Science 15-110, Fall 2010
Class Notes:  Getting Started with while Loops

1. The "while" statement
2. Examples
1. Read Until a Sentinel
2. Read Until a Condition
3. Find the Nth Positive Integer with Some Property
4. Misuse:  Iterate Over a Range of Integers
3. Practice Tracing
4. Practice Mystery Methods
5. Practice Problems

Getting Started with while Loops

1. The "while" statement
   
   while (test):
       body
   
   For example:
   
   def powersOfTwo(upperLimit):
       n = 1
       while (n < upperLimit):
           print n
           n *= 2
       print "After loop, n = ", n
   
   And a similar example:
   
   def dividingByTwo(n):
       count = 0
       while (n > 0):
           print n
           n /= 2
           count += 1
       print "After loop, n=", n, " and count=", count
   
   And a different example:
   
   def functionsRace(finishLine):
       print "Functions race!"
       print "Function 1: y1 = 10 * x + 50"
       print "Function 2: y2 = x * x - 100"
       print "x\ty1\ty2"
       x = 0
       y1 = 0
       y2 = 0
       # while loop with conjunct condition
       while (y1 < finishLine and y2 < finishLine):
           y1 = 10 * x + 50
           y2 = x * x - 100
           print "%d\t%d\t%d" % (x, y1, y2)
           x += 1
       if (y1 > y2):
           print "Function 1 wins!"
       elif (y1 < y2):
           print "Function 2 wins!"
       else:
           print "It's a tie!"
   
   
2. Examples
   
1. 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!"
   
   
2. 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
   
   
3. Find the Nth Positive Integer with Some Property
   
   # find the nth number that is a multiple of both 4 and 7
   
   def isMultipleOf4Or7(x):
       return ((x % 4) == 0) or ((x % 7) == 0)
   
   def nthMultipleOf4Or7(n):
       found = 0
       guess = -1
       while (found <= n):
           guess += 1
           if (isMultipleOf4Or7(guess)):
               found += 1
       return guess
   
   input = None
   while (input != -1):
       input = int(raw_input("Enter a # (or -1 to quit): "))
       if (input != -1):
           print "   nthMultipleOf4Or7(", input, ") =", nthMultipleOf4Or7(input)
   print "Bye!"
   
   
4. Misuse:  Iterate Over a Range of Integers
   
   # sum numbers from 1 to 10
   # note:  this works, but you should not use "while" here.
   #        instead, do this with "for" (once you know how)
   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 (which we'll soon learn about):
   
   # 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. Practice Tracing
   
   
1. # trace this code by hand, then run it to confirm your prediction
   x = 13
   y = 1
   while (y < x):
       print x,y
       x -= 2
       y += 3
   print x,y
   
   
2. # trace this code by hand, then run it to confirm your prediction
   s = "abcde"
   while (len(s) < 15):
       print s
       s += "-" + str(len(s))
   print "Done, and s =",s
   
   
4. Practice Mystery Methods
   
   
1. # State what this function does, in just a few words of plain English.
   # Do not state line-by-line what it does at a low level.  Find the general pattern.
   # Also note that there is a clearer, faster, better way to do the same thing!
   
   def mysteryA(x, y):
       # assume x and y are non-negative integers
       c = 0
       while (x > 0):
           c += y
           x -= 1
       return c
   
   
2. # State what this function does, in just a few words of plain English.
   # Do not state line-by-line what it does at a low level.  Find the general pattern.
   # Also note that there is a clearer, faster, better way to do the same thing!
   
   def mysteryB():
       c = 0
       input = "!"  # why do this?
       s = ""
       while (s.find(str(input)) < 0):
           s += str(input)
           c += 1
           input = "-" + raw_input("Enter an integer: ") + "-"
       return c
   
   
5. Practice Problems
   In recitation...

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