15-110: Principles of Computing - Week 4

"for" Loops and Infinite Loops

• "for" loops

  • "for" loop over a range
    
    
  • 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
    
    
  • 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
    
    
  • 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
    
    
  • 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
    
    
  • "for" loop over a String
    
    
  • Without an index
    
    # print each character in a String
    for ch in "Wow!":
        print ch
    
    
  • With an index
    
    
  • the old-fashioned way
    
    # print every other character in a String
    s = "Amazing!"
    for i in range(len(s)):
        if (i % 2 == 0):
            print s[i]
    
    
  • Optional:  the newfangled way (with enumerate)
    
    # print every other character in a String
    s = "Amazing!"
    for i, ch in enumerate(s):
        if (i % 2 == 0):
            print ch
    
    
  • Nested loops
    
    n = int(raw_input("How many rows? "))
    for row in range(n):
        for col in range(row+1):
            print "*",
        print
    
    
  • Infinite while loop with break
    
    while (True):
        response = raw_input("Enter a string (or 'done' to quit): ")
        if (response == "done"):
            break
        print "  You entered: ", response
    print "Bye!"
    
    

Readings from the book

Chapter 2 (pp. 36-38) (omit continue and pass statements)

Suggested exercises from the book

Chapter 2: problems 7, 9, 11, 19, 21, 24, 25, 26 (note:  random numbers are optional at this point)