Extract from Chapter 3 Hello, Little Turtles! about for loops

• The for Loop
  • iteration is one of the basic control structures
  • control structures determine which statements are executed and when
  • the default structure is the sequence (steps in order, all executed)
  • the loop changes that
    • syntax: for loopvariablename in list :
    • followed by indented statements which will be repeated = body
  • Flow of Execution of the for Loop (the semantics of the statement)
    • see flowchart in textbook
    • while the list given is not empty
      • assign loopvariablename to next thing in the list
      • do all the steps in the body with that value
      • go back to the top of the loop and get the next item from the list
      • when list becomes empty, get out of the loop and continue with following statement
    • Note that this says that a loop may NOT be executed at ALL, if the list that controls it is empty!
  • Iteration Simplifies our graphics Program

    from graphics import *
    
    def main():
        wn = GraphWin()
        x = 30
        y = 40
        for i in ['green','red','blue','white','black']:
            x = x + 10
            y = y + 20
            tt = Rectangle(Point(x,y),Point(x+10,y+10))
            tt.setFill(i)
            tt.draw(wn)
            wn.getMouse()
        wn.getMouse()
        wn.close()
        
    main() 
    

  • The range Function
    • a list of integers in order is so common and so useful, range function creates exactly that
    • has 3 forms, all useful
    • first syntax: range(integer)
    • semantics: generates a list starting at 0, going up to but NOT including the ending integer given
    • second syntax: range(integer, integer)
    • semantics: generates a list starting at first integer, going up to but NOT including the second integer given
    • third syntax: range(integer, integer, integer)
    • semantics: starts list at first integer, goes up to but NOT including the second integer, using third integer as the step size between values
• Examples of for loops
  • printing a column of stars

                   for i in range(5):
                       print("*")  # each print gives a newline character at the end
       	

  • repeating interactions

    		for i in range(3):
    		   n = int(input("Enter a number "))
                       print (n*3,"is three times as much as",n)
    	

• Uses for loops
  • used very often with numbers
  • count entries or accumulate some numbers
  • counters
    • must be initialized outside of the loop
    • must be changed inside the loop
    • statement looks like "ctr = ctr + 1" or "ctr += 1"
    • can be change by any amount, any mathematical operator
    • example

      			ctr = 3
      			for i in range(5):
      				ctr = ctr + 3
                	

    • results in ctr being 18
  • accumulators - used to find totals
    • must be initialized outside of the loop
    • must be changed inside the loop
    • statement looks like "total = total + num" or "total += num"
    • can be change by any amount, any mathematical operator

      			tot = 0
      			for i in range(5):
      				tot = tot + i 
                	

    • results in tot being 10
  • Factorial (written as an exclamation point !)
    • this is a mathematical idea which involves a product (multiplcation)
    • 1! = 0, 0! = 0
    • n! is 1 * 2 * 3 * ... * n
    • 5! = 1 * 2 * 3 * 4 * 5 = 120

    • 			fct = 1
       			n = int(input("Factorial of what number? "))
      			for i in range(2,n+1):
      				fct = fct * i
      		

    • fct is the factorial
    • note that the loop will not run at all if n is 1 or 0
  • A PROBLEM with Objects
    • to make an object, you should use one of the constructor methods, like "Line()"
    • if you just assign a name to an object, you do not create a new one
    • example:

      		tom = Line()
      		joe = tom
      
      

      
      This only creates ONE object, which has TWO names! the problem is called "aliasing". The same object has two names. Changing joe changes tom too!
    • You must use the constructor twice to have two objects
    • example:

      		tom = Line()
      		joe = Line()
      

    • or use the clone method