Chapter 7 Iteration Revisited

• Iteration = repetition - a basic control structure
• The for loop revisited
  • can use list to control a for loop
  • accumulator pattern
• Note: the questions 7.2a.1 and 7.2a.2 are premature, do them after you read the section on while statements. Also, 7.2a.2 has incorrect indentation and syntax for the print.
• The while Statement
  • has a condition at the top that controls when the loop stops
  • syntax: "while" followed by Boolean condition followed by colon
  • then indent body of loop statements
  • must have at least one statement in the body
  • called the "indefinite" loop (versus the for loop, which is the "definite" loop)
    • you don't know how many times the loop will execute, but you know the condition to stop
  • a while loop also called a "PRE-test" loop, because the test is done before the body is executed. It is possible the body will not be run at all on a particular run, depending on the test. If it is false at the start, the body is skipped and the program continues.
  • a while loop can be used to write an infinite loop; a for loop can't
    • to avoid infinite loops, make sure something inside the loop changes the variables involved in the condition at the top
  • can do anything with a while loop that you can do with a for loop
  • example: counter controlled loop

    	ctr = 0
    	while ctr < 5:
    		print(ctr)
    		ctr = ctr + 1
    

  • and can do MORE things with a while loop than with a for loop
• Sentinel Logic not in textbook!
  • a pattern that works when dealing with input which has a sentinel value
  • sentinel value
    • a special value chosen ahead of time which indicates the end of the data
    • can be -1, 0, 999, "stop", whatever you choose
    • is NOT real data!
    • do not want to process it like real data!
  • the pattern

    		input into a variable (say, x)
    		while x is not the sentinel value
    			process x 
    			input into x
    

  • a nice example: reading numbers and calculating their average. The sentinel value is going to be the empty string

    initialize sum to 0.0
    initialize count to 0
    input data item as a string, xStr
    while xStr is not empty
       convert xStr to a number, x
       add x to sum
       add 1 to count
       input next data item as a string, xStr
    Output sum / count
    

• Nested loops
  • one control structure can be nested inside another to any level, loops inside ifs, ifs inside loops, loops inside loops, ifs inside ifs, and of course all of them inside functions
  • very common to see nested loops
  • example:

    	for i in range(5):
    		for j in range(3):  # use a different variable!
    		    print (i,j, end = " ")
    	
    

    
    how does this print out? how could you make it a grid? that is, print each row on a line, then a carriage return
  • try printing out a multiplication table, from 1 to 10
• Flags not in the textbook
  • Read this
  • a flag is a boolean variable
  • has only values of True and False
  • give it a name that means something, "hasError", "inOrder"
  • what a flag means is up to you!
    "A True value means a negative number has been seen"
    "A False value means there was an error in the input"
    "A True means the data was in order, a False means it was not"
  • initialize the flag variable first (to True or False, whatever makes sense)
  • process the data as it needs to be done
  • test data for whatever condition the flag is meant to record
  • set the flag if needed
  • at the end of the data, test the flag to see what happened
  • usually used in loops, always needs if statements
• The 3n+1 Sequence
  • a sequence in mathematics
  • start with a number
  • if it is even, take half of it
  • if it is odd, form the number 3*number + 1
  • do this until you reach the value 1
  • unpredictable when that will happen, or if it will
  • perfect problem for a while loop
• Newton's Method
  • remember the lab with this problem?
  • it had a loop that printed out 20 approximations of the sqrt function using Newton's method
  • a while loop is better because it can stop as soon as the approximates get "close enough" or when the approximations stop changing much
  • note that the loop is controlled by a comparison of two floats (and two calculated floats at that!). How could you improve the control of the loop?
• Algorithms Revisited
  • algorithms can be expressed clearly enough that anyone, even a machine, can do them
  • a lot of things humans do (recognize faces, understand language, walk over rough terrain, create new things, ...) cannot be expressed by an algorithm
• Simple Tables
  • use \t for tab, \n for newline, \\ for a slash
  • "escaped" character
• 2-Dimensional Iteration: Image Processing