1 Chapter 4 The while loop and boolean operators Samuel Marateck ©2010

2 Boolean operators are used in boolean expressions and operate on other boolean expressions. They are the and, or and not. For instance: X>3 and y<=4 X>3 or y<=4 not(X>3 and y<=4)

3 If an and is used, both expressions that it connects must be true in order for the entire expression to be true.

4 If an or is used, if either expression that it connects is true, then the entire expression is true. The following tables show how the and and or work.

5 andTrueFalse True False

6 orTrueFalse True FalseTrueFalse

7 So for instance in: x>3 and y<=4 If x>3 is True and y<=4 is False, the entire expression is False.

8 However, in: x>3 or y<=4 If x>3 is True and y<=4 is False, the entire expression is True.

9 The not changes the boolean expression it precedes so that not True is False. The not must precede a boolean expression and operates on the expression it immediately precedes.

10 In not x > 3 what is the expression the not immediately precedes?

11 In not x > 3 the not immediately precedes the variable x which is not a boolean expression. What happens and how do you correct this?

12 It causes a compilation error. You write it as: not (x > 3) If the value of x is 3, what is the value of the entire expression?

13 not (x > 3) If the value of x is 3, what is the value of the entire expression? Since x > 3 is false, not (x > 3) is true.

14 The while loop The while loop differs from the for loop in that the variables in the while statement must be defined before the loop.

15 So in: while x < 3: the value of x must be defined before the loop –this is called priming the while --and execution of the loop will continue while the value of x < 3 is true. Here is an example:

16 x = 0 while x < 3: print(x) Since the value of x < 3 is true initially, the loop begins execution. The indented statements that follow the while are said to be in the scope of the while. How many times will the loop be executed?

17 x = 0 while x < 3: print(x) How many times will the loop be executed? An infinite number of times.

18 So unless a statement in the scope of the while statement changes the boolean expression in the while, execution of the while continues. x = 0 while x < 3: print(x).

19 So we can write, for instance, x = 0 while x < 3: print(x) x = x + 1.

20 Now the scope of the while, x = 0 while x < 3: print(x) x = x + 1 is. print(x) x = x + 1

21 Since x changes, the loop will only be executed a finite number of times. x = 0 while x < 3: print(x) x = x + 1 How many times will it be executed?

22 The loop, x = 0 while x < 3: print(x) x = x + 1 will be executed three times. What is its output?

23 The output of x = 0 while x < 3: print(x) x = x + 1 Is 0, 1 and 2.

24 One of the purposes of the while is to insure integrity of the input. In the following, the required input is between 0 and 9 Inclusively. So the while continues until the input is between 0 and 9.

25 invalid = True #primes the while while invalid : #continues if invalid is True n = input('type your number please \n') if 0 <= int(n) <= 9: invalid = False else: #continues the while print(n + ' is invalid') print('done')

26 In while invalid, the boolean expression is simply the expression invalid which is originally True. When the input is in the required range, invalid changes to False and execution quits the loop.

27 The ASCII Table All characters used by the computer are given a code. For instance, the digits ‘0’ through ‘9’ have the codes 48 through 57 in the ascii table. The letters ‘A’ through ‘Z’ have the codes 65 through 92. The function ord() gives the ascii code for a given character and chr() gives the character corresponding to the ascii code. So ord(‘A’) returns 65 and chr(65) returns ‘A’.

28 What’s the meaning of ‘0’<= c<=‘9’ where c is a character? How does the computer evaluate it?

29 What’s the meaning of ‘0’<= c<=‘9’ where c is a character? How does the computer evaluate it? It’s the equivalent of 48<=ord(c)<=57. So the computer tests if the ascii value of the character c is between the two limits 48 and 57.

30 The following counts the number of characters after the period. #counts # of characters after the period #Tests for no period also s = input('Type your sentence please \n') n = -1 #compensates for counting the period flag = False for c in s: if c == '.' : flag = True

31 A flag is initialized and set to False #counts # of characters after the period #Tests for no period also s = input('Type your sentence please \n') n = -1 #compensates for counting the period flag = False

32 Once the flag is True, the counting begins in n=n+1. s = input('Type your sentence please \n') n = -1 #compensates for counting the period flag = False for c in s: if c == '.' : flag = True if flag : n = n + 1 #counts the period also

33 If no period is encountered, the flag remains False. So an error message is printed after the loop is executed: if flag == False: print('No period found ') else: print('# of characters after the period is ', n)

34 Else the answer will be printed. The entire program is now:

35 #counts # of characters after the period #Tests for no period also s = input('Type your sentence please \n') n = -1 #compensates for counting the period flag = False for c in s: if c == '.' : flag = True if flag : n = n + 1 #counts the period also if flag == False: print('No period found ') else: print('# of characters after the period is ', n)