Conditionals and Recursion. Python Part 4

Transcription

1 Conditionals and Recursion Python Part 4

2 Modulus Operator Yields the remainder when first operand is divided by the second. >>>remainder=7%3 >>>print (remainder) 1

3 Boolean expressions An expression that is either true or false Operator == >>>5==5 True >>>5==6 False

4 Boolean expressions Type bool True and False >>>type (True) <type bool > >>>type (False) <type bool >

5 Boolean expressions Other operators: x!= y # x is not equal to y x > y # x is greater than y x < y # x is less than y x >= y # x is greater than or equal to y x <= y # x is less than or equal to y

6 Logical operators And, or, not Semantics similar to their meaning in English x>0 and x<10 not(x>y) Any nonzero number in Python is interpreted as true >>> 17 and True True

7 If statement Conditional execution if x>0: # CONDITION print ( x is positive ) Same structure as function definition - Header - Indented block - No limit on number of statements in the body (but at least one)

8 Alternative execution if else statement if x%2==0: print ( x is even ) else: print ( x is odd ) - Exactly one of the alternatives executed - Alternatives are called branches

9 Chained conditionals if-elseif statement if x<y: print ( x is less than y ) elif x>y: print ( x is greater than y ) else: print ( x and y are equal ) - Exactly one branch executed (no limit on number of elseif stmts). If there is else must be at the end

10 What is a loop for? Loops To repeat a piece of code over and over. Examples: Iterating through an array (sum, search, print, etc.) Run the main program loop (i.e. keep asking for user input until the program is over.) Etc. 10

11 MUST have indentation. New keyword while While Statement Syntax while ( condition ): expression1 expression2 MUST end with colon. 11

12 While execution: Perform test If test true, go to body. Execute body expressions. At the end of the block, go back to test. If test is false, go on. while ( condition ): expression1 expression2 next expression 12

13 The ingredients of a loop: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! 1. Initialize loop variable outside of the loop. 2. Define loop condition. 3. Do loop work. 4. Change the loop variable. 13

14 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! Execution: 1. Loop variable, n, set to 0 outside the loop. 2. Perform test: 0 <=5 True. So we enter loop. 3. Print n (so, we print 0) 4. Change n from 0 to Go back to test. OUTPUT 0 14

15 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! Execution: 6. Perform test: 1 <=5 True. So we enter loop. 7. Print n (so, we print 1) 8. Change n from 1 to Go back to test. OUTPUT

16 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! Execution: 10. Perform test: 2 <=5 True. So we enter loop. 11. Print n (so, we print 2) 12. Change n from 2 to Go back to test. OUTPUT

17 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! Execution: 14. Perform test: 3 <=5 True. So we enter loop. 15. Print n (so, we print 3) 16. Change n from 3 to Go back to test. OUTPUT

18 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! Execution: 18. Perform test: 4 <= 5 True. So we enter loop. 19. Print n (so, we print 4) 20. Change n from 4 to Go back to test. OUTPUT

19 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! Execution: 22. Perform test: 5 <= 5 True. So we enter loop. 23. Print n (so, we print 5) 24. Change n from 5 to Go back to test. OUTPUT

20 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Out Of LOOP! OUTPUT Out of LOOP! Execution: 26. Perform test: 6 <= 5 False. Skip the loop. 27. Print Out of LOOP! 20

21 Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print Blast off! Key points: 1. Initialize loop variable outside of the loop. 2. Determine loop condition. 3. Do loop work. 4. Change the test value. 21

22 Example 1: n = 1 while ( n <= 5 ): print n # n = n + 1 print Blast off! What would happen if we didn t change the loop variable? The loop condition would never become false. 22

23 Infinite loop When the test condition never has the chance to become False, you have an infinite loop. World s simplest infinite loop: while ( True ): print hi Other possible infinite loops? 23

24 Infinite loop n = 5 while ( n < 6 ): print n n = n - 1 print Blast off! n must always be less than 6. 24

25 Infinite loop n = 5 while ( n!= 0 ): print n n = n - 2 print Blast off! n will never reach the value 0: 5, 3, 1, -1, -3, -5, -7, etc. 25

26 Infinite loop n = 5 while ( n >= 0 ): print n n = n - 2 print Blast off! Not an infinite loop. When n reaches -1, the test wil no longer be true. 26

27 Recursion One function calls itself def countdown(n): if n <= 0: print ('Blastoff!' ) else: print (n) countdown(n-1) - What happens if we call - >>> coundown (3)

28 def print_n(s, n): if n <= 0: return print (s) print_n(s, n-1) Recursion - return statement exits the function - Base case - Recursive (general) case

29 Infinite recursion Recursion never reaches a base case def recurse(): recurse()

30 Keyboard input Built-in function called input (previous versions raw_input) Program stops and waits for the user to type something Value pressed returned to program as a string Good idea to print a prompt telling user what to input

31 Keyboard input >>>name =input ( What is your name?\n ) Arthur, King of the Britons! >>>print (name) Arthur, King of the Britons! \n represents a newline

32 Keyboard input >>> prompt = 'What is the velocity?\n' >>> speed = input(prompt) What is the velocity? 17 >>> int(speed) 17

33 End Part 4