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Python – Part 4

Python – Part 4. Conditionals and Recursion. Modulus Operator . Yields the remainder when first operand is divided by the second. >>>remainder=7%3 >>>print (remainder) 1. Boolean expressions. An expression that is either true or false Operator == >>>5==5 True >>>5==6 False.

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Python – Part 4

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  1. Python – Part 4 Conditionals and Recursion

  2. Modulus Operator • Yields the remainder when first operand is divided by the second. • >>>remainder=7%3 • >>>print (remainder) • 1 Prepared by Department of Preparatory year

  3. Boolean expressions • An expression that is either true or false • Operator == • >>>5==5 • True • >>>5==6 • False Prepared by Department of Preparatory year

  4. Boolean expressions • Type bool – True and False • >>>type (True) • <type ‘bool’> • >>>type (False) • <type ‘bool’> Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  7. Conditional execution • If statement 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) Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  10. Loops 10 • What is a loop for? • 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. Prepared by Department of Preparatory year

  11. While Statement MUST end with colon. MUST have indentation. 11 • New keyword while • Syntax while ( condition ): expression1 expression2 … Prepared by Department of Preparatory year

  12. While Statement • while ( condition ): expression1 expression2 … • next expression 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. Prepared by Department of Preparatory year

  13. While Statement • Initialize loop variable outside of the loop. • Define loop condition. • Do loop work. • Change the loop variable. 13 • The ingredients of a loop: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  14. While Statement Execution: Loop variable, n, set to 0 outside the loop. Perform test: 0 <=5 True. So we enter loop. Print n (so, we print 0) Change n from 0 to 1. Go back to test. OUTPUT 0 14 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  15. While Statement Execution: Perform test: 1 <=5  True. So we enter loop. Print n (so, we print 1) Change n from 1 to 2. Go back to test. OUTPUT 0 1 15 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  16. While Statement Execution: Perform test: 2 <=5  True. So we enter loop. Print n (so, we print 2) Change n from 2 to 3. Go back to test. OUTPUT 0 1 2 16 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  17. While Statement Execution: Perform test: 3 <=5  True. So we enter loop. Print n (so, we print 3) Change n from 3 to 4. Go back to test. OUTPUT 0 1 2 3 17 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  18. While Statement Execution: Perform test: 4 <= 5  True. So we enter loop. Print n (so, we print 4) Change n from 4 to 5. Go back to test. OUTPUT 0 1 2 3 4 18 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  19. While Statement Execution: Perform test: 5 <= 5  True. So we enter loop. Print n (so, we print 5) Change n from 5 to 6. Go back to test. OUTPUT 0 1 2 3 4 5 19 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  20. While Statement Execution: Perform test: 6 <= 5  False. Skip the loop. Print “Out of LOOP!” OUTPUT 0 1 2 3 4 5 Out of LOOP! 20 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” Prepared by Department of Preparatory year

  21. While Statement Key points: Initialize loop variable outside of the loop. Determine loop condition. Do loop work. Change the test value. 21 • Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Blast off!” Prepared by Department of Preparatory year

  22. While Statement 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. Prepared by Department of Preparatory year

  23. While Statement 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? Prepared by Department of Preparatory year

  24. While Statement 24 • Infinite loop n = 5 while ( n < 6 ): print n n = n - 1 print “Blast off!” • n must always be less than 10. Prepared by Department of Preparatory year

  25. While Statement 25 • Infinite loop n = 5 while ( n != 0 ): print n n = n - 2 print “Blast off!” • n will never reach the value 0: 9, 7, 5, 3, 1, -1, -2, etc. Prepared by Department of Preparatory year

  26. While Statement 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. Prepared by Department of Preparatory year

  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) Prepared by Department of Preparatory year

  28. Recursion • def print_n(s, n): if n <= 0: return print (s) print_n(s, n-1) • return statement exits the function • Base case • Recursive (general) case Prepared by Department of Preparatory year

  29. Infinite recursion • Recursion never reaches a base case def recurse(): recurse() Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  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 Prepared by Department of Preparatory year

  32. Keyboard input • >>> prompt = 'What is the velocity?\n' • >>> speed = input(prompt) • What is the velocity? • 17 • >>> int(speed) • 17 Prepared by Department of Preparatory year

  33. Part 4 End

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