 # Python – Part 4 Conditionals and Recursion. Modulus Operator Yields the remainder when first operand is divided by the second. >>>remainder=7%3 >>>print.

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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

Boolean expressions Type bool – True and False >>>type (True) >>>type (False)

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

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

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)

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

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

Loops 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. 10

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

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. 12 while ( condition ): expression1 expression2 … next expression

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

Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” 14 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 1. 5.Go back to test. 0 OUTPUT

Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” 15 Execution: 6.Perform test: 1 <=5  True. So we enter loop. 7.Print n (so, we print 1) 8.Change n from 1 to 2. 9.Go back to test. 0101 OUTPUT

Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” 16 Execution: 10.Perform test: 2 <=5  True. So we enter loop. 11.Print n (so, we print 2) 12.Change n from 2 to 3. 13.Go back to test. 012012 OUTPUT

Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” 17 Execution: 14.Perform test: 3 <=5  True. So we enter loop. 15.Print n (so, we print 3) 16.Change n from 3 to 4. 17.Go back to test. 01230123 OUTPUT

Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” 18 Execution: 18.Perform test: 4 <= 5  True. So we enter loop. 19.Print n (so, we print 4) 20.Change n from 4 to 5. 21.Go back to test. 0123401234 OUTPUT

Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” 19 Execution: 22.Perform test: 5 <= 5  True. So we enter loop. 23.Print n (so, we print 5) 24.Change n from 5 to 6. 25.Go back to test. 012345012345 OUTPUT

Example 1: n = 0 while ( n <= 5 ): print n n = n + 1 print “Out Of LOOP!” 20 Execution: 26.Perform test: 6 <= 5  False. Skip the loop. 27.Print “Out of LOOP!” 0 1 2 3 4 5 Out of LOOP! OUTPUT

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

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

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

Infinite loop n = 5 while ( n < 6 ): print n n = n - 1 print “Blast off!” – n must always be less than 10. 24

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. 25

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

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)

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

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

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

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

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

Part 4 End

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