Outline. Announcements. Homework 2. Boolean expressions 10/12/2007. Announcements Homework 2 questions. Boolean expression

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1 Outline ECS 10 10/8 Announcements Homework 2 questions Boolean expressions If/else statements State variables and avoiding sys.exit( ) Example: Coin flipping (if time permits) Announcements Professor Amenta will be back on Friday Dan Alcantara will be covering Monday and Wednesday lectures address: dfalcantara@gmail.com Slides will be on class website Temporary location: Homework 2 Homework 2 due Wednesday at 10PM Don t use sys.exit( ) Will explain why later today Questions? Boolean expressions Boolean expression Boolean algebra Named after George Boole ( ) 1864) Main idea: you can write down logic as mathematical formulas, as well as in sentences. Logic as a computational system. Python does some of this computation! 1

2 Boolean expressions Boolean is a new data type. Booleans can be either or. We now have four data types (and four kinds of expressions): Data type Examples Integer 1, 10, 53 Floating point , 6.29 String Hey,, String Boolean, Basic Boolean expressions Expression Expression in English X == Y X is equal to Y X!= Y X is not equal to Y X < Y X is less than Y X > Y X is greater than Y X <= Y X is less than or equal to Y X >= Y X is greater than or equal to Y not Whatever the condition, it s not Boolean expressions are if the expression in English is, and otherwise. Boolean expression examples Compound Boolean expressions Boolean expression Something == Something English expression The string Something is exactly the same as the string Something. 3 < 5 The integer 3 is less than 5. 3 > 5 The integer 3 is greater than 5. Cat == cat Cat!= cat The string Cat is exactly the same as the string cat. The string Cat is different from the string cat. Can combine multiple Boolean expressions with and and or keywords. Boolean expression 0 < 10 and 0 < 5 English expression 0 is less than 10 and 0 is less than 5 Boolean expression Dog == Dog or > 5 English expression Dog is exactly the same as Dog or 3+2 is greater than 5 Boolean expression == 4 and == 11 English expression 2+2 equals 4 and equals 11 If statements change the flow of the program Allows different things to happen under different conditions print Welcome,,, name,! If the player doesn t have a name, then: Call the player Anonymous Tell the player that he/she will be called Anonymous If the player does have a name, then: Welcome the player 2

3 print Welcome,,, name,! Check the player s name Check the player s name Player didn t give a name Player gave a name Player didn t give a name Player gave a name Rename the player Anonymous Tell the player they ve been renamed Welcome the player Rename the player Anonymous Tell the player they ve been renamed Continue the program Continue the program or and changes what the program does next. or and changes what the program does next. This is a block, which consists of a sequential set of lines indented the same way. The entire block is run when the condition is. or and changes what the program does next. This is a block, which consists of a sequential set of lines indented the same way. The entire block is run when the condition is. This is a condition, which is either or. This specific condition checks to see whether or not the variable name equals the empty string. Side note: Why sys.exit( ) is bad Some of you have been using it for program 2 Breaks the flow of the program Anything that is expected to happen after the sys.exit( ) call doesn t happen 3

4 Why sys.exit( ) is bad playeranswer = raw_input( What s your answer? ) if playeranswer == 42 : print You re right! playerscore = playerscore print You got it wrong! print Your final score is, playerscore Ask for the player s answer Check the player s answer Answered 42 Didn t answer 42 Why sys.exit( ) is bad playeranswer = raw_input( What s your answer? ) if playeranswer == 42 : print You re right! playerscore = playerscore sys.exit( You got it wrong! ) print Your final score is, playerscore Ask for the player s answer Check the player s answer Answered 42 Didn t answer 42 Congratulate the player Award the player 1000 points Print the final score Insult the player Congratulate the player Award the player 1000 points Print the final score Insult the player X Booleans as state variables Example program: Coin flipping State variables store information about the program s current state For homework 2, can be combined with if statements to determine if program should stop Their use avoids the need for sys.exit( ) call skiptherules = if skiptherules == : print Rule 1 print Rule 2 print On with the game! Objective: Python program will simulate flipping a coin Coin will land on either heads or tails User will guess either heads or tails before the flip Problem: Python doesn t have any way of randomly flipping a coin Need to add a module Our first module A module is a collection of Python random functions, module: maybe Generates other random stuff numbers thrown in (library in other computer languages) import the module to use its functions import random For random numbers, need the random module All the functions in module random are named random.something() Your program Random module Program that picks a number between 1 and 10 # get access to random number module import random # use function that picks a random # integer between 1 and 10 (inclusive) x = random.randint(1,10) 4

5 Example program: Coin flipping Use random.randint(1,2) to pick either 1 or 2 If 1, then coin is heads. If 2, then coin is tails. Start simple, then Now let user pick Heads or Tails User s choice has to match coin. Uses Boolean and and or. Key command if (choice == "H" and coin == 1)\ or (choice == "T" and coin == 2): \ means statement continues on next line Two ways to win; if either situation occurs, user wins. Each situation is an and or them together Or: Use to check input if (choice == "H") or (choice == "T"): And: if (choice!= "H") and (choice!= "T"): 5