Course A201: Introduction to Programming 09/16/2010

Outlines for today A new type of value: Boolean Concept of keyword and indentation Several basic concepts about Branching – IF - ELSE – IF and WHILE loop Keep trace: what’s the stop condition How and when to use “break” and “continue” Demonstration of “Guess My Number Game”

Boolean and Comparison Operators Boolean values: True or False Statements that will generate boolean values: 1 == 1True 2 <= 0False 4 > 1True 9 != 99True “1” == 1False These are Comparison operators

keyword In Assignment 1, question 2, the last variable is “for”, which seams legit but actually not This is a KEYWORD Keywords are a set of words that have special/predefined meanings: for, if, else, while, break, import, from They will appear in orange in.py files in IDLE.

Indentation In Python, proper indentations are crucial -> Python use indentations to determine the structure of if- else blocks, while blocks, etc. If you forget to have indentations, it will give you syntax error Enter tab if user == “bye”: print ( “bye!” ) elif user == “hi”: print( “Hi” ) else: print(“…”)

Conditional: IF if user == “bye”: print ( “bye!” ) elif user == “hi”: print( “Hi” ) else: print(“…”) Don’t forget the COLON You can have only IF, or IF- ELSE, or IF-ELIF, or IF-ELIF- ELSE You can have another IF block inside a IF block: if …: some statements

Conditional: WHILE gpa = 0.0 while gpa <= 4.0: gpa = float(raw_input(“Enter GPA: ” )) print(“Not high enough” ) While [this condition] is true, keep executing the [statements in here ] Creating loops, make repetitive tasks easier

Keep trace What’s wrong with this program: health = 10 trolls = 0 damage = 3 while health != 0: trolls += 1 health -= damage

Keep trace Healthtrollsdamagehealth!= True 71 3 True 4 23 True True True True True

Keep trace What’s wrong with this program: health = 10 trolls = 0 damage = 3 while health != 0: trolls += 1 health -= damage <- This condition will be true FOREVER!!!

Keep trace What’s wrong with this program: health = 10 trolls = 0 damage = 3 while health > 0: trolls += 1 health -= damage <- You have to confirm that this condition will be false at some time

Usage of break gpa = 0.0 while True: gpa = float(raw_input(“Enter GPA: ” )) if gpa > 4.0: break Break out of the loop

Usage of continue counter = 0 while counter <= 10: count += counter if count == 5 continue print(counter) print(“-” * 20) The output won’t include 5. When you reach continue, the rest of code (in the red rectangle) will be omitted, computer goes straight to next interation.

Guess My Number Game pick a random number while the player hasn't guessed the number, let the player guess If guess is right, congratulate the player

Lab work Write a program that flips a coin 100 times and then tells you the number of head and tails Modify your program so that it prompts the user for the number of times to flip the coin On paper in English first, on computer in Python next Optional team work

Have a nice evening! See you tomorrow~