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CS 177 Week 5 Recitation

CS 177 Week 5 Recitation. Booleans and Control Flow. Announcements. Contents. Booleans Decision structures While loop Project 2 preview. Booleans. Boolean (logical) expressions: An expression that can assume only the true or false value

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CS 177 Week 5 Recitation

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  1. CS 177 Week 5 Recitation Booleans and Control Flow

  2. Announcements

  3. Contents Booleans Decision structures While loop Project 2 preview

  4. Booleans • Boolean (logical) expressions: • An expression that can assume only the true or false value • We use logical expression in everyday language: • If today is raining, then bring an umbrella when you go out. • “today is raining” is a logical expression: its value can be either true or false. • Other examples: assume x=4 assume str=“abc” • x>3 type(str)==int • (true) (false)

  5. Booleans • Boolean operators: • And, or, not a and b a and true x > 0 and x <=2 y > 0 and y >= 3(overlapped)

  6. Booleans • Boolean operators: • And, or, not a or b a or true x <= 0 or x > 2 x > 5 or x < 10 (always true)

  7. Booleans • Boolean operators: • And, or, not not a not (not a) not x > 3 DeMorgan’s law not (a or b) == (not a) and (not b) not (a and b) == (not a) or (not b)

  8. Booleans • (P and (not Q)) or ((not P) and Q) • It has a name: XOR • Can you do this?

  9. Booleans • Does the result look familiar? • How about this: Let T=1, F=0 • Yes, it is the sum of binary numbers

  10. Decision Structures x = 5 if x>1: print(“print something”) The string is printed x = 0 if x>1: print(“print something”) Does nothing! • If statement • An if statement takes a logical expression and evaluates it. • If it is true, the statements in if block are executed, otherwise, they are not executed. Simple decision

  11. Decision Structures a = 45 if a < 100: print(“a is small”) else: print(“a is large”) >>> a is small a = 153 if a < 100: print(“a is small”) else: print(“a is large”) >>> a is large • Two-way decision

  12. Decision Structures no yes a < 100? “a is large” “a is small” • Two-way decision

  13. Decision Structures a = 1.5 if a > 2: print(“a>2”) else: if a > 1: print(“1<a<=2”) else: print(“a<=1”) >>> 1<a<=2 a = 1.5 if a > 2: print(“a>2”) elif a > 1: print(“1<a<=2”) else: print(“a<=1”) >>> 1<a<=2 • Multi-way decision

  14. Decision Structures no yes a >2 ? no yes a >1 ? “a>2” “a<1” “1<a<=2” Multi-way decision

  15. For vs While Loop • “How long do I have to shower?” You have two replies you could make: • a) 10 minutes • b) Until you are clean • a) When programming, the first answer would be portrayed as a for-loop because we focus on exactly how long the shower will continue: for minutes in range (0,9): shower • b) The second answer would be portrayed as a while-loop because the length of the shower is undetermined; we instead focus on the condition of cleanliness: while you are not clean: shower

  16. For vs While Loop • for count in range(0,9): • print(“The count is:”, count) • print(“for loop ended”) • count = 0 • while count < 9: • print(“The count is:”,count) • count = count + 1 • print(“while loop ended”)

  17. Rules of While Loops • Command name: while • Boolean condition (in the previous example: count < 9) • determines the termination of loop • A colon (“:”) • And a block (the indented lines of code)

  18. Interactive loop • count = 0 • str = “Yes” • while str == “Yes”: • print(“The count is:”,count) • count = count + 1 • str = input(“continue? Yes or No:”) • print(“while loop ended”) Using while loop, we can write interactive loops

  19. Project 2 Preview • Population Growth Simulation • The Geometric growth model • No limitation on the population • For loop • Show the population in characters • Limitation on the population • While loop • The limitation is the stopping criterion • Show the population in graphs

  20. Project 2 Preview Main function Function 1 Function 2 . . . The structure of a project should be like this:

  21. An Example from the Book def main(): sing("Fred") print() sing("Lucy") def sing(person): happy() happy() print("Happy birthday, dear", person + ".“) happy() def happy(): print("Happy birthday to you!")

  22. An Example from the Book f1 f2 parameter return value parameter return value f1 f2 • Functions link to each other by their parameters and return values • What does f2(f1()) mean? • Pass the function f1 to f2? No • Pass the return value of f1 to f2? Yes • v=f1() • f2(v)

  23. Questions?

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