1

CS 177 Week 5 Recitation

Booleans and Control Flow

2

Announcements

3

Contents

Booleans Decision structures While loop Project 2 preview

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)

Boolean operators: And, or, not

a and b

a and true

x > 0 and x <=2

y > 0 and y >= 3(overlapped)

Booleans

P Q P and Q

T T T

T F F

F T F

F F F

Boolean operators: And, or, not

a or b

a or true

x <= 0 or x > 2

x > 5 or x < 10 (always true)

Booleans

P Q P or Q

T T T

T F T

F T T

F F F

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)

Booleans

P not P

T F

F T

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

Booleans

P Q P xor Q

T T

T F

F T

F F

Does the result look familiar? How about this: Let T=1, F=0

Yes, it is the sum of binary numbers

Booleans

P Q P xor Q

1 1 0

1 0 1

0 1 1

0 0 0

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

Decision Structures

x = 5if x>1: print(“print something”) The string is printed

x = 0if x>1: print(“print something”) Does nothing!

Two-way decision

Decision Structures

a = 45if a < 100: print(“a is small”)else: print(“a is large”)

>>> a is small

a = 153if a < 100: print(“a is small”)else: print(“a is large”)

>>> a is large

Two-way decision

Decision Structures

a < 100?

“a is large” “a is small”

no yes

Multi-way decision

Decision Structures

a = 1.5if a > 2: print(“a>2”)else: if a > 1: print(“1<a<=2”) else: print(“a<=1”)

>>> 1<a<=2

a = 1.5if a > 2: print(“a>2”)elif a > 1: print(“1<a<=2”)else: print(“a<=1”)

>>> 1<a<=2

Decision Structures

Multi-way decision

14

a >2 ?

“a>2”

no yes

a >1 ?

“1<a<=2”“a<1”

no yes

“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

For vs While Loop

For vs While Loop

count = 0 while count < 9: print(“The count is:”,count) count = count + 1

print(“while loop ended”)

for count in range(0,9): print(“The count is:”, count)

print(“for loop ended”)

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)

17

Interactive loop

Using while loop, we can write interactive loops

18

count = 0str = “Yes” while str == “Yes”: print(“The count is:”,count) count = count + 1str = input(“continue? Yes or No:”)

print(“while loop ended”)

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

19

0pp tt

Project 2 Preview

The structure of a project should be like this:

20

Main function

Function 1

Function 2

.

.

.

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!")

21

An Example from the Book

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)

22

f1 f2

f1 f2parameter return value parameter return value

Questions?

23