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Conditional Expressions

and Decision Statements

June 1, 2015Brian A. Malloy

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1. Modulus

• We have introduced 5 operators for addi-tion, subtraction, multiplication, division,and exponentiation:+, -, *, /, **• We now introduce a very useful operator

called modulus, which returns the remain-der after division.

• Use operator for conditional expressions

• The modulus operator is %• For example, 10%3=1, 13%2=1, 1%2=1

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1.1. What good is modulus?

• Useful for lots of conditions:

– Determine if a number is even

– Limit the range of a number

– Wrapping around

– Finding the value of a digit

– . . .

• How to determine even/odd:

– Assume number is stored in x

– if x % 2 is 0, then x is even,otherwise x is odd.

– Example: 5 % 2 == 1 ⇒ 5 is odd

– Example: 6 % 2 == 0 ⇒ 6 is even

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2. Boolean Values

• We have described three data types:integer, float, string.

• We now add a fourth data type, bool, whichevaluates to True or False.

• Named after mathematician, George Boole

• A boolean expression evaluates to eitherTrue or False.

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3. Boolean Expressions

• There are 6 boolean operators:==, !=, <, >, ≤, ≥• These operators are interpreted as follows:

(a) == compares two values; returns Trueif operands are equal, False otherwise.

(b) != compares two values; returns Trueif they are not equal, False otherwise

(c) etc.

• When used in a Python program, ≤ mustbe written <=, and ≥ must be written >=

• Examples:

(a) 3 < 3 is False, but

(b) 3 <= 3 is True

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3.1. Boolean Expressions in Python

• Can print the type of a Boolean expression:print type(True)<type ’bool’>

• Can print the value of Boolean expression:

$ python

Python 2.7.6 (default, Mar 22 2014, 22:59:56)

[GCC 4.8.2] on linux2

>>> x = 5

>>> x != 7

True

>>> x >= 12

False

>>> x > 5

False

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3.2. Determine if a number is evenPython 2.7.6 (default, Mar 22 2014, 22:59:56)

[GCC 4.8.2] on linux2

Type "help", "copyright", "credits"

>>> x = input("Type a number: ")

Type a number: 7

>>> x % 2

1

>>>

Since x % 2 is 1, this means x is odd

Explanation: for the above program: if the valueof x % 2 == 0 is True then the number is even,otherwise the number is odd.

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4. Decisions

• Much of the power of computer program-ming, and computers in general, derivesfrom the ability to make decisions and loop.

• Decision structures in Python, and otherlanguages, take many forms:

1. Conditional execution

2. Alternative execution

3. Chained conditionals

4. Nested Conditionals

• We’ll describe each decision structure next

• We’ll save loops for future presentations

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4.1. Conditional Execution

• Means that a sequence of 1 or more state-ments will execute only if a certain condi-tion is satisfied.

• Provides the ability to change the bahaviorof a program for each execution.

• Format:

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4.1.1. Conditional Execution: input

• Example, the following program:

– has output if x is greater than zero

– has no output if x is zero or negative

– Notice use of colon :

x = input("Type a number: ")

if x > 0:

print x, "is positive"

$ python if.py

Type a number: 34

34 is positive

$ python if.py

Type a number: -4

$ python if.py

Type a number: 0

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4.1.2. Conditional Execution: spaces

• In the following, if x > 0, then 2 statementsare executed, numbered (3) & (4)

• Spaces must be the same on, (3) & (4)

This one works:

(1) x = input("Type a number: ")

(2) if x > 0:

(3) print x, "is positive"

(4) print x, "is not zero or negative"

This one doesn’t work:

(1) x = input("Type a number: ")

(2) if x > 0:

(3) print x, "is positive"

(4) print x, "is not zero or negative"

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4.1.3. Conditional Execution: Flowchart

The flowchart on the title page of these slidesaptly describes conditional execution. That is,statement(s) are executed only if condition is true:

If condition is False, statement(s) are skipped,and execution goes straight to rest of program

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4.2. Alternative Execution

• The problem with Conditional Execution isthat statement(s) are only executed if thecondition is True.

• We may want output in either case:statement(s) if condition is True.statement(s) if condition is False.

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4.2.1. Alternative Execution: input

Now we get output in either case:

x = input("Type a number: ")

if x % 2 == 0:

print x, "is even"

else:

print x, "is odd"

$ python ifelse.py

Type a number: 4

4 is even

$ python ifelse.py

Type a number: 7

7 is odd

$ python ifelse.py

Type a number: 0

0 is even

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4.3. Chained Conditionals

• Use this if more than 2 possible outcomes

• Notice keywords elif and else

• Consider a trichotomy example:

1 x = input("Type a number: ")

2 if x > 0:

3 print x, "is greater than zero"

4 elif x == 0:

5 print x, "is zero"

6 else:

7 print x, "is less than zero"

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4.4. Nested Conditionals

• Can “nest” conditional within a conditional

• spacing is intended to make it clear whatgets executed

• But Nested Conditionals can be confusing

• Consider an alternative to trichotomy; state-ments 5–8 happen only if x == 0 is false.

1 x = input("Type a number: ")

2 if x == 0:

3 print x, "is zero"

4 else:

5 if x > 0:

6 print x, "is greater than zero"

7 else:

8 print x, "is less than zero"

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5. Logical Operators

• These include and, or, and not

• They can simplify conditional execution

• The following examples do the same thing;Which is easier to understand:

if x > 0:

if x < 10:

print x, "is a single digit number"

if x > 0 and x < 10:

print x, "is a single digit number"

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6. return statement• It’s possible, sometimes necessary, to re-

turn from function before last statement.

• Remember, division by 0 is undefined

1 def findAverage(sum, n):

2 if n <= 0:

3 return 0

4 return sum/n

5

6 print findAverage(25, 7)

7 print findAverage(25, 0)

3

0

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6.1. return statement

• If a function contains no return statement,there is always one implied at the end of afunction

• The following functions are the same:

1 def display(n):

2 print "n is:", n

1 def display(n):

2 print "n is:", n

3 return

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7. Now What?• Need to write programs

• Use conditionals/decisions tosolve problems

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7.1. Evaluate

• Try to evaluate the following expressionsin your head, then check your answer withPython interpreter:

1. >>> 5 % 2

2. >>> 9 % 5

3. >>> 15 % 12

4. >>> 12 % 15

5. >>> 6 % 6

6. >>> 0 % 7

7. >>> 7 % 0

• What happened with the last example?

• Explore modulus until you’re convinced youunderstand how it works

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7.2. True/False ⇒ 1/0

• What’s the output? Why?

1 if ’A’:

2 print ’yes’

3 else:

4 print ’no’

• What’s the output? Why?

1 if 1:

2 print ’yes’

3 else:

4 print ’no’

• 0 ⇒ False

• 1 ⇒ True, 2 ⇒ True, 3 ⇒ True, . . .

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7.3. Problems

• Are negative numbers True or False

• Write code to determine if:

1. A number is even

2. If a number is a multiple of 3?

3. If a number is a multiple of 17?

4. If user input is a digit?

5. If user input is a letter?

6. If x is off a screen that’s 640x480?

7. If user input is a vowel?

8. . . .