Python – Part 4

Conditionals and Recursion

Modulus Operator

• Yields the remainder when first operand is divided by the second.

• >>>remainder=7%3

• >>>print (remainder)

• 1

Boolean expressions

• An expression that is either true or false

• Operator ==

• >>>5==5

• True

• >>>5==6

• False

Boolean expressions

• Type bool – True and False

• >>>type (True)

• <type ‘bool’>

• >>>type (False)

• <type ‘bool’>

Boolean expressions

Other operators:

x != y # x is not equal to y

x > y # x is greater than y

x < y # x is less than y

x >= y # x is greater than or equal to y

x <= y # x is less than or equal to y

Logical operators

• And, or, not

• Semantics similar to their meaning in English

• x>0 and x<10

• not(x>y)

• Any nonzero number in Python is interpreted as “true”

• >>> 17 and True

• True

Conditional execution

• If statementif x>0: # CONDITION

print (‘x is positive’)

Same structure as function definition

- Header

- Indented block

- No limit on number of statements in the body (but at least one)

Alternative execution

if – else statement

if x%2==0:

print (‘x is even’)

else:

print (‘x is odd’)

- Exactly one of the alternatives executed

- Alternatives are called branches

Chained conditionals

if-elseif statementif x<y:

print (‘x is less than y’)elif x>y:

print (‘x is greater than y’)else:

print (‘x and y are equal’)

- Exactly one branch executed (no limit on number of elseif stmts). If there is else must be at the end

Loops

• What is a loop for?

– To repeat a piece of code over and over.

– Examples:

• Iterating through an array (sum, search, print, etc.)

• Run the main program loop (i.e. keep asking for user input until the program is over.)

• Etc.

10

While Statement• New keyword

while

• Syntaxwhile ( condition ):

expression1

expression2

…

11

MUST end with colon.

MU

ST have in

den

tation

.

• While execution:

– Perform test

– If test true, go to body.

• Execute body expressions.

• At the end of the block, go back to test.

– If test is false, go on.

12

while ( condition ):

expression1

expression2

…

next expression

• The ingredients of a loop:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

13

1. Initialize loop variable outsideof the loop.

2. Define loop condition.3. Do loop work.4. Change the loop variable.

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

14

Execution:1. Loop variable, n, set to 0

outside the loop.2. Perform test: 0 <=5 True. So

we enter loop.3. Print n (so, we print 0)4. Change n from 0 to 1.5. Go back to test.

0

OUTPUT

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

15

Execution:6. Perform test: 1 <=5 True. So

we enter loop.7. Print n (so, we print 1)8. Change n from 1 to 2.9. Go back to test.

0

1

OUTPUT

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

16

Execution:10. Perform test: 2 <=5 True. So

we enter loop.11. Print n (so, we print 2)12. Change n from 2 to 3.13. Go back to test.

0

1

2

OUTPUT

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

17

Execution:14. Perform test: 3 <=5 True. So

we enter loop.15. Print n (so, we print 3)16. Change n from 3 to 4.17. Go back to test.

0

1

2

3

OUTPUT

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

18

Execution:18. Perform test: 4 <= 5 True. So

we enter loop.19. Print n (so, we print 4)20. Change n from 4 to 5.21. Go back to test.

0

1

2

3

4

OUTPUT

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

19

Execution:22. Perform test: 5 <= 5 True. So

we enter loop.23. Print n (so, we print 5)24. Change n from 5 to 6.25. Go back to test.

0

1

2

3

4

5

OUTPUT

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Out Of LOOP!”

20

Execution:26. Perform test: 6 <= 5 False.

Skip the loop.27. Print “Out of LOOP!”

0

1

2

3

4

5

Out of LOOP!

OUTPUT

• Example 1:n = 0

while ( n <= 5 ):

print n

n = n + 1

print “Blast off!”

21

Key points:1. Initialize loop variable outside

of the loop.2. Determine loop condition.3. Do loop work.4. Change the test value.

• Example 1:n = 1

while ( n <= 5 ):

print n

# n = n + 1

print “Blast off!”

– What would happen if we didn’t change the loop variable?• The loop condition would never become false.

22

• Infinite loop

– When the test condition never has the chance to become False, you have an infinite loop.

– World’s simplest infinite loop:while ( True ):

print “hi”

• Other possible infinite loops?

23

• Infinite loopn = 5

while ( n < 6 ):

print n

n = n - 1

print “Blast off!”

– n must always be less than 6.

24

• Infinite loopn = 5

while ( n != 0 ):

print n

n = n - 2

print “Blast off!”

– n will never reach the value 0: 5, 3, 1, -1, -3, -5, -7, etc.

25

• Infinite loopn = 5

while ( n >= 0 ):

print n

n = n - 2

print “Blast off!”

– Not an infinite loop. When n reaches -1, the test wil no longer be true.

26

Recursion

• One function calls itselfdef countdown(n):

if n <= 0: print ('Blastoff!' )

else: print (n)countdown(n-1)

- What happens if we call- >>> coundown (3)

Recursion

• def print_n(s, n):if n <= 0:

return

print (s)

print_n(s, n-1)

- return statement exits the function

- Base case

- Recursive (general) case

Infinite recursion

• Recursion never reaches a base case

def recurse():

recurse()

Keyboard input

• Built-in function called input (previous versions raw_input)

• Program stops and waits for the user to type something

• Value pressed returned to program as a string

• Good idea to print a prompt telling user what to input

Keyboard input

• >>>name =input (‘What is your name?\n’)

• Arthur, King of the Britons!

• >>>print (name)

• Arthur, King of the Britons!

• \n represents a newline

Keyboard input

• >>> prompt = 'What is the velocity?\n'

• >>> speed = input(prompt)

• What is the velocity?

• 17

• >>> int(speed)

• 17

Part 4

End