python – part 4 conditionals and recursion. modulus operator yields the remainder when first...

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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)Prepared by

Department of Preparatory year

Alternative execution

if – else statementif 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 statement if 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.

10Prepared by Department of Preparatory year

While Statement• New keyword

while

• Syntaxwhile ( condition ): expression1 expression2 …

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MUST end with colon.

MU

ST have indentation.


• 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.

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while ( condition ): expression1 expression2 …

next expression

While Statement


• The ingredients of a loop:n = 0while ( n <= 5 ): print n n = n + 1print “Out Of LOOP!”

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1. Initialize loop variable outside of the loop.

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

While Statement


• Example 1:n = 0while ( n <= 5 ): print n n = n + 1

print “Out Of LOOP!”

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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.

While Statement


0

OUTPUT



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


While Statement


01

OUTPUT



16



While Statement


012

OUTPUT



17



While Statement


0123

OUTPUT



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


While Statement


01234

OUTPUT



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


While Statement


012345

OUTPUT



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

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

012345Out of LOOP!

OUTPUT

While Statement



print “Blast off!”

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Key points:1. Initialize loop variable outside

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

While Statement


• Example 1:n = 1while ( 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.

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While Statement


• 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?

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While Statement


• Infinite loopn = 5while ( n < 6 ): print n n = n - 1


– n must always be less than 10.

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While Statement


• Infinite loopn = 5while ( n != 0 ): print n n = n - 2


– n will never reach the value 0: 9, 7, 5, 3, 1, -1, -2, etc.

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While Statement


• Infinite loopn = 5while ( n >= 0 ): print n n = n - 2


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

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While Statement


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 casedef 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

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python – part 4 conditionals and recursion. modulus operator yields the remainder when first...

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