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Defining Functions Academic Integrity Quiz Remember : quiz about - - PowerPoint PPT Presentation

Aug 20, 2022 284 likes •766 views

Lecture 4 Defining Functions Academic Integrity Quiz Remember : quiz about the course AI policy Have posted grades for completed quizes Right now, missing ~130 enrolled students If did not receive at least 9/10, take it again If

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SLIDE 1

Defining Functions

Lecture 4

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SLIDE 2

Academic Integrity Quiz

  • Remember: quiz about the course AI policy

§ Have posted grades for completed quizes § Right now, missing ~130 enrolled students § If did not receive at least 9/10, take it again

  • If you are not aware of the quiz

§ Go to http://www.cs.cornell.edu/courses/cs11110/ § Click Academic Integrity in side bar § Read and take quiz in CMS

9/4/18 Defining Functions 2

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SLIDE 3

Recall: Modules

  • Modules provide extra functions, variables

§ Example: math provides math.cos(), math.pi § Access them with the import command

  • Python provides a lot of them for us
  • This Lecture: How to make modules

§ Atom Editor to make a module § Python to use the module

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Two different programs

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SLIDE 4

We Write Programs to Do Things

  • Functions are the key doers

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Function Call Function Definition

  • Command to do the function

>>> plus(23) 24 >>>

  • Defines what function does

def plus(n): return n+1

  • Parameter: variable that is listed within

the parentheses of a method header.

  • Argument: a value to assign to the method

parameter when it is called

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SLIDE 5

We Write Programs to Do Things

  • Functions are the key doers

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Function Call Function Definition

  • Command to do the function

>>> plus(23) 24 >>>

  • Defines what function does

def plus(n): return n+1

  • Parameter: variable that is listed within

the parentheses of a method header.

  • Argument: a value to assign to the method

parameter when it is called

Function Header

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SLIDE 6

We Write Programs to Do Things

  • Functions are the key doers

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Function Call Function Definition

  • Command to do the function

>>> plus(23) 24 >>>

  • Defines what function does

def plus(n): return n+1

  • Parameter: variable that is listed within

the parentheses of a method header.

  • Argument: a value to assign to the method

parameter when it is called

Function Header Function Body (indented)

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SLIDE 7

We Write Programs to Do Things

  • Functions are the key doers

9/4/18 Defining Functions 7

Function Call Function Definition

  • Command to do the function

>>> plus(23) 24 >>>

  • Defines what function does

def plus(n): return n+1

  • Parameter: variable that is listed within

the parentheses of a method header.

  • Argument: a value to assign to the method

parameter when it is called

Function Header Function Body (indented) declaration of parameter n argument to assign to n

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SLIDE 8

Anatomy of a Function Definition

def plus(n): """Returns the number n+1 Parameter n: number to add to Precondition: n is a number""" x = n+1 return x

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Function Header name parameters Docstring Specification Statements to execute when called

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SLIDE 9

Anatomy of a Function Definition

def plus(n): """Returns the number n+1 Parameter n: number to add to Precondition: n is a number""" x = n+1 return x

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Function Header name parameters Docstring Specification Statements to execute when called The vertical line indicates indentation

Use vertical lines when you write Python

  • n exams so we can see indentation
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SLIDE 10

The return Statement

  • Format: return <expression>

§ Used to evaluate function call (as an expression) § Also stops executing the function! § Any statements after a return are ignored

  • Example: temperature converter function

def to_centigrade(x): """Returns: x converted to centigrade""" return 5*(x-32)/9.0

9/4/18 Defining Functions 10

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SLIDE 11

A More Complex Example

Function Definition def foo(a,b): """Return something Param a: number Param b: number""" x = a y = b return x*y+y Function Call >>> x = 2 >>> foo(3,4)

9/4/18 Defining Functions 11

? x

What is in the box?

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SLIDE 12

A More Complex Example

Function Definition def foo(a,b): """Return something Param a: number Param b: number""" x = a y = b return x*y+y Function Call >>> x = 2 >>> foo(3,4)

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? x

What is in the box?

A: 2 B: 3 C: 16 D: Nothing! E: I do not know

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SLIDE 13

A More Complex Example

Function Definition def foo(a,b): """Return something Param a: number Param b: number""" x = a y = b return x*y+y Function Call >>> x = 2 >>> foo(3,4)

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? x

What is in the box?

A: 2 B: 3 C: 16 D: Nothing! E: I do not know CORRECT

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SLIDE 14
  • Number of statement in the

function body to execute next

  • Starts with 1

Draw parameters as variables (named boxes)

Understanding How Functions Work

  • Function Frame: Representation of function call
  • A conceptual model of Python

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function name local variables (later in lecture) parameters instruction counter

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SLIDE 15

Text (Section 3.10) vs. Class

Textbook This Class

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def to_centigrade(x): return 5*(x-32)/9.0

Call: to_centigrade(50.0) Definition:

to_centigrade 1 50.0 x to_centigrade

x –> 50.0

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SLIDE 16

Example: to_centigrade(50.0)

  • 1. Draw a frame for the call
  • 2. Assign the argument value

to the parameter (in frame)

  • 3. Execute the function body

§ Look for variables in the frame § If not there, look for global variables with that name

  • 4. Erase the frame for the call

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def to_centigrade(x): return 5*(x-32)/9.0 to_centigrade 1 x Initial call frame (before exec body) next line to execute 1 50.0

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SLIDE 17

Example: to_centigrade(50.0)

  • 1. Draw a frame for the call
  • 2. Assign the argument value

to the parameter (in frame)

  • 3. Execute the function body

§ Look for variables in the frame § If not there, look for global variables with that name

  • 4. Erase the frame for the call

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def to_centigrade(x): return 5*(x-32)/9.0 to_centigrade 50.0 x Executing the return statement Return statement creates a special variable for result 1

RETURN 10.0

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SLIDE 18

Example: to_centigrade(50.0)

  • 1. Draw a frame for the call
  • 2. Assign the argument value

to the parameter (in frame)

  • 3. Execute the function body

§ Look for variables in the frame § If not there, look for global variables with that name

  • 4. Erase the frame for the call

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def to_centigrade(x): return 5*(x-32)/9.0 to_centigrade 50.0 x Executing the return statement The return terminates; no next line to execute 1

RETURN 10.0

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SLIDE 19

Example: to_centigrade(50.0)

  • 1. Draw a frame for the call
  • 2. Assign the argument value

to the parameter (in frame)

  • 3. Execute the function body

§ Look for variables in the frame § If not there, look for global variables with that name

  • 4. Erase the frame for the call

9/4/18 Defining Functions 19

def to_centigrade(x): return 5*(x-32)/9.0 E R A S E W H O L E F R A M E But don’t actually erase on an exam 1

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SLIDE 20

Call Frames vs. Global Variables

The specification is a lie:

def swap(a,b): """Swap global a & b""" tmp = a a = b b = tmp

>>> a = 1 >>> b = 2 >>> swap(a,b)

9/4/18 Defining Functions 20

1 a 2 b 1 2 3 swap 1 1 a 2 b

Global Variables Call Frame

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SLIDE 21

Call Frames vs. Global Variables

The specification is a lie:

def swap(a,b): """Swap global a & b""" tmp = a a = b b = tmp

>>> a = 1 >>> b = 2 >>> swap(a,b)

9/4/18 Defining Functions 21

1 a 2 b swap 2 1 a 2 b

Global Variables Call Frame

1 tmp 1 2 3

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SLIDE 22

Call Frames vs. Global Variables

The specification is a lie:

def swap(a,b): """Swap global a & b""" tmp = a a = b b = tmp

>>> a = 1 >>> b = 2 >>> swap(a,b)

9/4/18 Defining Functions 22

1 a 2 b swap 3 1 a 2 b

Global Variables Call Frame

1 tmp 2 1 2 3

x

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SLIDE 23

Call Frames vs. Global Variables

The specification is a lie:

def swap(a,b): """Swap global a & b""" tmp = a a = b b = tmp

>>> a = 1 >>> b = 2 >>> swap(a,b)

9/4/18 Defining Functions 23

1 a 2 b swap 1 a 2 b

Global Variables Call Frame

1 tmp 2 1 1 2 3

x x

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SLIDE 24

Call Frames vs. Global Variables

The specification is a lie:

def swap(a,b): """Swap global a & b""" tmp = a a = b b = tmp

>>> a = 1 >>> b = 2 >>> swap(a,b)

9/4/18 Defining Functions 24

1 a 2 b

Global Variables Call Frame

1 2 3 ERASE THE FRAME

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SLIDE 25

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

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1 2 3

What does the frame look like at the start?

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SLIDE 26

Which One is Closest to Your Answer?

A: B:

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C: D:

foo 1 3 a 4 b 3 x foo 1 3 a 4 b x y foo 3 a 4 b foo 1 3 a 4 b

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SLIDE 27

Which One is Closest to Your Answer?

A: B:

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C: D:

foo 1 3 a 4 b 3 x foo 1 3 a 4 b x y foo 3 a 4 b foo 1 3 a 4 b

E:

¯\_()_/¯

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SLIDE 28

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

B:

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1 2 3 foo 1 3 a 4 b

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SLIDE 29

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

B:

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1 2 3 foo 1 3 a 4 b

What is the next step?

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SLIDE 30

Which One is Closest to Your Answer?

A: B:

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C: D:

foo 2 3 a 4 b 3 x foo 2 3 a 4 b 3 x y foo 2 3 a 4 b foo 1 3 a 4 b 3 x

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SLIDE 31

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

C:

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1 2 3 foo 2 3 a 4 b 3 x

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SLIDE 32

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

C:

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1 2 3 foo 2 3 a 4 b 3 x

What is the next step?

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SLIDE 33

Which One is Closest to Your Answer?

A: B:

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C: D:

foo 3 3 a 4 b 3 x 4 y foo 3 a 4 b 3 x 4 y

RETURN

3 foo 3 a 4 b 3 x 4 y

RETURN

16 ERASE THE FRAME

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SLIDE 34

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

A:

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1 2 3 foo 3 3 a 4 b 3 x 4 y

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SLIDE 35

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

A:

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1 2 3 foo 3 3 a 4 b 3 x 4 y

What is the next step?

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SLIDE 36

Which One is Closest to Your Answer?

A: B:

9/4/18 Defining Functions 36

C: D:

foo 3 foo 3 a 4 b 3 x 4 y

RETURN

16 3 foo 3 a 4 b 3 x 4 y

RETURN

16 ERASE THE FRAME

RETURN

16

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SLIDE 37

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

C:

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1 2 3 foo 3 a 4 b 3 x 4 y

RETURN

16

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SLIDE 38

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

C:

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1 2 3 foo 3 a 4 b 3 x 4 y

RETURN

16

What is the next step?

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SLIDE 39

Which One is Closest to Your Answer?

A: B:

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C: D:

16 x foo foo 16 x

RETURN

16 ERASE THE FRAME ERASE THE FRAME

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SLIDE 40

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

D:

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1 2 3 16 x ERASE THE FRAME

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SLIDE 41

Exercise Time

Function Definition def foo(a,b): """Return something Param x: a number Param y: a number""" x = a y = b return x*y+y Function Call

>>> x = foo(3,4)

D:

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1 2 3 16 x ERASE THE FRAME Variable in global space

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SLIDE 42

Visualizing Frames: The Python Tutor

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SLIDE 43

Visualizing Frames: The Python Tutor

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Global Space Call Frame

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SLIDE 44

Visualizing Frames: The Python Tutor

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Global Space Call Frame

Variables from second lecture go in here

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SLIDE 45

Visualizing Frames: The Python Tutor

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Missing line numbers!

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SLIDE 46

Visualizing Frames: The Python Tutor

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Missing line numbers! Line number marked here (sort-of)

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SLIDE 47

Next Time: Text Processing

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