61a lecture 2 - university of california,...

61A Lecture 2

Friday, January 23, 2015

61A Lecture 2

Thursday, January 22, 2015

Announcements

2

Announcements

• Starting next week, submitting labs & attending section will provide a midterm safety net

2

Announcements


• Homework 1 is due next Wednesday 1/28

2

Announcements



§All homework is graded on effort; you must make progress on each problem to earn 2/2

2

Announcements




§Homework Party on Tuesday 1/27 5-6:30pm in 2050 VLSB

2

Announcements





• Quiz 1 released next Wednesday 1/28 is due next Thursday 1/29 (graded on correctness)

2

Announcements






• Ask questions about lab and homework assignments in office hours! (cs61a.org/weekly.html)

2

Announcements







§2 locations in Bechtel Engineering Center (Map: http://goo.gl/dAcHXf)

2

Announcements








§11-2 & 3-5 on Monday, 11-6 on Tuesday & Thursday, 11-2 & 3-4 on Wednesday, 11-1 on Friday

2

Announcements









• You need to register a class account (Lab 0); that's how we track assignments

2

Announcements









• You need to register a class account (Lab 0); that's how we track assignments

§Please register even if you're on the waitlist or applying for concurrent enrollment

2

Names, Assignment, and User-Defined Functions

(Demo)

Types of Expressions

4


Primitive expressions:

4


Primitive expressions: 2

Number or Numeral

4


Primitive expressions: 2 add

Number or Numeral Name

4


Primitive expressions: 2 add 'hello'

Number or Numeral Name String

4



Call expressions:

2 add 'hello'


4



Call expressions:

2 add 'hello'

max ( 2 , 3 )


4



Call expressions:

2 add 'hello'

max ( 2 , 3 )

Operator


4



Call expressions:

2 add 'hello'

max ( 2 , 3 )

Operator Operand Operand


4



Call expressions:

2 add 'hello'

max ( 2 , 3 )


max(min(pow(3, 5), -4), min(1, -2))


4



Call expressions:

2 add 'hello'

max ( 2 , 3 )


max(min(pow(3, 5), -4), min(1, -2))


4

An operand can also be a call expression

Discussion Question 1

5


What is the value of the final expression in this sequence?

5



>>> f = min

5



>>> f = min

>>> f = max

5



>>> f = min

>>> f = max

>>> g, h = min, max

5



>>> f = min

>>> f = max

>>> g, h = min, max

>>> max = g

5



>>> f = min

>>> f = max

>>> g, h = min, max

>>> max = g

>>> max(f(2, g(h(1, 5), 3)), 4)

5



>>> f = min

>>> f = max

>>> g, h = min, max

>>> max = g

>>> max(f(2, g(h(1, 5), 3)), 4)

???

5

Environment Diagrams


Environment diagrams visualize the interpreter’s process.

7Interactive Diagram


Frames (right):Code (left):




Statements and expressions





Import statement






Import statement




7

Assignment statement

Interactive Diagram


Import statement


Arrows indicate evaluation order



7


Interactive Diagram


Import statement





7

Just executed


Interactive Diagram


Import statement





7

Just executed

Next to execute Assignment statement

Interactive Diagram


Import statement

Each name is bound to a valueStatements and expressions




7

Just executed


Interactive Diagram


Name

Import statement





7

Just executed


Interactive Diagram


Name Value

Import statement





7

Just executed


Interactive Diagram


Name Value

Import statement

Each name is bound to a value

Within a frame, a name cannot be repeated





7

Just executed


Interactive Diagram


(Demo)

Name Value

Import statement

Each name is bound to a value

Within a frame, a name cannot be repeated





7

Just executed


Interactive Diagram

Assignment Statements



8

Just executed

Interactive Diagram


8

Just executed

Next to execute

Interactive Diagram


Execution rule for assignment statements:

8

Just executed

Next to execute

Interactive Diagram



1. Evaluate all expressions to the right of = from left to right.

8

Just executed

Next to execute

Interactive Diagram




2. Bind all names to the left of = to those resulting values in the current frame.

8

Just executed

Next to execute

Interactive Diagram




2. Bind all names to the left of = to those resulting values in the current frame.

8

Just executed

Just executed

Next to execute

Interactive Diagram

Discussion Question 1 Solution

9

(Demo)

Interactive Diagram


9

func min(...)

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

func max(...) 2

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

g(h(1, 5), 3) func max(...) 2

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

g(h(1, 5), 3) func max(...) 2

func min(...)

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

g(h(1, 5), 3) func max(...) 2

h(1, 5) func min(...)

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

g(h(1, 5), 3) func max(...) 2

h(1, 5) func min(...)

5 func max(...) 1

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

g(h(1, 5), 3) func max(...) 2

h(1, 5) func min(...) 5

5 func max(...) 1

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

g(h(1, 5), 3) func max(...) 2

3 h(1, 5)

func min(...) 5

5 func max(...) 1

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

g(h(1, 5), 3) 3 func max(...) 2

3 h(1, 5)

func min(...) 5

5 func max(...) 1

(Demo)

Interactive Diagram


9

func min(...) f(2, g(h(1, 5), 3))

3

g(h(1, 5), 3) 3 func max(...) 2

3 h(1, 5)

func min(...) 5

5 func max(...) 1

(Demo)

Interactive Diagram


9

func min(...) 4 f(2, g(h(1, 5), 3))

3

g(h(1, 5), 3) 3 func max(...) 2

3 h(1, 5)

func min(...) 5

5 func max(...) 1

(Demo)

Interactive Diagram


9

func min(...) 4

3

f(2, g(h(1, 5), 3)) 3

g(h(1, 5), 3) 3 func max(...) 2

3 h(1, 5)

func min(...) 5

5 func max(...) 1

(Demo)

Interactive Diagram


9

func min(...) 4

3

f(2, g(h(1, 5), 3)) 3

g(h(1, 5), 3) 3 func max(...) 2

3 h(1, 5)

func min(...) 5

5 func max(...) 1 3

(Demo)

Interactive Diagram

Defining Functions

Defining Functions

Assignment is a simple means of abstraction: binds names to values

Function definition is a more powerful means of abstraction: binds names to expressions

11

Defining Functions



<name>(<formal parameters>):

return <return expression>

>>> def

11

Defining Functions





>>> def

Function signature indicates how many arguments a function takes

11

Defining Functions





>>> def


Function body defines the computation performed when the function is applied

11

Defining Functions





>>> def

Execution procedure for def statements:



11

Defining Functions





>>> def


1. Create a function with signature <name>(<formal parameters>)



11

Defining Functions





>>> def



2. Set the body of that function to be everything indented after the first line



11

Defining Functions





>>> def



2. Set the body of that function to be everything indented after the first line

3. Bind <name> to that function in the current frame



11

Calling User-Defined Functions



Procedure for calling/applying user-defined functions (version 1):




1. Add a local frame, forming a new environment




1. Add a local frame, forming a new environment2. Bind the function's formal parameters to its arguments in that frame




1. Add a local frame, forming a new environment2. Bind the function's formal parameters to its arguments in that frame3. Execute the body of the function in that new environment





Built-in function





Built-in function

User-defined function





Local frame

Built-in function






Local frame

Original name of function called

Built-in function






Local frame


Formal parameter bound to argument

Built-in function






Local frame


Formal parameter bound to argument Return value

(not a binding!)

Built-in function




13


1. Add a local frame, forming a new environment 2. Bind the function's formal parameters to its arguments in that frame 3. Execute the body of the function in that new environment

Interactive Diagram


A function’s signature has all the information needed to create a local frame

13


1. Add a local frame, forming a new environment 2. Bind the function's formal parameters to its arguments in that frame 3. Execute the body of the function in that new environment

Interactive Diagram

Looking Up Names In Environments

14


Every expression is evaluated in the context of an environment.

14



So far, the current environment is either:

14




• The global frame alone, or

14





• A local frame, followed by the global frame.

14






Most important two things I’ll say all day:

14







An environment is a sequence of frames.

14








A name evaluates to the value bound to that name in the earliest frame of the current environment in which that name is found.

14









E.g., to look up some name in the body of the square function:

14










• Look for that name in the local frame.

14











• If not found, look for it in the global frame.(Built-in names like “max” are in the global frame too, but we don’t draw them in environment diagrams.)

14











• If not found, look for it in the global frame.(Built-in names like “max” are in the global frame too, but we don’t draw them in environment diagrams.)

(Demo)14

Print and None

(Demo)

None Indicates that Nothing is Returned

16


The special value None represents nothing in Python

16



A function that does not explicitly return a value will return None

16




Careful: None is not displayed by the interpreter as the value of an expression

16





16

>>> def does_not_square(x):

... x * x

...





16


... x * x

... No return





16


... x * x

...

>>> does_not_square(4)

No return





16


... x * x

...


No return

None value is not displayed





16


... x * x

...


>>> sixteen = does_not_square(4)

No return






16


... x * x

...


>>> sixteen = does_not_square(4)The name sixteen is now bound to the value None

No return






16


... x * x

...


>>> sixteen = does_not_square(4)

>>> sixteen + 4

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

The name sixteen is now bound to the value None

No return


Pure Functions & Non-Pure Functions

Pure Functions just return values

Non-Pure Functions have side effects

17

abs




17

abs


-2Pure Functions just return values


17

abs


-22



17

abs


-22



Argument

17

abs


-22



Argument

Return value

17

abs


-22

pow



Argument

Return value

17

abs


-22

2, 100 pow



Argument

Return value

17

abs


-22

2, 100 pow



Argument

Return value

17

2 Arguments

abs


-22

2, 1001267650600228229401496703205376

pow



Argument

Return value

17

2 Arguments

abs


-22

print

2, 1001267650600228229401496703205376

pow



Argument

Return value

17

2 Arguments

abs


-22

-2 print

2, 1001267650600228229401496703205376

pow



Argument

Return value

17

2 Arguments

abs


-22

-2None

print

2, 1001267650600228229401496703205376

pow



Argument

Return value

17

2 Arguments

abs


-22

-2None

print

Python displays the output “-2”

2, 1001267650600228229401496703205376

pow



Argument

Return value

17

2 Arguments

abs


-22

-2None

print


2, 1001267650600228229401496703205376

pow



Argument

Return value

Returns None!

17

2 Arguments

abs


-22

-2None

print


2, 1001267650600228229401496703205376

pow



Argument

Return value

A side effect isn't a value; it's anything that happens as a consequence of

calling a function

Returns None!

17

2 Arguments

Nested Expressions with Print

18


print(print(1), print(2))

18



func print(...)

18



func print(...)print(1)

func print(...) 1

18



func print(...)

print(...):1None

display “1”

print(1)

func print(...) 1

18



func print(...)

print(...):1None

display “1”

print(1)

func print(...) 1

None

18



func print(...)

print(...):1None

display “1”

print(1)

func print(...) 1

Noneprint(2)

2

18

func print(...)



func print(...)

print(...):1None

display “1”

print(...):2None

display “2”

print(1)

func print(...) 1

Noneprint(2)

2

18

func print(...)



func print(...)

print(...):1None

display “1”

print(...):2None

display “2”

print(1)

func print(...) 1

Noneprint(2)

2

None

18

func print(...)



func print(...)

print(...):1None

display “1”

print(...):2None

display “2”

print(...):None, NoneNone

display “None None”

print(1)

func print(...) 1

Noneprint(2)

2

None

18

func print(...)


Noneprint(print(1), print(2))

func print(...)

print(...):1None

display “1”

print(...):2None

display “2”



print(1)

func print(...) 1

Noneprint(2)

2

None

18

func print(...)


Noneprint(print(1), print(2))

func print(...)

print(...):1None

display “1”

print(...):2None

display “2”



print(1)

func print(...) 1

Noneprint(2)

2

None

18

Does not get displayed

func print(...)

61a lecture 2 - university of california,...

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