CS 7 : Introduction to Programming and Computer Science Python is TIOBE's programming language of the year 2018! www.tiobe.com/tiobe-index “The Python programming language has won the title "programming language of the year"! Python has received this title because it has gained most ranking points in 2018 if compared to all other languages. The Python language has won 3.62%, followed by Visual Basic .NET and Java. Python has now definitely become part of the big programming languages. For almost 20 years, C, C++ and Java are consistently in the top 3, far ahead of the rest of the pack. Python is joining these 3 languages now. It is the most frequently taught first language at universities nowadays, it is number one in the statistical domain, number one in AI programming, number one in scripting and number one in writing system tests. Besides this, Python is also leading in web programming and scientific computing (just to name some other domains).” In summary, Python is everywhere. Computing in the news
Transcript
CS 7 : Introduction to Programming and Computer Science
Python is TIOBE's programming language of the year 2018! www.tiobe.com/tiobe-index“The Python programming language has won the title "programming language of the year"! Python has received this title because it has gained most ranking points in 2018 if compared to all other languages. The Python language has won 3.62%, followed by Visual Basic .NET and Java. Python has now definitely become part of the big programming languages. For almost 20 years, C, C++ and Java are consistently in the top 3, far ahead of the rest of the pack. Python is joining these 3 languages now. It is the most frequently taught first language at universities nowadays, it is number one in the statistical domain, number one in AI programming, number one in scripting and number one in writing system tests. Besides this, Python is also leading in web programming and scientific computing (just to name some other domains).” In summary, Python is everywhere.
Computingin the news
This material is an adaptation from CS61A material at UC Berkeley.
Credits to Professor John DeNero and the entire CS61A staff.
Acknowledgements
Parts of the Course
Lecture: Lecture is on Mon and Tues
Lab section: The most important part of this course
Staff office hours: The most important part of this course
Online textbook: http://composingprograms.com
Optional Discussion section: The most important part of this course
Weekly lab, homework assignments, three programming projects (hopefully)
Lots of optional special events to help you complete all this work
Assignment is a simple means of abstraction: binds names to values
Function definition is a more powerful means of abstraction: binds names to expressions
<name>(<formal parameters>):
return <return expression>
>>> def
Execution procedure for def statements:
1. Create a function with signature <name>(<formal parameters>)
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
Function signature indicates how many arguments a function takes
Function body defines the computation performed when the function is applied
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Calling User-Defined Functions
Procedure for calling/applying user-defined functions (version 1):
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
A function’s signature has all the information needed to create a local frame
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Procedure for calling/applying user-defined functions (version 1):
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
Every expression is evaluated in the context of an environment.
So far, the current environment is either:
• The global frame alone, or
• A local frame, followed by the global frame.
Most important two things I’ll say all day:
An environment is a sequence of frames.
A name evaluates to the value bound to that name in the earliest frame of the current environment in which that name is found.
E.g., to look up some name in the body of the square function:
• Look for that name in the local frame.
• 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.)
(Demo5)29
Print and None
(Demo1)
None Indicates that Nothing is Returned
The special value None represents nothing in Python
A function that does not explicitly return a value will return None
Careful: None is not displayed by the interpreter as the value of an expression
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>>> def does_not_return_square(x):
... x * x
...
>>> does_not_return_square(4)
>>> sixteen = does_not_return_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 sixteenis now bound tothe value None
No return
None value is not displayed
abs
Pure Functions & Non-Pure Functions
-22
-2None
print
Python displays the output “-2”
2, 1001267650600228229401496703205376
pow
Pure Functionsjust return values
Non-Pure Functionshave side effects
Argument
Return value
A side effect isn't a value; it's anything that happens as a consequence of
calling a function
Returns None!
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2 Arguments
Nested Expressions with Print
Noneprint(print(1), print(2))
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
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Does not get displayed
func print(...)
Multiple Environments
Life Cycle of a User-Defined Function
Def statement:
Call expression:
square( x ):
return mul(x, x)
>>> def
square(2+2)
Calling/Applying: square( x ):
Def statement
Formal parameter
Body
Return expression
(return statement)
A new function is created!
Name bound to that function in the current frame
operand: 2+2argument: 4
Operator & operands evaluated
Function (value of operator) called on arguments (values of operands)
2. If it is a true value, execute the (whole) suite,then return to step 1.
1 2 31 3 6
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(Demo4)
Execution Rule for While Statements:
fib
n
predcurr
k
5
def fib(n):"""Compute the nth Fibonacci number, for N >= 1."""pred, curr = 0, 1 # 0th and 1st Fibonacci numbersk = 1 # curr is the kth Fibonacci numberwhile k < n:
The next Fibonacci number is the sum of the current one and its predecessor
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Designing Functions
Describing Functions
A function's domain is the set of all inputs it might possibly take as arguments.
A function's range is the set of output values it might possibly return.
A pure function's behavior is the relationship it creates between input and output.
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def square(x):"""Return X * X."""
x is a number
square returns a non-negative real number
square returns the square of x
A Guide to Designing Function… Generalization!
Give each function exactly one job, but make it apply to many related situations
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Don’t repeat yourself (DRY). Implement a process just once, but execute it many times.
>>> round(1.23, 1)1.2
>>> round(1.23, 0)1
>>> round(1.23, 5)1.23
>>> round(1.23)1
Generalization
Shape:
Generalizing Patterns with Arguments
Regular geometric shapes relate length and area.
Area:
Finding common structure allows for shared implementation
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(Demo1)
Higher-Order Functions
Generalizing Over Computational Processes
The common structure among functions may be a computational process, rather than a number.
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(Demo2)
hof.py Page 2
return total
def identity(k): return k
def cube(k): return pow(k, 3)
def summation(n, term): """Sum the first n terms of a sequence. >>> summation(5, cube) 225 """ total, k = 0, 1 while k <= n: total, k = total + term(k), k + 1 return total
def make_adder(n): """Return a function that takes one argument k and returns k + n.
>>> add_three = make_adder(3) >>> add_three(4) 7 """ def adder(k): return k + n return adder
def compose1(f, g): """Return a function that composes f and g.
f, g �� functions of a single argument """ def h(x): return f(g(x)) return h
@maindef run(): interact()
Summation Example
Function of a single argument (not called "term")
A formal parameter that will be bound to a function
The function bound to term gets called here
The cube function is passed as an argument value
0 + 1 + 8 + 27 + 64 + 125
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Functions as Return Values
(Demo3)
hof.py Page 2
return total
def identity(k): return k
def cube(k): return pow(k, 3)
def summation(n, term): """Sum the first n terms of a sequence. >>> summation(5, cube) 225 """ total, k = 0, 1 while k <= n: total, k = total + term(k), k + 1 return total
def make_adder(n): """Return a function that takes one argument k and returns k + n.
>>> add_three = make_adder(3) >>> add_three(4) 7 """ def adder(k): return k + n return adder
def compose1(f, g): """Return a function that composes f and g.
f, g �� functions of a single argument """ def h(x): return f(g(x)) return h
@maindef run(): interact()
Locally Defined Functions
A function that returns a function
A def statement within another def statement
The name add_three is bound to a function
Can refer to names in the enclosing function
Functions defined within other function bodies are bound to names in a local frame
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make_adder( n ):
Call Expressions as Operator Expressions
make_adder(1) ( 2 )
Operator Operand
An expression that evaluates to a function
An expression that evaluates to its argument
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2
3
make_adder(1)func adder(k)
func make_adder(n) 1
func adder(k)
Lambda Expressions
(Demo4)l
Lambda Expressions
>>> x = 10
>>> square = x * x
>>> square = lambda x: x * x
>>> square(4)16
An expression: this one evaluates to a number
Also an expression: evaluates to a function
that returns the value of "x * x"with formal parameter x
A function
Lambda expressions are not common in Python, but important in general
Important: No "return" keyword!
Must be a single expression
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Lambda expressions in Python cannot contain statements at all!
Lambda Expressions Versus Def Statements
square = lambda x: x * x def square(x):return x * xVS
• Both create a function with the same domain, range, and behavior.
• Both bind that function to the name square.
• Only the def statement gives the function an intrinsic name, which shows up in environment diagrams but doesn't affect execution (unless the function is printed).
