Discussion 10: Iterators, Generators and Streams Nancy Shaw - - PowerPoint PPT Presentation

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Discussion 10: Iterators, Generators and Streams Nancy Shaw - - PowerPoint PPT Presentation

Dec 19, 2023 282 likes •700 views

Discussion 10: Iterators, Generators and Streams Nancy Shaw (nshaw99@berkeley.edu) Caroline Lemieux (clemieux@berkeley.edu) April 18th, 2019 Iterators and Generators Iterators vs. Iterables s = am a string i = iter(s) Iterators vs.

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Discussion 10:

Iterators, Generators and Streams

Nancy Shaw (nshaw99@berkeley.edu) Caroline Lemieux (clemieux@berkeley.edu)

April 18th, 2019

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Iterators and Generators

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Iterators vs. Iterables

s = “am a string” i = iter(s)

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Iterators vs. Iterables

s = “am a string” i = iter(s)

str is an iterable

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Iterators vs. Iterables

s = “am a string” i = iter(s)

str is an iterable get its iterator with iter()

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Iterators vs. Iterables

s = “am a string” i = iter(s)

“am a string”

i

str is an iterable get its iterator with iter()

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Iterators vs. Iterables

s = “am a string” i = iter(s)

“am a string”

i

str is an iterable get its iterator with iter()

next(s)

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Iterators vs. Iterables

s = “am a string” i = iter(s)

“am a string”

i

str is an iterable get its iterator with iter()

next(s) ERROR

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SLIDE 9

Iterators vs. Iterables

s = “am a string” i = iter(s)

“am a string”

i

str is an iterable get its iterator with iter()

next(s) ERROR next(i)

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SLIDE 10

Iterators vs. Iterables

s = “am a string” i = iter(s)

“am a string”

i

str is an iterable get its iterator with iter()

next(s) ERROR next(i) “a”

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SLIDE 11

Iterators vs. Iterables

s = “am a string” i = iter(s)

“am a string”

i

str is an iterable get its iterator with iter()

next(s) ERROR next(i) “a” next(i)

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Iterators vs. Iterables

s = “am a string” i = iter(s)

“am a string”

i

str is an iterable get its iterator with iter()

next(s) ERROR next(i) “a” “m” next(i)

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How to go through iterables without calling next

for i in iterable goes through all the things in iterable, by calling next Calling list(iterable) makes a list of all the things we get by calling next

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Do 1.1

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Pausing vs. stopping a video

Kind of like yield Kind of like return

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Generators

def generate_up_to(n): for i in range(0, n): yield i

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Generators

def generate_up_to(n): for i in range(0, n): yield i >>> generate_up_to(5) <generator object ...>

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Generators

def generate_up_to(n): for i in range(0, n): yield i >>> generate_up_to(5) <generator object ...> When python sees a yield in a function, calling that function returns a generator

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Generators

def generate_up_to(n): for i in range(0, n): yield i >>> generate_up_to(5) <generator object ...> >>> g = generate_up_to(5) >>> next(g) When python sees a yield in a function, calling that function returns a generator Calling next on the generator “plays” that function, until yield, where it “pauses”

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Generators

def generate_up_to(n): for i in range(0, n): yield i >>> generate_up_to(5) <generator object ...> >>> g = generate_up_to(5) >>> next(g) When python sees a yield in a function, calling that function returns a generator Calling next on the generator “plays” that function, until yield, where it “pauses”

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Generators

def generate_up_to(n): for i in range(0, n): yield i >>> generate_up_to(5) <generator object ...> >>> g = generate_up_to(5) >>> next(g) >>> next(g) 1 When python sees a yield in a function, calling that function returns a generator Calling next on the generator “plays” that function, until yield, where it “pauses”

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Concept check: yield vs return

def generate_up_to(n): for i in range(0, n): return i >>> generate_up_to(5)

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Concept check: yield vs return

def generate_up_to(n): for i in range(0, n): return i >>> generate_up_to(5)

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Concept check: yield vs return

def generate_up_to(n): for i in range(0, n): return i >>> generate_up_to(5) Calling a regular functions “plays” that function, until return, where it “stops”

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Concept check: yield vs return

def generate_up_to(n): for i in range(0, n): return i >>> generate_up_to(5) >>> generate_up_to(5) Calling a regular functions “plays” that function, until return, where it “stops”

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Concept check: yield vs return

def generate_up_to(n): for i in range(0, n): return i >>> generate_up_to(5) >>> generate_up_to(5) Calling a regular functions “plays” that function, until return, where it “stops”

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SLIDE 27

Concept check: yield vs return

def generate_up_to(n): for i in range(0, n): return i >>> generate_up_to(5) >>> generate_up_to(5) Calling a regular functions “plays” that function, until return, where it “stops” When we call it again, it “plays” from the start

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Yield from

def generate_up_to(n): for i in range(0, n): yield i def generate_up_to(n): yield from range(0,n) Same thing!

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Recursive generator

def generate_down_to_zero(n): if n == 0: yield 0 else: yield n yield from generate_down_to_zero(n-1)

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Do 1.1 (the other 1.1)

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Attendance

links.cs61a.org/caro-disc next(cats)

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Streams (back to scheme)

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An infinite natural number generator in Python

(demo)

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An infinite natural number generator… in scheme?

(demo)

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What’s a stream:

A “lazy” scheme list

  • Lazy because it evaluates its first element….
  • … but then is lazy and doesn’t evaluate the second
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How do I make a stream?

(cons-stream <operand1> <operand2>) Another special form! 1. Evaluate operand1 to get val1 2. Construct promise containing operand2 3. Return a pair (val1, promise of operand2)

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How do I make a stream?

(cons-stream <operand1> <operand2>) Another special form! 1. Evaluate operand1 to get val1 2. Construct promise containing operand2 3. Return a pair (val1, promise of operand2) (demo)

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How do I make a stream?

(cons-stream <operand1> <operand2>) Another special form! 1. Evaluate operand1 to get val1 2. Construct promise containing operand2 3. Return a pair (val1, promise of operand2) Need special (cdr-stream s) to get the cdr properly Important: cdr-stream evalutes its value once, then saves that for later calls

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How do I make a stream?

(cons-stream <operand1> <operand2>) Another special form! 1. Evaluate operand1 to get val1 2. Construct promise containing operand2 3. Return a pair (val1, promise of operand2) Need special (cdr-stream s) to get the cdr properly Important: cdr-stream evalutes its value once, then saves that for later calls (demo)

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Stream Recap

1. nil is the empty stream 2. cons-stream constructs a stream 3. car gets the first element of the stream 4. cdr-stream computes and returns the rest of the stream (it only computes

  • nce, and saves the value)

a. Promise is “forced” if we’ve computed its value