Untitled - HackMD

Scratch and Python 2018 Summer - Python Lecture 3
交大公開課
來源：https://hackmd.io/5oUx5oXUSEGuCrZG1Pm4kQ?both

===

Iterator and Iterables

Iterables: bags of items

list
tuple
str
generator

range

range(n) sample code


for i in range(10):
    print(i)

range(L,U) sample code


for i in range(3,9):
    print(i)

range(L,U,step) sample code


for i in range(3,9,2):
    print(i)

Iterator: an access to a certain item and the iterator to its next item

it = iter(iterables)
Next element: next(it) or it.__next__()
- StopIteration will be raised if no more elements.
- next(it,default) gives default if no more elements, otherwise the next element will be given.

Sample code 1







lst = [1,3,7,9]
it = iter(lst)
print(next(it))
print(next(it))
print(next(it))
print(next(it))
print(next(it))

Sample code 2





lst = [1,3,7,9]
it = iter(lst)
print('outside for-loop:',next(it))
for i in it:
    print('inside for-loop:',i)

Sample code 3









lst = [1,3,7,9]
it = iter(lst)
print('outside while-loop:',next(it))
while True:
    try:
        i = next(it)
    except StopIteration:
        break
    print('inside while-loop:',i)

Use except Type: to catch certain kind of error or exceptionType

Sample code 4

































def merge(a, b):
    it_a, it_b = iter(a), iter(b)
    item_a = next(it_a,None)
    item_b = next(it_b,None)
    while item_a != None and item_b != None:
        if item_a <= item_b:
            yield item_a
            item_a = next(it_a,None)
        else:
            yield item_b
            item_b = next(it_b,None)
    if item_a != None: yield item_a
    if item_b != None: yield item_b
    for item in it_a: yield item
    for item in it_b: yield item

def mergesort(a):
    L = len(a)
    if L<=1:
        for x in a:
            yield x
        return
    for x in merge(mergesort(a[:L//2]),mergesort(a[L//2:])):
        yield x

print('merge',[1,3,5,6],'and',[2,4,7,8])
for item in merge([1,3,5,6],[2,4,7,8]):
    print(item)

print()
print('mergesort',[1,5,2,4,3,8,7,6])
for item in mergesort([1,5,2,4,3,8,7,6]):
    print(item)

Use pass to do nothing.

in operator

Sample code
















def gen():
    i = 0
    while True:
        yield i
        i = i + 1

if 5 in gen():
    print('5 is in gen()')
if 5000 in gen():
    print('5000 is in gen()')
if 5000000 in gen():
    print('5000000 is in gen()')
if 0.5 in gen():
    print('0.5 is in gen()')

print('This is the last line')

Some useful tools

zip






lst_a = [1,3,5,7,9]
lst_b = ['a','b','C','D']
for item in zip(lst_a,lst_b):
    print(item)
for a, b in zip(lst_a,lst_b):
    print('a:',a,'b:',b)

enumerate



lst = [1,3,5,7,9]
for item in enumerate(lst):
    print(item)

map



scores = (40,50,60,70,80,90,100)
for new_score in map(lambda x: int(10*x**0.5), scores):
    print(new_score)

lambda x: int(10*x**0.5) is a shorter and anonymous definition of the following function.


def noname(x):
    return int(10*x**0.5)

sum


scores = (40,50,60,70,80,90,100)
print(sum(scores)/len(scores))

str: use .join()


characters = ('a','b','c','d','e','f','g')
print(sum(characters))

min
max

filter


print(*filter(lambda x: x % 2 == 0, range(20)))

lambda x: x % 2 == 0 is True if x is even.

all
any

List comprehension
- type(int(10*x**0.5) for x in range(0,100,5) if x % 2 == 0)
- [int(10*x**0.5) for x in range(0,100,5) if x % 2 == 0]
More tools
- itertools
  - import itertools
    - itertools.combinations
- functools
  - import functools
    - functools.reduce

String processing

Escape Characters: Table
Multiline Strings: triple quotes '''
Integers, characters and strings
- chr
  - try chr(65)
- ord
  - try ord('A')
- Python uses str's of length 1 to represent characters.
Indexing:
- print('123'[0])
- print('123'[-1])
index()
- print('123123123'.index('312'))
Slicing:
- print('abcde'[2:])
- print('abcde'[:2])
- print('abcde'[1:3])
in and not in
- print('abc' in 'zabcy')
- print('gg' not in 'zabcy')
upper() and lower()
isupper(), islower(), isdecimal(), isalpha(), isalnum()
- try '123.0'.isdecimal()
join(list_str)
- try ', '.join([str(i) for i in range(10)])
split(string)
- try '<a href="https://automatetheboringstuff.com/">'.split('"')
startswith(string) and endswith(string)
- Many URLs start with http and end with html
strip(), rstrip(), lstrip()
.format(): Replace placeholder {} with corresponding variables.
- Ex: 'Case {}: {}, {}'.format(1,68.7,'passed')
URL
- Almost everything is specified: https://en.wikipedia.org/w/api.php?action=rsd
- Same protocol: //en.wikipedia.org/w/api.php?action=rsd
- Same address: /w/api.php?action=rsd
- Relative path: api.php?action=rsd

Random

import random
- Random integer from a to b: random.randint(a,b)
- Random floating point number in [0.0,1.0): random.random()
- Randomly shuffle a list lst: random.shuffle(lst)
- Random choice from a sequence: random.choice(seq)
- Random k samples from a population: random.sample(population,k)

Game: Guess a secret number

Guess a secret number from 0 to 9

Sample code























# Read an integer which is in {0,...,9}
def read_a_digit():
    ret = -1
    while True:
        try:
            ret = int(input('Input a digit: '))
        except ValueError:
            print('Not an integer')
            continue
        if ret >= 0 and ret <= 9:
            return ret
        print(ret,'is not in {0,...,9}.')

from random import randint

ans = randint(0,9)

guess = read_a_digit()
while guess != ans:
    print(guess,'is not the answer')
    guess = read_a_digit()

print('Congrats! The answer is',ans)

Guess a secret number from 0 to 9 with hints

Sample code


























# Read an integer which is in {0,...,9}
def read_a_digit():
    ret = -1
    while True:
        try:
            ret = int(input('Input a digit: '))
        except ValueError:
            print('Not an integer')
            continue
        if ret >= 0 and ret <= 9:
            return ret
        print(ret,'is not in {0,...,9}.')

from random import randint

ans = randint(0,9)

guess = read_a_digit()
while guess != ans:
    if ans < guess:
        print(guess,'is greater than the answer.')
    else:
        print(guess,'is less than the answer.')
    guess = read_a_digit()

print('Congrats! The answer is',ans)

Bulls and Cows

Sample code































from random import sample
from itertools import combinations

# Check whether a input is valid
def valid(s):
    if len(s)!=4:
        return False
    if any(x not in '0123456789' for x in s):
        return False
    if any(x==y for x, y in combinations(s,2)):
        return False
    return True

# Read string which has four distinct digits
def read_four_distinct_digits():
    while True:
        ret = input('Input four distinct digits: ')
        if valid(ret):
            return ret
        print(ret,'is not valid.')

ans = ''.join(sample('0123456789',4))

guess = read_four_distinct_digits()
while guess != ans:
    A = sum(1 for x,y in zip(guess,ans) if x==y)
    B = sum(1 for x in guess if x in ans)-A
    print(guess,'is {}A{}B.'.format(A,B))
    guess = read_four_distinct_digits()

print('Congrats! The answer is',ans)

Task: Assistant

Write a program to helps you play Bulls and Cows.

There are many approaches to help you to play the game. But the following strategy will help you to guess the number within 10 steps.

Maintain a list of candidates.
Randomly pick a candidate from the list, and guess it.
If the answer is what you guess, then you win. Otherwise, you would know how many bulls and cows.
Remove the candidates which do not match the result from the list. Go to step 2.

Unfinished sample code





























import itertools
import random

# return guess and ans is ?A?B
def check( guess, ans ):
    A = #?
    B = #?
    return '{}A{}B'.format(A,B)

# S = [0123, 0124, 0125, ... , 9876] (4 distinct digit)
S = #?

while True:
    if len(S) == 0:
        print("No solution!")
        break

    # ans = random pick one element from S
    ans = #?
    print(ans)

    # read ?A?B
    cmd = input()

    if cmd == '4A0B' :
        break
    else:
        # update S: remove all element x in S such that check(x,ans) ≠ cmd
        S = #?

More on `list` and `tuple`

List
- Empty list: []
- May have elements of different types
  - [12, 3.4, None, True, 'string', print]
- Access (zero-based) i-th element of list x: x[i]
  - try [123,456][1]
  - try [12, 3.4, None, True, 'string', print][5]('hello world')
  - try [123,456][1.0]
- Lists are mutable: you may change the contents!
Tuple
- Empty tuple: ()
- Single element tuple: (element,)
  - Try type(('str')) and type(('str',))
- Comprehension
  - Try type((x for x in range(5)))
  - Use tuple(x for x in range(5)) to generate (0,1,2,3,4)
- (12, 3.4, None, True, 'string', print)
- The elements are immutable: you may not change the contests!
list() and tuple()
- Try list(('a','b','c'))
- Try tuple(['d','e','f'])
- Try list('hello')
- Try tuple('hello')
Negative index: a_list_or_tuple[-1]
Slice
- print(['a','b','c','d','4'][1:3])
- print(['a','b','c','d','4'][2:])
- print(['a','b','c','d','4'][:4])
- print(['a','b','c','d','4'][-3:])
- print(['a','b','c','d','4'][:-3])
- print(['a','b','c','d','4'][3:1])
- Inclusive, exclusive
- print(['a','b','c','d','4'][1:5:3])
in
not in
Concatenation +
Replication *
index()
- try print([0,1,2,0].index(0))
Auto boxing & auto unboxing
- code

Lists are Mutable

insert(pos,val): insert val to position pos
- Let a = [0,1,2]. What will happen if we perform a.insert(1,3)?
append(val)
pop(): remove the last element and return its value
pop(pos): remove the element of position pos
remove(val): remove an element of value val
- Let a = [1,0,0,1]. What will happen if we perform a.remove(1)?
sort(): sort the list ascendingly
type(value) and id(name)
- Concept
- If you modify an object, then all references will be changed.
Copy and Deepcopy
- import copy
  + copy.copy(): Shallow copy
  + copy.deepcopy(): Deep copy

Higher Dimension

How to define a
$5$ -by-
$5$ matrix with all entries set to zero?
- Correct: [[0]*5,[0]*5,[0]*5,[0]*5,[0]*5]
- Incorrect: [[0]*5] * 5
- Correct: [[0]*5 for _ in range(5)]

More on `dict`

list is indexed by int
- From 0 to len()-1
Dictionary is indexed by keys (words).
- Word: key
- Definition: value
- A set of key-value pairs
instructor = {'Name': 'MZ', 'Height': 178, 'Weight': 108}
- try print(instructor['Name'])
- try print(instructor['age'])
Empty dictionary: {}
dict is mutable
- Add a new item via assignment
  - a_dict[key]=val
- Modify an item via assignment
  - a_dict[key]=val
- Remove an item via del
  - del a_dict[key]
  - del can also undefine a variable.
Iterate a dictionary
- keys()
- values()
- items()
in
- try print('MZ' in instructor)
- try print('Name' in instructor)
- try print(('Name','MZ') in instructor)
get(key,val): If key is missing, then return val
- Example code: keyword arguments
- Try to use an undefined item to compute.

setdefault(key,val): if there is no such key, initialize the key-value pair to (key,val) and return val. Otherwise return the value corresponding key.

Example: word count








some_list = [0,3,5,7,1,2,1,2,1]
counter = {}
for x in some_list:
    if x in counter.keys():
        counter[x] += 1
    else:
        counter[x] = 1
print(counter)

can be simplified into






some_list = [0,3,5,7,1,2,1,2,1]
counter = {}
for x in some_list:
    counter.setdefault(x,0)
    counter[x] += 1
print(counter)