python基本数据结构（八）

• 1. 逐个打印元素

weekdays=['Monday','Tuesday','Wednesday','Thursday','Friday']
print(weekdays)
i=0
while i< len(weekdays):
    print(weekdays[i])
    i+=1
# range method
list(range(len(weekdays))) # 返回sequence，用于loop
# [0, 1, 2, 3, 4] 抱歉不报后
for day in weekdays:
    print(day) # 返回list中的每一个元素
#type(enumerate(weekdays))
for key,day in enumerate(weekdays):
    print(str(key)+" "+day) # enumerare return a tuple,including index and value
my_list=['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec']
for c,value in enumerate(my_list,1): # 指定index的起始位置
    print(c,value)

pizza={
    "size":"medium",
    "type":"pepperoni",
    "crust":"Thick",
    "qty":1,
    "deliver":True
      }
for k,v in pizza.items(): # 调用函数 + （），items返回key&value pair
    print("key is {},value is {}".format(k,v))

# range returns a sequence
num_list=[i for i in range(0,10)] # list comprehensions
num_list 

num_list=list(range(1,10,2))
num_list # return a odd list 

num_list=[i for i in range(0,10) if i%2==1] 
num_list # 添加filter的list comprehensions

num_list=[]
num_list=[i**2 for i in range(500) if i%3==2]
#num_list
#type(num_list)

gen=(i*i for i in range(500) if i%3==2)
gen
type(gen) # data type generator

# print generator number
for num in gen:
    print(num) # print generator
# random function
import random
num_list=random.sample(range(10),10) # pick k unique elements from population
target=5
print("list=",num_list)
print("target=",target)

l1=[x for x in num_list if x< target]
l2=[x for x in num_list if x>=target]
#list comprehensions default return a generator
print("l1 is ",l1)
print("l2 is {}".format(l2))

l2=[x for x in num_list if x not in l1]
print("l2:",l2) # not in syntax
print("num_list:",num_list)

rows=range(1,4)
cols=range(1,3)

cells=[]
for row in rows:
    for col in cols:
        cells.append([row,col]) # appedn only takes exactly one argument
cells # nested method 

# Above loop can be writtern in the 
cells=[[r,c] for r in rows for c in cols]
cells # list comprehension method

builder_list=[]
string_build=""
for data in container:
    builder_list.append(str(data))
"".join(builder_list)

# antother way is to use a list comprehension
"".join([str(data) for data in container])

# or use map function
"".join(map(str,container))

Python code structures

possibility_to_rain=0.7
if possibility_to_rain> 0.8:
    print("Do take your umberalla with you.")
elif possibility_to_rain>0.3:
    print("Take your umberalla just in case. hhhaha")
else:
    print("Enjoy the sunshine!")

card_type="debit"
account_type="checking"
if card_type=="debit":
    if account_type=="checking":
        print("Checkings selected.")
    else:
        print("Saving selected.")
else:
    print("Credit card.")

x=int(input("Please enter an integer:\n"))
if x < 0:
    x=0
    print("Negative changed to zero")
elif x==0:
    print("Zero")
elif x==1:
    print("Single")
else:
    print("More")

switch case

class SMTP:
    def lookupMethod(self,command):
        return getattr(self,'do_'+ command.upper(),None)
    def do_HELO(self,rest):
        return 'Howdy '+rest
    def do_QUIT(self,rest):
        return 'Bye'
    
SMTP().lookupMethod('HELO')('foo.bar.com') #=>'howdy foo.bar.com'
SMTP().lookupMethod('QUIT')('') # =>'Bye'

# while loop
cnt=1
while cnt<=5:
    print('cnt=%d' % cnt)
    cnt+=1
print("finish")

# 每次询问，用户决定何时退出
cnt=1
while True:
    print('cnt=%d' % cnt)
    ch=input('Do you want to continue? [y|n]:')
    if ch=='y':
        cnt+=1
    else:
        break

# for loop
# ?range——查询build-in function
# for else ,如果遇到 break打印，没有遇到不打印。
for i in range(10):
    print(i)
    if i==15:
        break
else:
    print("no break")
---
list(range(1,100)) # range(a,b),从a开始，到b-1结束生成数列

# 猜数
from random import randint
mystery=randint(1,100)
times=3
print("I've thought a number from 1 to 100. You have %s times to guest it out" % times)

def guess_number(times):
    for i in range(0,times):
        user_input=int(input("Please enter a number:\n>>>"))
        if user_input==mystery:
            print("Great, you are right, the number is %s." % mystery)
            break
        elif user_input>mystery:
            print("Your input is too big.")
            print("You have %s times left. Good luck!" %(times-i))
        else:
            print("Your input is too small.")
            print("You have %s times left. Good luck!" % (times-i))
    print("")
    print("Game is over...")

guess_number(5)
---
# nested loop
for i in range(4):
    for j in range(10):
        print("x",sep='',end="")# 指定结尾符号
    print("")
for i in range(4):
    for j in range(i+1):
        print("x",sep='',end="")# 指定结尾符号
    print("") # 阶梯型
 
for i in range(5):
    for j in range(5+1):
        if i+j>4:
            print("x",end='')
        else:
            print('T',end='')
    print("") # 指定换行

# list comprehensions
[[[0] for x in range(5)] for x in range(5)] # 初始化5*5list
[[0]*5 for x in range(5)] #总结，墙边的操作可以list comprehension
---
# dictionary comprehension
# Count number of each base in the dna sequence
dna='ACCGAATTAGT'
dna_cnt={b : dna.count(b) for b in set(dna)}
dna_cnt # set() constructure
# {'A': 4, 'C': 2, 'T': 3, 'G': 2}

# function 
# An empty function that does nothing
def do_nothing():
    pass
type(do_nothing)

# An fucntion without parameters and returns values
def greeting():
    print("Hello Pyhton")
# Call the function
a=greeting()

#  positional arguments
# A function with 3 parameters
def menu(wine,entree,dessert):
    return {'wine':wine,'entree':entree,'dessert':dessert}
# Get a menu
menu('chardonnay','chicken','cake')

# To avoid positional argument confusion. you can specify arguments by the names of their corresponding parameters. even in a different order from their definition in the function.
menu(entree='beef',dessert='bagel',wine='bordeaux') # keyword argument
# You can even mix positional and keyword argument
# Note:You have to provide all positional argument before feed any keyword argumen
---

# default dessert is pudding
def menu(wine,entree,dessert='pudding'):
    return {'wine':wine,'entree':entree,'dessert':dessert}
# call menu without providing dessert
menu('chardonnay','chicken')
# default parameter 必须放在最后
# Default value will be overwritten if caller provide a value
menu('chardonnay','chicken','doughnut')

# 少输入参数的默认处理
def nonbuggy(arg,result=None):
    if result is None:
        result=[]
    print(id(result))
    result.append(arg)
    print(result)

nonbuggy('a')
nonbuggy('b')

# *args 输入多个参数，similat to varargin in matlab.
def print_args(*args):
    print('Positional args:',args)
#If your function has require positional arguments as well.write all require arguments before you use *. which will gather all arguments until the very last one.
def print_args_with_require(req1,req2,*args):
    print('require arg 1:',req1)
    print('require arg 1:',req2)
    print('other arguments:',args)

---
def print_kwargs(**kwargs):
    print('keyword args:',kwargs)

print_kwargs(first=1,second=1)
# keyword args: {'first': 1, 'second': 1}

def print_all_args(req1,*args,**kwargs):
    print('require arg1:',req1)
    print('Positional args:',args)
    print('keyword args:',kwargs)
print_all_args(1,2,3,s='hello') 
#require arg1: 1
#Positional args: (2, 3)
#keyword args: {'s': 'hello'}


# Like we talked eariler. We can
# attach documentation to a 
# function definition by including a # string at the beginning of the 
# function body. This can be super 
# helpful when you are working 
# with others or when you're using 
# IDE.
def print_if_true(thing,check):
    '''
    Print the first argument if the second argument is true.
    The operation is:
        1. check whther the second argument is true
        2. If it is, print the first argument.
    '''
    if check:
        print(thing)

# use help to get the docstring of a function
help(print_if_true) # 显示说明信息

# lambda function
double= lambda x: x*3
double(3) # similar f=@(x) x*3 in matlab