略去地球大气层的影响,已知地球半径RE=6.37×106m,地球轨道半径r=1.50×1011m,太阳质量Ms=1.99×1030kg。
(1)在地球引力作用下,贴近地面沿圆轨道运动的飞行器速度υ1,称为第一宇宙速度,试求之;
(2)从地面向上发射太空飞行器,为使它能远离地球而去的最小发射速度υ2,称为第二宇宙速度,试求之;
(3)从地面向上发射太空飞行器,为使它能相继脱离地球和太阳的引力束缚远离太阳系而去的最小发射速度υ3,称为第三宇宙速度,试求之。
(2) database.htm
(3) program.htm
(4) network.htm
(5) mysql.htm
(6) python.htm
(7) java.htm
from bs4 import BeautifulSoup import urllib.request def spider(url): try: data=urllib.request.urlopen(url) data=data.read() data=data.decode() soup=BeautifulSoup(data,"lxml") print(soup.find("h3").text) ____________________________________ for link in links: href=link["href"] ___________________________________ spider(url) except Exception as err: print(err) start_url="http://127.0.0.1:5000" spider(start_url) print("The End") 递归调用
A、links=soup.select("a");url=start_url+href
B、links=soup.select("li");url=start_url+"/"+href
C、links=soup.select("a");url=start_url+"/"+href
D、links=soup.select("li");url=start_url+href
from bs4 import BeautifulSoup import urllib.request class Stack: def __init__(self): self.st=[] def pop(self): return self.st.pop() def push(self,obj): self.st.append(obj) def empty(self): return len(self.st)==0 def spider(url): stack=Stack() stack.push(url) while not stack.empty(): url=stack.pop() try: data=urllib.request.urlopen(url) data=data.read() data=data.decode() soup=BeautifulSoup(data,"lxml") print(soup.find("h3").text) links=soup.select("a") for i in _______________________________: href=links[i]["href"] url=start_url+"/"+href stack.push(url) except Exception as err: print(err) start_url="http://127.0.0.1:5000" spider(start_url) print("The End")
A、range(len(links)-1,-1,-1)
B、range(len(links),-1,-1)
C、range(len(links)-1,0,-1)
D、range(len(links),0,-1)
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