' ' ' lo == 0, hi == len(cards) - 1 ' ' '和' ' ' lo, hi = 0,

Question

我目前正在练习二进制搜索，我很难理解lo == 0, hi == len(cards) - 1和lo, hi = 0, len(cards) - 1之间的语法差异。代码适用于第二个方法，但不适用于第一个方法。任何帮助都会很感激。谢谢你。

代码如下:

def binary_search(cards, query):
# lo == 0, hi == len(cards) - 1
lo, hi = 0, len(cards) - 1
while lo <= hi:
mid = (lo+hi) // 2
result = locate_card(cards, query, mid)
mid_card = cards[mid]
print('lo: ', lo, 'hi: ', hi)
print('mid: ', mid)
print('mid_card: ', mid_card)
print('result: ', result)
print('n')
if result == 'found':
return mid
elif result == 'left':
hi = mid - 1
else:
lo = mid + 1
return -1
def locate_card(cards, query, mid):
if cards[mid] == query:
if mid > 0 and cards[mid-1] == query:
return 'left'
else:
return 'found'
elif cards[mid] < query:
return 'left'
else:
return 'right'

return binary_search(cards, query)
if __name__ == '__main__':
cards = [13, 11, 10, 7, 4, 3, 1, 0]
query = 1
print(binary_search(cards, query))

Answer 1

双等号(==)是比较两个对象是否相等的操作符。lo == 0, hi == len(cards) - 1会比较lo和0,hi和len(cards)-1。

lo, hi = 0, len(cards) - 1等同于写

lo = 0
hi = len(cards) - 1

Answer 2

我建议在python解释器中玩一下，以便更好地理解=(赋值)，==(相等性测试)和,(用于构建元组)这三个符号。

使用==和,:元组相等，或元组相等

>>> 3 == 3, 4 == 4
(True, True)
>>> 3, 4 == 3, 4
(3, False, 4)
>>> (3, 4) == (3, 4)
True
>>> 3 == 3 and 4 == 4
True

玩=和,:分配与解包

>>> 0, 10
(0, 10)
>>> lo, hi = (0, 1)
>>> print(lo, hi)
0 1
>>> lo, hi = 1, 2
>>> print(lo, hi)
1 2
>>> lo = 3
>>> hi = 4
>>> print(lo, hi)
3 4

赋值元组:语法不正确

>>> lo = 5, hi = 6
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

:=赋值表达式(python>= 3.8)

>>> (lo := 7, hi := 8)
(7, 8)
>>> print(lo, hi)
7 8

Answer 3

==操作符用于使用条件表达式并比较两个表达式，如果表达式为真，则执行操作，但=操作符用于为变量赋值。例如:

if lo == 0 and hi == len(cards) - 1:
print("Hello")