为什么这不起作用？
# to reverse a part of the string in place
a = [1,2,3,4,5]
a[2:4] = reversed(a[2:4]) # This works!
a[2:4] = [0,0] # This works too.
a[2:4].reverse() # But this doesn't work
至于问的称号,是open('...','w').write('...')和urllib.urlopen('..')异步调用？
以下类型签名用于查找表示为元组的向量的大小似乎不起作用:
mag :: (Floating b, Num a) => (a,a) -> b
mag (x,y) = sqrt (x**2 + y**2)
错误:
Couldn't match expected type `b' against inferred type `a'
`b' is a rigid type variable bound by
the type signature for `mag' at tone.hs:1:17
`a' is a rigid type variable bound by
the type signature for `mag' at tone.hs:1:24
In the expression: sqrt (x ** 2 + y ** 2)
In the definition of `mag': mag (x, y) = sqrt (x …for如果满足某个条件,是否可以阻止循环进入列表/迭代器中的下一个值？
lst = list('abcde')
for alphabet in lst:
if some_condition:
# somehow prevent the for loop from advancing so that in the
# next iteration, the value of alphabet remains the same as it is now
# do something