Sep 3, 2018
Python
def check_equal(str1, str2):
if len(str1) != len(str2):
return False;
i = 0
j = 0
for i, j in zip(str1, str2):
if i != j:
return False;
return True
前天学习Rabin-Karp之后没有自己看Tushar写的代码,今天一下,嗯?骚操作?注意到上一段代码有这样一个操作 for i, j in zip(str1, str2):
Iterating through two lists in parallel using zip()
zip()
From the Python docs,
zip
returns a list of tuples, where thei_th
tuple contains thei_th
element from each of the argument sequences or iterables. This is useful for iterating over two lists in parallel. For example, if I have two lists, I can get the first element of both lists, then the second element of both lists, then the third, etc.
Python 2.5.1 (r251:54863, May 18 2007, 16:56:43)
[GCC 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> a = [1,2,3]
>>> b = ['a','b','c']
>>> for i,j in zip(a,b):
... print i, j
...
1 a
2 b
3 c
>>>
If the lists are different lengths, zip
truncates to the length of the shortest list. Using map
with None
is similar to zip
except the results are padded with None
.
>>> a = [1,2,3]
>>> b = ['a','b','c','d']
>>> zip(a,b)
[(1, 'a'), (2, 'b'), (3, 'c')]
>>> map(None,a,b)
[(1, 'a'), (2, 'b'), (3, 'c'), (None, 'd')]
>>>
If I have a list of keys and a list of values, I can create a dictionary by passing the output of zip
to dict
.
>>> mykeys = ['a', 'b', 'c']
>>> myvalues = [1, 2, 3]
>>> dict(zip(mykeys, myvalues))
{'a': 1, 'c': 3, 'b': 2}
>>>
Closures
A Closure is a function object that remembers values in enclosing scopes even if they are not present in memory. Let us get to it step by step
Firstly, a Nested Function is a function defined inside another function. It's very important to note that the nested functions can access the variables of the enclosing scope. However, at least in python, they are only read only. However, one can use the "nonlocal" keyword explicitly with these variables in order to modify them.
For example:
def transmit_to_space(message):
"This is the enclosing function"
def data_transmitter():
"The nested function"
print(message)
data_transmitter()
print(transmit_to_space("Test message"))
Output
Test message
None
This works well as the 'data_transmitter' function can access the 'message'. To demonstrate the use of the "nonlocal" keyword, consider this
def print_msg(number):
def printer():
"Here we are using the nonlocal keyword"
nonlocal number
number = 3
print(number)
printer()
print(number)
print_msg(9)
Output:
3
3
Without the nonlocal keyword, the output would be "3 9", however, with its usage, we get "3 3", that is the value of the "number" variable gets modified.
Now, how about we return the function object rather than calling the nested function within. (Remember that even functions are objects. (It's Python.))
def transmit_to_space(message):
"This is the enclosing function"
def data_transmitter():
"The nested function"
print(message)
return data_transmitter
And we call the function as follows:
def transmit_to_space(message):
"This is the enclosing function"
def data_transmitter():
"The nested function"
print(message)
return data_transmitter
fun2 = transmit_to_space("Burn the Sun!")
fun2()
Output:
Burn the Sun!
Even though the execution of the "transmit_to_space()" was completed, the message was rather preserved. This technique by which the data is attached to some code even after end of those other original functions is called as closures in python
ADVANTAGE : Closures can avoid use of global variables and provides some form of data hiding.(Eg. When there are few methods in a class, use closures instead).
Also, Decorators in Python make extensive use of closures.
References
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