map-function | 易学教程

Python 3 Map function is not Calling up function

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问题 Why doesn't following code print anything: #!/usr/bin/python3 class test: def do_someting(self,value): print(value) return value def fun1(self): map(self.do_someting,range(10)) if __name__=="__main__": t = test() t.fun1() I'm executing the above code in Python 3. I think i'm missing something very basic but not able to figure it out. 回答1: map() returns an iterator, and will not process elements until you ask it to. Turn it into a list to force all elements to be processed: list(map(self.do

Python 3 vs Python 2 map behavior

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问题 In Python 2, a common (old, legacy) idiom is to use map to join iterators of uneven length using the form map(None,iter,iter,...) like so: >>> map(None,xrange(5),xrange(10,12)) [(0, 10), (1, 11), (2, None), (3, None), (4, None)] In Python 2, it is extended so that the longest iterator is the length of the returned list and if one is shorter than the other it is padded with None . In Python 3, this is different. First, you cannot use None as an argument for the callable in position 1: >>> list

Python: Something like `map` that works on threads [closed]

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I was sure there was something like this in the standard library, but it seems I was wrong. I have a bunch of urls that I want to urlopen in parallel. I want something like the builtin map function, except the work is done in parallel by a bunch of threads. Is there a good module that does this? There is a map method in multiprocessing.Pool . That does multiple processes. And if multiple processes aren't your dish, you can use multiprocessing.dummy which uses threads. import urllib import multiprocessing.dummy p = multiprocessing.dummy.Pool(5) def f(post): return urllib.urlopen('http:/

Pass multiple parameters to concurrent.futures.Executor.map?

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问题 The concurrent.futures.Executor.map takes a variable number of iterables from which the function given is called. How should I call it if I have a generator that produces tuples that are normally unpacked in place? The following doesn't work because each of the generated tuples is given as a different argument to map: args = ((a, b) for (a, b) in c) for result in executor.map(f, *args): pass Without the generator, the desired arguments to map might look like this: executor.map( f, (i[0] for i

Why do generators not support map()?

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It seems utterly natural to me that generators, which function very much like Arrays, should support the very basic list operations, like map() , filter() , and reduce() . Am I missing something? I wrote the code for map and it seems simple enough, but it would be much better to have all the functions embedded in all the generators: let fancyGen = g => { let rv = function*() { for (let x of g) yield x; } rv.map = function*(p) { for (let x of g) yield p(x); } return rv; } I'm new to generators, so any comments on the code are welcome. In particular, is that the best way to write "the identity

Java: is there a map function?

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问题 I need a map function. Is there something like this in Java already? (For those who wonder: I of course know how to implement this trivial function myself...) 回答1: There is no notion of a function in the JDK as of java 6. Guava has a Function interface though and the Collections2.transform(Collection<E>, Function<E,E2>) method provides the functionality you require. Example: // example, converts a collection of integers to their // hexadecimal string representations final Collection<Integer>

What is the difference between .map, .every, and .forEach?

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问题 I've always wondered what the difference between them were. They all seem to do the same thing... 回答1: The difference is in the return values. .map() returns a new Array of objects created by taking some action on the original item. .every() returns a boolean - true if every element in this array satisfies the provided testing function. An important difference with .every() is that the test function may not always be called for every element in the array. Once the testing function returns

Is there a difference between foreach and map?

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Ok this is more of a computer science question, than a question based on a particular language, but is there a difference between a map operation and a foreach operation? Or are they simply different names for the same thing? Different. foreach iterates over a list and applies some operation with side effects to each list member (such as saving each one to the database for example) map iterates over a list, transforms each member of that list, and returns another list of the same size with the transformed members (such as converting a list of strings to uppercase) The important difference

Passing multiple parameters to pool.map() function in Python [duplicate]

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问题 This question already has an answer here: Python multiprocessing pool.map for multiple arguments 18 answers I need some way to use a function within pool.map() that accepts more than one parameter. As per my understanding, the target function of pool.map() can only have one iterable as a parameter but is there a way that I can pass other parameters in as well? In this case, I need to pass in a few configuration variables, like my Lock() and logging information to the target function. I have

Function application over numpy's matrix row/column

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问题 I am using Numpy to store data into matrices. Coming from R background, there has been an extremely simple way to apply a function over row/columns or both of a matrix. Is there something similar for python/numpy combination? It's not a problem to write my own little implementation but it seems to me that most of the versions I come up with will be significantly less efficient/more memory intensive than any of the existing implementation. I would like to avoid copying from the numpy matrix to