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_someting,range(10))) or use collections.deque() with the length set to 0 to not produce a list if you don’t need the map output: from collections import deque deque(map(self.do_someting, range(10))) but … Read more
Yes, this is an anti-pattern. Although you could argue it is not and it is just plain misuse. I would call it an anti-pattern because for some reason there is a frequent widespread incidents of people using .map() when they should not. The term comes from mathematics where you can map from one category to … Read more
Different. foreach iterates over a list and performs 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 … Read more
Array.prototype.map() takes a second argument to set what this refers to in the mapping function, so pass this as the second argument to preserve the current context: someList.map(function(item) { … }, this) Alternatively, you can use an ES6 arrow function to automatically preserve the current this context: someList.map((item) => { … })
map isn’t particularly pythonic. I would recommend using list comprehensions instead: map(f, iterable) is basically equivalent to: [f(x) for x in iterable] map on its own can’t do a Cartesian product, because the length of its output list is always the same as its input list. You can trivially do a Cartesian product with a … Read more
The following are rough guidelines and educated guesses based on experience. You should timeit or profile your concrete use case to get hard numbers, and those numbers may occasionally disagree with the below. A list comprehension is usually a tiny bit faster than the precisely equivalent for loop (that actually builds a list), most likely … Read more
I had a task that I only knew the length of the array and needed to transform the items. I wanted to do something like this: let arr = new Array(10).map((val,idx) => idx); To quickly create an array like this: [0,1,2,3,4,5,6,7,8,9] But it didn’t work because: (see Jonathan Lonowski’s answer a few answers above) The … Read more
This is usually called maplist/3 and is part of the Prolog prologue. Note the different argument order! :- meta_predicate(maplist(2, ?, ?)). maplist(_C_2, [], []). maplist( C_2, [X|Xs], [Y|Ys]) :- call(C_2, X, Y), maplist( C_2, Xs, Ys). The different argument order permits you to easily nest several maplist-goals. ?- maplist(maplist(test),[[1,2],[3,4]],Rss). Rss = [[1,4],[9,16]]. maplist comes in … Read more
Do this: list(map(chr,[66,53,0,94])) In Python 3+, many processes that iterate over iterables return iterators themselves. In most cases, this ends up saving memory, and should make things go faster. If all you’re going to do is iterate over this list eventually, there’s no need to even convert it to a list, because you can still … Read more
map function for objects (instead of arrays)