map-function

问题 I'm reading through the Python documentation to really get in depth with the Python language and came across the filter and map functions. I have used filter before, but never map, although I have seen both in various Python questions here on SO. After reading about them in the Python tutorial, I'm confused on the difference between the two. For example, from 5.1.3. Functional Programming Tools: >>> def f(x): return x % 2 != 0 and x % 3 != 0 ... >>> filter(f, range(2, 25)) [5, 7, 11, 13, 17,

问题 I have some understanding of list comprehension. I understand that the expression: [x * x | x <- [1..10]] should output [1,4,9,16,25,36,49,64,81,100] and that the effect of that expression is the same as: map power [1..10] power x = x * x Now, I have to find out the other method (just like the above) for the following function: [(x,y+z) | x <- [1..10], y <- [1..x], z <- [1..y]] I can't figure it out by myself without errors, please help me 回答1: The Haskell Report tells us how to translate

问题 I have some understanding of list comprehension. I understand that the expression: [x * x | x <- [1..10]] should output [1,4,9,16,25,36,49,64,81,100] and that the effect of that expression is the same as: map power [1..10] power x = x * x Now, I have to find out the other method (just like the above) for the following function: [(x,y+z) | x <- [1..10], y <- [1..x], z <- [1..y]] I can't figure it out by myself without errors, please help me 回答1: The Haskell Report tells us how to translate

问题 The following code: let resultsArray = Array.apply(null, Array(10)).map(Number.prototype.valueOf,0); creates the following array [0, 0, 0, 0, 0, 0, 0, 0, 0, 0] Why does map() need Number.prototype.valueOf just to push the number 0 in to each position of this array. Is there a different (more efficient) way to achieve this result, or is this the best way? 回答1: If you read the map documentation you can read this: The map() method creates a new array with the results of calling a provided

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 3 years ago . 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? 回答1: There is a

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 3 years ago . 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? 回答1: There is a

问题 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? 回答1: 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

问题 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? 回答1: 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

问题 map2_List :: (a -> b -> c) -> [a] -> [b] -> [c] map2_List f [] _ = [] map2_List f (a:as) bs = map (f a) bs ++ map2_List f as bs This is an example from my lecture, which try to apply a binary function to all pairs of elements of two lists. The part (f a) makes me confused. Does it suppose to be a value but not a function? Then what does map value bs do? 回答1: "The part (f a) makes me confused." What is happening here is called currying and if you are coming to Haskell from imperative languages

问题 In Scala programming use an anonymous function is a usual thing . when i decide to creat a vector as out put of an anonymous function from two different ways way one : var hold1=(1 to 5).map(_*2) way two: var hold2=(1 to 5).map(2*) I want to know what is the difference between those two declaration ? 回答1: In short - they are exactly the same. First approach: var hold1 = (1 to 5).map(_*2) Let's rewrite this another way to demonstrate what's really happening under the hood (no syntactic sugar)