NB: The high memory usage issue is a bug in the GHC RTS, where upon stack overflow and allocation of new stacks on the heap it was not checked whether garbage collection is due. It has been already fixed in GHC HEAD. I was able to get much better performance by CPS-converting ack: module Main … Read more
The dirs and files lists are all always relative to root – i.e., they are the basename() of the files/folders, i.e. they don’t have a / in them (or \ on windows). You need to join the dirs/files to root to get their whole path if you want your code to work to infinite levels … Read more
2**n -1 is also 1+2+4+…+2n-1 which can made into a single recursive function (without the second one to subtract 1 from the power of 2). Hint: 1+2*(1+2*(…)) Solution below, don’t look if you want to try the hint first. This works if n is guaranteed to be greater than zero (as was actually promised in … Read more
Writing a recursive function without naming it is a problem that is as old as computer science itself (even older, actually, since λ-calculus predates computer science), since in λ-calculus all functions are anonymous, and yet you still need recursion. The solution is to use a fixpoint combinator, usually the Y combinator. This looks something like … Read more
There’s a simple ad hoc proof for this. Since you can build a Turing complete language using strictly iterative structures and a Turing complete language using only recursive structures, then the two are therefore equivalent.
Here is my take: def nest_dict(flat): result = {} for k, v in flat.items(): _nest_dict_rec(k, v, result) return result def _nest_dict_rec(k, v, out): k, *rest = k.split(‘_’, 1) if rest: _nest_dict_rec(rest[0], v, out.setdefault(k, {})) else: out[k] = v flat = {‘X_a_one’: 10, ‘X_a_two’: 20, ‘X_b_one’: 10, ‘X_b_two’: 20, ‘Y_a_one’: 10, ‘Y_a_two’: 20, ‘Y_b_one’: 10, ‘Y_b_two’: … Read more
Using .filter() and making a recursive call as I described in the comment above is basically what you need. You just need to update each .children property with the result of the recursive call before returning. The return value is just the .length of the resulting .children collection, so if there’s at least one, the … Read more
def total_subsets_matching_sum(numbers, sum): array = [1] + [0] * (sum) for current_number in numbers: for num in xrange(sum – current_number, -1, -1): if array[num]: array[num + current_number] += array[num] return array[sum] assert(total_subsets_matching_sum(range(1, 10), 9) == 8) assert(total_subsets_matching_sum({1, 3, 2, 5, 4, 9}, 9) == 4) Explanation This is one of the classic problems. The idea … Read more
You may write a recursive algorithm in Batch that gives you exact control of what you do in every nested subdirectory: @echo off call :treeProcess goto :eof :treeProcess rem Do whatever you want here over the files of this subdir, for example: for %%f in (*.tif) do echo %%f for /D %%d in (*) do … Read more
constexpr functions have no side-effects and can thus be memoized without worry. Given the disparity in runtime the simplest explanation is that the compiler memoizes constexpr functions during compile-time. This means that fibonacci(n) is only computed once for each n, and all other recursive calls get returned from a lookup table.