lazy-evaluation

I have the following methods, part of the logic for performing stratified k-fold crossvalidation. private static IEnumerable<IEnumerable<int>> GenerateFolds( IClassificationProblemData problemData, int numberOfFolds) { IRandom random = new MersenneTwister(); IEnumerable<double> values = problemData.Dataset.GetDoubleValues(problemData.TargetVariable, problemData.TrainingIndices); var valuesIndices = problemData.TrainingIndices.Zip(values, (i, v) => new { Index = i, Value = v }); IEnumerable<IEnumerable<IEnumerable<int>>> foldsByClass = valuesIndices.GroupBy(x => x.Value, x => x.Index) .Select(g

More and more features of Python move to be "lazy executable", like generator expressions and other kind of iterators. Sometimes, however, I see myself wanting to roll a one liner "for" loop, just to perform some action. What would be the most pythonic thing to get the loop actually executed? For example: a = open("numbers.txt", "w") (a.write ("%d " % i) for i in xrange(100)) a.close() Not actuall code, but you see what I mean. If I use a list generator, instead, I have the side effect of creating a N-lenght list filled with "None"'s. Currently what I do is to use the expression as the

I'm trying to learn me a Haskell (for great good), and one of the many different things I'm doing is trying to tackle some Project Euler problems as I'm going along to test my mettle. In doing some of the Fibonacci based problems, I stumbled on and started playing around with the recursive infinite list version of the Fibonacci sequence: fibs = 1 : 2 : zipWith (+) fibs (tail fibs) For one of the PE problems, I needed to extract the subsequence of even Fibonacci numbers less than 4,000,000. I decided to do this with a list comprehension, and in my playing around with the code I stumbled on to

I just started learning Haskell. Below is some code written in an imperative style that implements a simple server -- it prints out the HTTP request headers. Besides the fact that I need to rethink it in Haskell, to work with lazy lists and higher order functions, I'd like to see clearly why it does not do what I intended. It is always one behind -- I hit it with a request, nothing happens, hit it again, it prints the first request, hit it the 3rd time, it prints the 2nd request, etc. Why is that? And what is a minimal change to this code that would cause it to print right when the request

Several years ago I took an algorithms course where we were giving the following problem (or one like it): There is a building of n floors with an elevator that can only go up 2 floors at a time and down 3 floors at a time. Using dynamic programming write a function that will compute the number of steps it takes the elevator to get from floor i to floor j . This is obviously easy using a stateful approach, you create an array n elements long and fill it up with the values. You could even use a technically non-stateful approach that involves accumulating a result as recursively passing it