You need to use an accumulator. While you could do something like this: which will recurse all the way down to the end of the list and then, as each invocation returns, add one to the length, until it gets back to the top level with the correct result. The problem with this approach is that … Read more
Your solution explained: If we reverse the empty list, we obtain the empty list. If we reverse the list [H|T] , we end up with the list obtained by reversing T and concatenating with [H] . To see that the recursive clause is correct, consider the list [a,b,c,d] . If we reverse the tail of … Read more
Use list comprehension for this
There are four errors in your approach. First, file.names <- dir(path, pattern =”.csv”) will extract just file names, without path. So, when you try to import then, read.csv() doesn’t find. Building the path You can build the right path including paste0(): Or file.path(), which add slashes automaticaly. And another way to create the path, for … Read more
The others are right that the problem is the : operator. I would say that your areTheySame function that returns a list is the wrong approach anyway, though. Rather than switch to the ++ operator, a better implementation of that function would be: As you can see, this is a pretty simple implementation. Also, consing … Read more
Use frozenset, To represent a set of sets, the inner sets must be frozenset objects for the reason that the elements of a set must be hashable (all of Python’s immutable built-in objects are hashable). frozenset is immutable and set is mutable.
The reason this doesn’t work is that res only has the value of the first node you give it appended to it; each time you recursively recall the function, it just makes a new res. It is a simple fix though, as follows: In this, it returns the left branch, the value, and then the right. This … Read more
The code as you’ve posted it is (almost) OK. The order of clauses just needs to be swapped (in order to make this predicate definition productive, when used in a generative fashion): This defines a relationship between the three arguments, let’s say A, B and C. Your first line says, ” C is the result of appending A and B if A and C are non-empty lists, they both have … Read more
Appending to an object in a for loop causes the entire object to be copied on every iteration, which causes a lot of people to say “R is slow”, or “R loops should be avoided”. As BrodieG mentioned in the comments: it is much better to pre-allocate a vector of the desired length, then set the element … Read more