Recursion or Iteration?

Question

Is there a performance hit if we use loop instead of recursion or vice versa in algorithms where both can serve the same purpose? Eg : Check if given string is palindrome. I have seen many programmers using recursion as a means to show off when a simple iteration algorithm can fit the bill. Does the compiler play a vital role in deciding what to use?

Answer 1

Stack overflow will only occur if you're programming in a language that doesn't have in built memory management.... Otherwise, make sure you have something in your function (or a function call, STDLbs, etc). Without recursion it would simply not be possible to have things like... Google or SQL, or any place one must efficiently sort through large data structures (classes) or databases.

Recursion is the way to go if you want to iterate through files, pretty sure that's how 'find * | ?grep *' works. Kinda dual recursion, especially with the pipe (but don't do a bunch of syscalls like so many like to do if it's anything you're going to put out there for others to use).

Higher level languages and even clang/cpp may implement it the same in the background.

Answer 2

Comparing recursion to iteration is like comparing a phillips head screwdriver to a flat head screwdriver. For the most part you could remove any phillips head screw with a flat head, but it would just be easier if you used the screwdriver designed for that screw right?

Some algorithms just lend themselves to recursion because of the way they are designed (Fibonacci sequences, traversing a tree like structure, etc.). Recursion makes the algorithm more succinct and easier to understand (therefore shareable and reusable).

Also, some recursive algorithms use "Lazy Evaluation" which makes them more efficient than their iterative brothers. This means that they only do the expensive calculations at the time they are needed rather than each time the loop runs.

That should be enough to get you started. I'll dig up some articles and examples for you too.

Link 1: Haskel vs PHP (Recursion vs Iteration)

Here is an example where the programmer had to process a large data set using PHP. He shows how easy it would have been to deal with in Haskel using recursion, but since PHP had no easy way to accomplish the same method, he was forced to use iteration to get the result.

http://blog.webspecies.co.uk/2011-05-31/lazy-evaluation-with-php.html

Link 2: Mastering Recursion

Most of recursion's bad reputation comes from the high costs and inefficiency in imperative languages. The author of this article talks about how to optimize recursive algorithms to make them faster and more efficient. He also goes over how to convert a traditional loop into a recursive function and the benefits of using tail-end recursion. His closing words really summed up some of my key points I think:

"recursive programming gives the programmer a better way of organizing code in a way that is both maintainable and logically consistent."

http://www.ibm.com/developerworks/linux/library/l-recurs/index.html

Link 3: Is recursion ever faster than looping? (Answer)

Here is a link to an answer for a stackoverflow question that is similar to yours. The author points out that a lot of the benchmarks associated with either recursing or looping are very language specific. Imperative languages are typically faster using a loop and slower with recursion and vice-versa for functional languages. I guess the main point to take from this link is that it is very difficult to answer the question in a language agnostic / situation blind sense.

Is recursion ever faster than looping?

Answer 3

You have to keep in mind that utilizing too deep recursion you will run into Stack Overflow, depending on allowed stack size. To prevent this make sure to provide some base case which ends you recursion.

Answer 4

If the iterations are atomic and orders of magnitude more expensive than pushing a new stack frame and creating a new thread and you have multiple cores and your runtime environment can use all of them, then a recursive approach could yield a huge performance boost when combined with multithreading. If the average number of iterations is not predictable then it might be a good idea to use a thread pool which will control thread allocation and prevent your process from creating too many threads and hogging the system.

For example, in some languages there are recursive multithreaded merge sort implementations.

But again, multithreading can be used with looping rather than recursion, so how well this combination will work depends on more factors including the OS and its thread allocation mechanism.

Answer 5

There are many cases where it gives a much more elegant solution over the iterative method, the common example being traversal of a binary tree, so it isn't necessarily more difficult to maintain. In general, iterative versions are usually a bit faster (and during optimization may well replace a recursive version), but recursive versions are simpler to comprehend and implement correctly.

Answer 6

I believe tail recursion in java is not currently optimized. The details are sprinkled throughout this discussion on LtU and the associated links. It may be a feature in the upcoming version 7, but apparently it presents certain difficulties when combined with Stack Inspection since certain frames would be missing. Stack Inspection has been used to implement their fine-grained security model since Java 2.

http://lambda-the-ultimate.org/node/1333