Recursion and Iteration

Question

What is the difference? Are these the same? If not, can someone please give me an example?

MW: Iteration - 1 : the action or a process of iterating or repeating: as

Answer 1

For a good example of the above, consider recursive v. iterative procedures for depth-first search. It can be done using language features via recursive function calls, or in an iterative loop using a stack, but the process is inherently recursive.

Answer 2

Here's a Lisp function for finding the length of a list. It is recursive:

(defun recursive-list-length (L)
  "A recursive implementation of list-length."
  (if (null L)
      0
    (1+ (recursive-list-length (rest L)))))

It reads "the length of a list is either 0 if that list is empty, or 1 plus the length of the sub-list starting with the second element).

And this is an implementation of strlen - the C function finding the length of a nul-terminated char* string. It is iterative:

size_t strlen(const char *s)
{
    size_t n;

    n = 0;
    while (*s++)
        n++;
    return(n);
}

You goal is to repeat some operation. Using iteration, you employ an explicit loop (like the while loop in the strlen code). Using recursion, your function calls itself with (usually) a smaller argument, and so on until a boundary condition (null L in the code above) is met. This also repeats the operation, but without an explicit loop.

Answer 3

As per the definitions you mentioned, these 2 are very different. In iteration, there is no self-calling, but in recursion, a function calls itself

For example. Iterative algorithm for factorial calculation

fact=1
For count=1 to n
fact=fact*count
end for

And the recursive version

function factorial(n)
if (n==1) return 1
else
n=n*factorial(n-1)
end if
end function

Generally

Recursive code is more succinct but uses a larger amount of memory. Sometimes recursion can be converted to iterations using dynamic programming.

Answer 4

Recursion: Eg: Take fibonacci series for example. to get any fibonacci number we have to know the previous one. So u will berform the operation (same one) on every number lesser than the given and each of this inturn calling the same method.

fib(5) = Fib (4) + 5

fib(4) = Fib (3) + 4 . . i.e reusing the method fib

Iteration is looping like you add 1+1+1+1+1(iteratively adding) to get 5 or 3*3*3*3*3 (iteratively multiplying)to get 3^5.

Answer 5

[Hurry and trump this!]

One form can be converted to the other, with one notable restriction: many "popular" languages (C/Java/normal Python) do not support TCO/TCE (tail-call-optimization/tail-call-elimination) and thus using recursion will 'add extra data to the stack' each time a method calls itself recursively.

So in C and Java, iteration is idiomatic, whereas in Scheme or Haskell, recursion is idiomatic.

Answer 6

For difference between recursive vs non-recursive; recursive implementations are a bit easier to verify for correctness; non- recursive implementations are a bit more efficient.

Algorithms (4th Edition)