Thread: recoursion or loops?

  1. #1
    Registered User
    Join Date
    Dec 2001

    recoursion or loops?

    My Uni-teacher seems to love recoursive functions, and so I get strange counting-exercises frequently. I've heard where ever I can solve an exercise with a recoursive function I could also use loops. Loops are said to be executed more quickly than recoursive functions, and they also need less storage. Is there any good reason to prefer recoursive functions? (My teacher says they're easier to understand. - ???)

  2. #2
    It's full of stars adrianxw's Avatar
    Join Date
    Aug 2001
    A loop is often faster than recursion. Each time a recursive function calls itself, it needs to set up stack, branch then unwind the stack again. A loop does not have that overhead, (and incidentally, does not run the risk of overflowing the stack).

    >>> My teacher says they're easier to understand.

    I've encountered that argument before, and would simply disagree. "Easy to understand" is not an objective statement. Whilst some people may find it easier, others find it more difficult. Using recursion for the sake of it is bad practice, use what is less likely to cause confusion.

    Another factor is that some operating systems require you to specify in advance your stack size. This can be difficult if you are using recursive functions and you can't specify in advance how deep the recursion will be - you tend to end up requesting excessively large stacks to cover the worst case scenario, that can frequently lead to wasted resources.
    Wave upon wave of demented avengers march cheerfully out of obscurity unto the dream.

  3. #3
    Join Date
    Aug 2001
    Alot of c compilers can optimize a function that is written this way

    void print_hello(int n)
        if (n != 0) {
            print_hello(n - 1);
    The output from gcc with optimization is

    	pushl	%ebp
    	movl	%esp, %ebp
    	pushl	%ebx
    	pushl	%eax
    	movl	8(%ebp), %ebx
    	testl	%ebx, %ebx
    	je	.L18
    	subl	$12, %esp
    	pushl	$.LC0
    	call	printf
    	decl	%ebx
    	addl	$16, %esp
    	jmp	.L19
    	.p2align 2
    	movl	-4(%ebp), %ebx
    So a iterative solution written recursively doesn't always eat up stack space.

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