quartz/content/notes/analysis-of-recursive-algorithms.md

title	tags
analysis-of-recursive-algorithms	cosc201

analysis-of-recursive-algorithms

• induction and recursion are linked
• inductive approach is esential for understanding time-complexity of resursive algorithms

1 Proof by induction

Induction Find a (positive integer) parameter that gets smaller in all recursive calls Prove inductively that "for all values of the parameter, the result computed is correct" To do that:

• check correctness is all non-recursive cases
• check correctness in recursive cases assuming correcness in the recursive calls

2 Examples

2.1 Quicksort

divide and conquer algorithm sorts a range in an array (a group of elements between some lower index, lo inclusive and some upper index hi exclusive) as follows:

• If length of range (hi - lo) is at most 1 -> do nothing
• otherwise, choose a pivot p (e.g., the element at lo) and:
  • place all items less that p in positions lo to lo +r
  • place all items >= p in positions lo +r+1 to hi
  • place p in position lo+r
  • call quicksort on the ranges lo to lo + r and lo+r+1 to hi

2.1.1 Proof

parameter is hi - lo

the parameter gets smaller in all recusive call because we always remove the element p so, even if it is the smallest or largest element of the range ,,the recursive call has a range of size at most hi - lo - 1

the non-recursive case is correct because if we have 1 or fewer elements in a range they are already sorted

in the recirsive case, since all the elements before p are smaller than it and we assume they get sorted correctly be quicksort, and the same happens for the elements larger than p, we will get a correctly sorted array

2.2 Fibonacci 1

	def fib(n)
		if n <= 1
			return 1
	return fib(n-1) + fib(n-2)

line 1 -> always executed line 2 -> executed if n<=1 line 4 -> executed if n>1, cost equal to cost of callling fib(n-1), fib(n-2), and some constant cost for the addition and return

2.2.1 Cost bounds/Proof

if we let T(n) denote the time required for evaluating fib(n) using this algorithm this analysis gives:

T(0) = T(1) = C

T(n) = D + T(n-1) + T(n-2)

where c and d are some positive (non-zero) constants.

• this shows that T(n) grows at least as quick as fib(n)
• even if D=0 we'd get T(n) = C \times fib(n)
• growth rates are the same \therefore exponential (at least 1.6^n) and far too slow

A recurive algorithm that makes two or more recurive calls with parameter values close to the original will generally have exponential time complexity

2.3 Fibonacci 2

	def fibPair()
		if n == 1
			return 1, 1
		a,b = fibpair(n-1)
		return b, a+b

line 1 -> always executed some constant cost line 2-> executed if n=1, some constant cost line 4-> executed if n>1, cost equal to cost of calling fibPair(n-1) line 5 -> executed if n>1, some constant cost

2.3.1 Proof

it's true for $n-1 by design$ If it's true at n-1 then the result of computing fibpair(n) is:

(f_{n-1}, f_{n-1} + f_{n-1}) = (f_{n-1}, f_n)

which is what we want

2.3.2 Cost bounds

if we let P(n) denote the time required for evaluating fib(n) using this algorithm this analysis gives:

$P(1) = C$ P(n) = P(n-1) + D\ for\ n>1

where C and D are some positive (non-zero) constants.

Claim: P(n) = C + D(n-1)

By induction: it's true for n = 1 since,

$P(1) = C$ C+D\times(1-1)=C

suppose that it's true for n-1. Then it's true for n as well because

$P(n) = P(n-1) + D$ $\ \ \ \ \ \ \ \ \ = C+D\times(n-2)+D$ \ \ \ \ \ \ \ \ \ = C+D\times(n-1)

\therefore By induction it's true for all n>=1

P(n) is the time for evaluating fibPair(n) using this algorithm. This analysis gives:

$P(1) = C$ P(n) = P(n-1) +D

where C and D are some positive constants

#theorem

P(n) = C+D\times(n-1)

in particular, P(n) = \theta(n)

A recursive algorithm that make one recurive call with a smaller value and a constant amount of additional work will have at most linear time complexity