CS 312 Lecture 17
Analyzing running time with recurrence relations

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Analyzing Running Times of Procedures

While asymptotic complexity has its limitations, it is still a useful tool for thinking about the performance of programs. We can use asymptotic complexity to express and to analyze the performance of SML functions. The use of big-O notation simplifies our task. We assume that the primitive operations of our language, such as arithmetic operations and pattern matching, all take no more than a certain constant time. And all reductions performed during evaluation similarly take constant time.

This assumption may be surprising if you think about the substitution work that seems to be required by the substitution model of evaluation, but the SML implementation avoids doing the work of substitution, instead keeping track of all the in-scope substitutions in a separate environment in which variables are looked up when needed.

Now, consider the following multiplication routine:

fun times1 (a:int, b:int):int = 
  if (b = 0) then 0 else a + times1(a,b-1)

What is the order of growth of the time required by times1 as a function of n, where n is the magnitude of the parameter b? Note that the "size" of a number can be measured either in terms of its magnitude or in terms of the number of digits (the space it takes to write the number down). Often the number of digits is used, but here we use the magnitude. Note that it takes only about log10 x digits to write down a number of magnitude x, thus these two measures are very different.

We assume that all the primitive operations in the times1 function if, +, =, and -) and the overhead for function calls take constant time. Thus if n=0, the routine takes constant time. If n>0, the time taken on an input of magnitude n is constant time plus the time taken by the recursive call on n-1. In other words, there are constants c1 and c2 such that T(n) satisfies

T(n) = T(n-1) + c1 for n > 0
T(0) = c2

This is called a recurrence relation. It simply states that the time to multiply a number a by another number b of size n > 0 is the time required to multiply a by a number of size n-1 plus a constant amount of work (the primitive operations performed). 

This recurrence relation has a unique closed form solution, namely

T(n) = c2 + c1n

which is O(n), so the algorithm is linear in the magnitude of b. One can obtain this equation by generalizing from small values of n, then prove that it is indeed a solution to the recurrence relation by induction on n.

Now consider the following procedure for multiplying two numbers:

fun times2(a:int, b:int):int = 
    if (b = 0) then
    else if even(b) then
      times2(double(a), half(b))
      a + times2(a, b-1)

Again we want an expression for the running time in terms of n, the magnitude of the parameter b. We assume that double and half operations are constant time (these could be done in constant time using arithmetic shift) as well as the standard primitives. The recurrence relation for this problem is more complicated than the previous one:

T(n) = T(n-1) + c1 if n > 0 and n is odd
T(n) = T(n/2) + c2 if n > 0 and n is even
T(0) = c3 

We somehow need to figure out how often the first versus the second branch of this recurrence relation will be taken. It's easy if n is a power of two, i.e. if n = 2m for some integer m. In this case, the second branch of will only get taken when n = 1, because 2m is even except when m = 0, i.e. when n = 1. Note further that T(1) = O(1) because T(1) = T(0) + O(1) = O(1) + O(1) = O(1). Thus, for this special case we get the recurrence relation:

T(n) = T(n/2) + c2 if n > 0 and n is a power of 2
T(0) = c3


T(n) = T(n/2) + c2 for n > 0 and n is a power of 2
T(0) = c3

for some constants c2 and c3. For powers of 2, the closed form solution of this is:

T(n) = c3 + c2 log2 n

which is O(log n).

What if n is not a power of 2? The running time is still O(log n) even in this more general case. Intuitively, this is because if n is odd, then n-1 is even, so on the next recursive call the input will be halved. Thus the input is halved at least once in every two recursive calls, which is all you need to get O(log n).

A good way to handle this formally is to charge to the cost of a call to times2 on an odd input the cost of the recursive call on an even input that must immediately follow it. We reason as follows: on an even input n, the cost is the cost of the recursive call on n/2 plus a constant, or

T(n) = T(n/2) + c2

as before. On an odd input n, we recursively call the procedure on n-1, which is even, so we immediately call the procedure again on (n-1)/2. Thus the total cost on an odd input is the cost of the recursive call on (n-1)/2 plus a constant. In this case we get

T(n) = T((n-1)/2) + c1+ c2

In either case,

T(n) ≤ T(n/2) + (c1 + c2)

whose solution is still O(log n). This approach is more or less the same as explicitly unwinding the else clause that handles odd inputs:

fun times2(a:int, b:int):int = 
    if (b = 0) then
    else if even(b) then
      times2(double(a), half(b))
      a + times2(double(a), half(b-1))

then analyzing the rewritten program, without actually doing the rewriting.

Charging one operation to another (bounding the number of times one thing can happen by the number of times that another thing happens) is a common technique for analyzing the running time of complicated algorithms.

Order notation is a useful tool, and should not be thought of as being just a theoretical exercise. For example, the practical difference in running times between the logarithmic times1 and the linear times2 is noticeable even for moderate values of n

The key points are:

Let's take a look at a useful algorithm in more detail and show that it is not only correct but that its worst-case performance is O(n lg n). The algorithm we'll look at is merge sort, a recursive algorithm for sorting a list of items. Merge sort is an example of a divide-and-conquer algorithm. It sorts a list by dividing it into two smaller sublists, recursively sorting the sublists, and then merging the two sorted lists together to produce the final result. Merging two lists is pretty simple if they themselves are already sorted. To prove the correctness and run time of merge sort we will want a stronger proof technique: strong induction.

Strong induction

Strong induction has the same 5 steps as ordinary induction, but the induction hypothesis is a little different:

  1. State the proposition to be proved in terms of P(n)
  2. Base case: show P(n0) is true
  3. Induction hypothesis: Assume that P(m) is true for all n0mn. This is different from ordinary induction where we only get to assume that P(m) is true for m=n.
  4. Induction step: Using the induction hypothesis, prove P(n+1) is true.
  5. Conclusion:  P(n) is true for all nn0

It is often easier to prove asymptotic complexity bounds using strong induction than it is using ordinary induction, because you have a stronger induction hypothesis to work with when trying to prove P(n+1).

Implementation and correctness of merge sort

(* split(xs) is a pair (ys,zs) where half (rounding up) of
 * the elements of xs are found in ys and the rest are in zs.
fun split (xs: int list): int list * int list = 
    let fun loop(xs:int list, left:int list, right:int list)
            : int list * int list =
	case xs of
	    nil => (left, right)
	  | x::nil => (x::left, right)
	  | x::y::rest => loop(rest, x::left, y::right)
	loop(xs, [], [])

(* A simpler way to write split. Recall the definition of
   foldl. What is the asymptotic performance of foldl f lst0
   lst where f is an O(1) function and lst is an n-element
   list? O(n). *)

fun split'(xs:int list) : int list * int list = 
  foldl (fn (x, (left,right)) => (x::right,left)) ([],[]) xs
(* merge(left,right) is a sorted list (in ascending order)
 * containing all the elements of left and right.
 * Requires: left and right are sorted lists *)
fun merge (left: int list, right: int list): int list =
  case (left, right) of
    (nil,_) => right
  | (_,nil) => left
  | (x::left_tail, y::right_tail) => 
      (if x > y then y::(merge(left, right_tail))
                else x::(merge(left_tail, right))))

How do we know that merge works? By induction on the sum of the length of the two input lists (i.e., length(left)+length(right)). Clearly if that minimum length is zero, the function works because one of the first two cases are used and they are trivially correct. What about the general case? We are trying to show that merge works on lists left and right whose total length is n+1, and we are allowed to assume that it works on lists left and right whose total length is n or less. If one of the two lists is empty the function works. What if both lists are non-empty? By the precondition (requires clause) we know that x is the smallest element of left and y the smallest element of right, and that rest_left and rest_right are sorted lists. Our inductive hypothesis lets us assume that merge works correctly in the recursive calls because the total length of the two lists is smaller than the total length of left and right, and the precondition of merge in the recursive calls is satisfied (it is being applied to sorted lists). If the then branch executes, y must be smaller than any element in either list; therefore, y::(merge(left, rest_right)) is a sorted list. Conversely, in the else branch x is smaller than or equal to any element in either list; therefore x::(merge(rest_left, right)) is also a sorted list. And we can see that merge doesn't "lose" any elements of left or right assuming that the recursive calls don't either.

Now we can write the merge-sort function itself. Note how we explicitly separate the specification of the function from the description of the algorithm that implements it. With merge and split specified as above, we don't really need even this much description of how merge_sort works.

(* merge_sort(xs) is a list containing the same elements as xs but in
 * ascending (nondescending) sorted order.
 * Implementation: lists of size 0 or 1 are already sorted. Otherwise,
 * split the list into two lists of equal size, recursively sort
 * them, and then merge the two lists back together. *)
fun merge_sort (xs: int list) : int list =

  case xs of
    [] => []   
  | [x] => [x]
  | _ => let val (left, right) = split xs
           merge (merge_sort(left), merge_sort(right))

Again, we can see by induction on the length of the input list that this function works. For lists of length 0 or 1 it clearly works. For larger lists we observe from the specification for split that both left and right must contain some elements and together they contain all the elements of xs. By the inductive hypothesis, merge_sort applied to each of these lists results in sorted lists. From the specification for merge the result must be a sorted list containing all the elements of xs. Therefore the merge_sort function will work correctly.

Merge sort asymptotic timing analysis

Now let's show that merge_sort is not only a correct but also an efficient algorithm for sorting lists of numbers. We start by observing without proof that the performance of the split function is linear in the size of the input list. This can be shown by the same approach we will take for merge, so let's just look at merge instead.

The merge function too is linear-time -- that is, O(n) -- in the total length of the two input lists. We will first find a recurrence relation for the execution time. Suppose the total length of the input lists is zero or one. Then the function must execute one of the two O(1)  arms of the case expression. These take at most some time c0 to execute. So we have

T(0) = c0
T(1) = c0

Now, consider lists of total length n. The recursive call is on lists of total length n−1, so we have

T(n) = T(n1) + c1

where c1 is an constant upper bound on the time required to execute the if statement and the operator :: (which takes constant time for usual implementations of lists). This gives us a recurrence relation to solve for T.  We can apply the iterative method to solve the recurrence relation by expanding out the recurrence relation inequalities for the first few steps. 

T(0) = c0
T(1) = c0
T(2) = T(1) + c1 = c0 + c1
T(3) = T(2) + c1 = c0 + 2c1
T(4) = T(3) + c1 = c0 + 3c1
T(n) = T(n−1) + c1 = c0 + (n1)c1 = (c0 + c1) + c1n

We notice a pattern which the last line captures. Recall that T(n) is O(n) if for all n greater than some n0, we can find a constant k such that T(n) < kn. For n at least 1, this is easily satisfied by setting k = c0 + 2c1. Or we can just remember that any first-degree polynomial is O(n) and also Θ(n). An even simpler way to find the right bound is to observe that the choice of constants c0 and c1 doesn't matter; if we plug in 1 for both of them we get T(1) = 1, T(2)=2, T(3)=3, etc., which is clearly O(n).

Now let's consider the merge_sort function itself. Again, for zero- and one-element lists we compute in constant time. For n-element lists we make two recursive calls, but to sublists that are about half the size, and calls to split and merge that each take Θ(n) time. For simplicity we'll pretend that the sublists are exactly half the size. The recurrence relation we obtain has this form:

T(0) = c0
T(1) = c0
T(n) = 2 T(n/2) + c1n +  c2n + c3

Let's use the iterative method to figure out the running time of merge_sort. We know that any solution must work for arbitrary constants c0 and c4, so again we replace them both with 1 to keep things simple. That leaves us with the following recurrence equations to work with:

T(1) = 1
T(n) = 2 T(n/2) + n

Starting with the iterative method, we can start expanding the time equation until we notice a pattern:

T(n) = 2T(n/2) + n
     = 2(2T(n/4) + n/2) + n
     = 4T(n/4) + n + n
     = 4(2T(n/8) + n/4) + n + n
     = 8T(n/8) + n + n + n
     = nT(n/n) + n + ... + n + n + n
     = n + n + ... + n + n + n

Counting the number of repetitions of n in the sum at the end, we see that there are lg n + 1 of them.  Thus the running time is n(lg n + 1) = n lg n + n. We observe that n lg n + n < n lg n + n lg n = 2n lg n for n>0, so the running time is O(n lg n).  So now we've done the analysis by using the iterative method, let's use induction to verify that the bound is correct. It will be convenient to use a slightly different version of the induction proof technique known as strong or course-of-values induction.

Merge sort analysis using strong induction

Consider n0 = 2.

Property of n to prove:

For n>n0, there exists
T(n) = n lg n + n

Proof by strong (course-of-values) induction on n

Base case: n = 1
    T(1) = 1 = 1 lg 0 + 1

Induction Step:

Induction Hypothesis:
T(k) = k lg k + k         for all kn

Property to prove for n+1:
T(n+1) = (n+1) lg (n+1) + (n+1)


T(n+1) = 2 T((n+1)/2) + (n+1)

    = 2 ((n+1)/2 lg ((n+1)/2) + (n+1)/2) + (n+1)             (by induction hypothesis)

    = (n+1)(lg ((n+1)/2)) + (n+1) + (n+1)

    = (n+1)(lg(n+1) − 1) + 2(n+1)

    = (n+1) lg(n+1) + (n+1)

Thus we have shown that merge sort is Θ(n lg n).