Session 17

Variable Decrease and Conquer

CS 3530
Design and Analysis of Algorithms

The Fake Coin

an old-fashioned balance scale

You are given n coins. They all look identical. They should all be the same weight, too -- but one is a fake, made of a lighter metal.

Your neighbor has an old-fashioned balance scale that enables you to compare any two sets of coins. If it tips either to the left or to the right, you will know that the one of the sets is heavier than the other. Sadly, you aren't on speaking terms with the neighbor, so he charges you each time you weigh anything.

Your task is this:

Design an algorithm to find the fake coin in the fewest number of weighings.

How many times must you use the scale?

Finding the Fake Coin

We have been studying decrease-and-conquer, so it's not too surprising that a decrease-and-conquer algorithm works here.

You may well have realized that you can divide the pile in half, weigh the halves, and narrow your focus to the pile that is lighter. But that sounds a lot like binary search -- isn't binary search the prototypical divide-and-conquer approach?

As some of you discovered while studying partitioning and search earlier this semester, some people prefer not to call binary search "divide-and-conquer" because it doesn't solve both sub-problems. Instead, it discards have the space and solves only one of the sub-problems. From this perspective, it is really a decrease(-by-half)-and-conquer algorithm. That sort of approach works for finding the fake coin.

But we can do better than a factor of 2.

Suppose we divide the coins into three piles, where at least two of them contain the same number of coins. After weighing the equal-sized piles, we can eliminate ~2/3 of the coins!

To design an algorithm, we need to be more precise.

Here is an algorithm:

    INPUT    : integer n

    if n = 1 then
       the coin is fake
       divide the coins into piles of A = ceiling(n/3), B = ceiling(n/3),
           and C = n-2*ceiling(n/3)
       weigh A and B
       if the scale balances then
          iterate with C
          iterate with the lighter of A and B

How many weighings does this require? Approximately log3 n. But you don't have to settle for an approximate answer...

Quick Exercise: Construct and solve the recurrence relation for this algorithm. Simplify your work by assuming n = 3k for an integer k.

How much does this improve on a decrease-by-half approach, in which we split the coins into two piles?

    log2 n
    log3 n

In case you haven't worked with logarithms in a while, I'll drop some arithmetic on you:

    k = log2 nn = 2k
    m = log3 nn = 3m

            2k = 3m
       log2 2k = log2 3m
    k * log2 2 = m * log2 3
             k = m * log2 3

             - = log2 3


    log2 n
    ------ = log2 3 = 1.584963...
    log3 n

This means that, on top of the log2 n speedup, the three-group split gives another 1.6x speedup. Very nice.

This algorithm and binary search can be classified more generally as decrease-by-constant-factor algorithms. The larger the factor, generally the more efficient the algorithm.

Majority by Subtraction

Speaking of decrease-and-conquer, how did you like my decrease-and-conquer algorithm for the Election puzzle? Sometimes there's gold hidden in them thar hills.

Russian Peasant Multiplication

Back in Session 14, we looked at some optimizations in divide-and-conquer-multiplication. Suppose that n is even. Then, n * m can be rewritten (n/2) * (m*2). This can be quite efficient at the machine level, because doubling and halving can be implemented as shifting the bits of the number left and right, respectively.

We do have to handle the case in which n is odd. To do so, we can keep track of values that are lost when we divide by 2. For example:

     n   *     m      LOST

    26   *    42
    13   *    84
     6   *   168        84
     3   *   336
     1   *   672       336

So, n * m = 672 + 336 + 84 = 1092.

In order to design a concise algorithm, we need to identify a simple invariant. Take another look at our sequence of halves-and-doubles... The values we add to find our product are exactly the values of m when n is odd -- including the final case, where n = 1.

Let's rewrite our table to make this invariant clearer:

     n   *     m       ADD

    26   *    42
    13   *    84        84
     6   *   168
     3   *   336       336
     1   *   672       672

This leads to a straightforward algorithm.

Quick Exercise: Write it.

Recursively, we might start with:

    multiply(n, m)
      if n = 1 then
         return m
      else if n is odd then
         return m + multiply(n/2, m*2)
         return multiply(n/2, m*2)

... and make it better. But we can also write this using a loop in a straightforward way, too:

    multiply(n, m)
      sum = 0
      while true
         if n is odd then
            sum += m
         if n = 1 then
            return sum
         n = n / 2
         m = m * 2

As I said when we saw this the first time...

Again: Beautiful.

Interpolation Search

Consider again binary search, the prototypical decrease-by-half algorithm:

    input : k, a target value
            v[left..right], a sorted list of values
    output: the index of k in v, or failure

    if right < left
       then fail

    middle ← (left+right)/2      *** the key line
    if k = v[middle]
       then return middle
    if k < v[middle]
       then return search(left, middle-1)
       else return search(middle+1, right)

As noted in Session 12, we can let middle be any index such that (1 ≤ middlen),

... but we get our best performance with middle in the very middle:
We eliminate half of the list on each pass!

This is certainly true for arrays such as

    3 14 27 31 39

... in which the values in the array are evenly distributed. But what about arrays such as this one:

    1 2 3 4 5 ... 40 41 91 99

When we search for 3, the algorithm will partition the array into ...

    1 2 3 4 5 ... 20 21

It will then work its way down to 3, looking in slots 11, 6, and 3. We can't do much better.

But what if we are looking for 91? The left half of every subarray along the way has values that much smaller than 91. Couldn't we take advantage of this to speed things up a bit?

Recurring theme alert: partition by position versus partition by value...

Instead of splitting the array in half each time using the indices of the left and right values, why not let the values themselves tell us where to go, allowing us to jump farther into subarrays?

The idea is this: Compute the new value of middle using a ratio based on the relative distance between our target value k and the leftmost and rightmost values of the array being searched. Let's replace the key line above:

    middle ← (left+right)/2

... with this:

             |                   k - v[left]      |
    middle ← | (right-left) * ------------------  | + left
             |                v[right] - v[left]  |

For example, if we are looking for 91 in the original array above, then on our first pass we compute middle as:

                       91 - 1
    middle ←  (43-1) * ------ + 1
                       99 - 1

           = (42 * 0.918) + 1
           = 39.571
           = 39

Because 91 > v[39] = 39, the algorithm focuses on the subarray v[40 .. 43] -- an array of size 4! That's much faster than the standard approach.

This is often called interpolation search. To in·ter·po·late is to "insert (something) between fixed points" or "to estimate values of (data or a function) between two known values". Many of you may recognize the idea of interpolation from your high school math courses, where it is a useful technique for simulating a continuous function when all you have is a discrete set of ordered pairs.

We can call interpolation search a decrease-by-variable-amount algorithm. The size of the portion of the original problem discarded on each pass depends on the values in the problem. This can make for an efficient and concise algorithm. It also complicates efficiency analysis, which now must include probabilities in the computation.

Quick Exercise: For what inputs does this algorithm perform better than straight binary search? In such cases, how much better can it perform? What is the worst case input for this algorithm? (In the worst case, interpolation search performs worse than straight binary search!)

Aside about the key line: The computation of middle in the straight binary search hides a nasty potential error...

Solving Homework 3

Small steps. Greedy in Ruby.

"Sorting columns". Surprised so few asked Qs. Start earlier? Work slower. Some possibilities in Python.

First, come up with an idea. Then find the syntax or library functions you need to implement it.

Oh, and be sure to read the assignment!

See today's zip file for some illustrative code.

Wrap Up

Eugene Wallingford ..... ..... March 11, 2014