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Handout 17: Homework #4 Nearest-Neighbor Classification using kd-Trees

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CpSc 2120: Algorithms and Data Structures

Webpage: http://www.cs.clemson.edu/~bcdean/ MWF 9:05-9:55
Handout 17: Homework #4 Vickery 100
1 Nearest-Neighbor Classification using kd-Trees
This assignment gives us a chance to work with multi-dimensional data by building and searching
kd-trees. We will also gain experience with nearest-neighbor classification, a common technique in
the domain of machine learning that can be quite relevant in many situations in practice.
The dataset we will use for this exercise, derived from the study in [1], is the following:
/group/course/cpsc212/f14/hw04/wine.txt
It contains data on several thousand brands of white wine, each with a real-valued human-assessed
quality rating (in the range 0-10) and 11 real-valued physiochemical attributes (pH, alcohol content,
sulphate concentration, sugar content, etc.). We can think of this data as a collection of points in
11-dimensional space, each labeled with a real number in the range 0-10. Our goal is to see if we
can predict the quality of a wine just based on physiochemical properties alone. As is typical in
machine learning, it is not known how successful we will be at either goal. We may later test our
code on other datasets as well, depending on their availability.
2 Reading the Data
The first line of each data file contains two integers, n and d, specifying the number of data points
and the number of attributes (dimensions) of each point. The next n lines each contain d + 1 real
values that describe a single data point: the first number is a “label” for the data point (i.e., the
quality score); the remaining d values describe the d attributes of the data point. Our dataset
therefore has n labeled points in d-dimensional space. Let xi(1). . . xi(d) denote the d coordinate
values for data point i. All the points in the dataset are guaranteed to be distinct.
Since we will be computing distances between points, we want to make sure each dimension contributes the roughly the same. For example, if one dimension was measured in millimeters and
another in meters, distances could end up being dominated by the first dimension, being orders
of magnitude times the scale of the second. We therefore apply the following two steps to each
dimension j = 1 . . . d:
1. Translate the data so that it has zero mean: P
i
xi(j) = 0.
2. Then rescale the data so it has unit variance: P
i
xi(j)
2 = 1.
1
These two steps will ensure that the data has roughly the same scale in each dimension, so all
dimensions will figure in equally to our distance calculations.
After rescaling the data, you should build the data into a kd-tree. When deciding where to split in
each dimension, feel free to choose a random point as the splitting point (ideally, we would use the
median, but random choices should also lead to reasonable performance). Be sure to be consistent
about how ties are broken; for example, if you split on a point with coordinate value v, then you
may want to adopt the convention that points with values less than or equal to v should go in the
left subtree.
3 Classification
To see how well nearest neighbor classification works, we will use “leave one out” testing, where we
guess the label of each point by temporarily pretending that it is absent from the data set. If we
are using nearest neighbor classification, then we would guess that each point should be labeled the
same as its nearest neighbor (other than itself). The choice of how we compute distance is often
an important consideration in nearest neighbor clustering; for simplicity, we will use the standard
Euclidean distance in this assignment, where the distance between points xi and xj is given by
sX
k
[xi(k) − xj (k)]2
.
If we use k-nearest neighbor classification (discussed in lecture), we will guess that the label of a
data point should be a weighted average of the labels of its k nearest points (not including itself,
again, of course). Mathematically, a weighted average of values v1 . . . vn using weights w1 . . . wn is
given by
P
P
i wivi
i wi
.
When computing this weighted average, we should assign higher weight to closer neighbors. For
example, we can set the weight of a neighbor to e
−αd, where d is the distance to the neighbor, and
α is a parameter of our choosing, selected to make our final results as good as possible.
4 Running and Validating your Code
Your program should take two parameters on the command line: the name of an input file (e.g.,
wine.txt, and a value of k (up to 10). It should then perform k-nearest neighbor classification
using a kd-tree on each of the n data points, and it should print out the average squared error of
the final classification. That is, if ai
is the actual label of data point i, and gi
is our guess at the
label of data point i, then you should output
1
n
X
i
(ai − gi)
2
.
To assess whether this is a good result, you may wish to compare it to what you would get if you
simply guessed randomly – here, the optimal choice that minimizes the squared output error is to
guess that each gi
is just equal to the average of the ai
’s. Does your classifier have a smaller error
2
than this? Does the value of k have a large impact on this error? You do not need to include
answers to these questions in your submission, but you will certainly be interested in knowing the
answers to these questions in any case.
Note that the wine data set is sufficiently small that you should be able to check that your code is
working properly by finding nearest neighbors using the kd-tree and also finding the same nearest
neighbors by “brute force”, simply by sorting all the other points according to distance. This can
be very helpful for debugging your code.
5 Submission and Grading
Please name your submitted file wine.cpp.
Your final grade will be out of 20 points, as with the previous homework assignments. Approximately 15 points will be given for correctness and efficiency, with roughly 5 points given for clarity
of code. Programs that do not compile on the lab machines will receive zero points, so please be
sure to check that your code compiles properly.
Final submissions are due by 11:59pm on the evening of Friday, November 21. No late submissions
will be accepted.
References
[1] P. Cortez, A. Cerdeira, F. Almeida, T. Matos and J. Reis, Modeling wine preferences by data
mining from physicochemical properties. In Decision Support Systems 47(4):547-553, 2009
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