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COMP9517 Lab 3 solved

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The goal of this lab is to become familiar with the training/testing process for pattern
recognition (week 4 lectures). A simple k-nearest neighbour (kNN) algorithm is to be
developed. Background information is provided AFTER all the questions.
Submission instruction will be posted prior to the lab.

Preliminaries

The following experiments will be based on a provided dataset of handwritten digits (500
samples of each digit 0 to 9). This dataset was derived from the ‘digits.png’ file included with
OpenCV installations. This dataset is designed to test classification algorithms.

Some Python libraries you might find useful for this lab include scikit-learn, numpy and
opencv. For reading images from disk, the os and glob libraries will be useful.
It is recommended that you use the “KNeighborsClassifier” class from scikit-learn instead of
the kNN classifier from opencv, because scikit-learn functions give you more control and you
can set different parameters to optimise the kNN classifier.

The “metrics” module from scikit-learn is useful for generating the confusion matrix.
You are free to use other programming languages that you prefer, such as Matlab.

Lab Tasks (1 mark)

Develop a program to perform handwritten digit recognition using the kNN classifier. The
program should contain the following steps:
 import packages
 read the dataset (images and class labels)
 split images dataset into 80% training and 20% test sets
 initialize kNN model (use k=5)

 fit the KNN model using the training data (i.e. construct a search tree)
 perform handwritten digit recognition using the test data
 evaluate the recognition performance by calculating accuracy, confusion matrix, and
precision and recall for each digit class
Submit the results (accuracy, confusion matrix, per-class precision and recall) in a pdf file
and code for marking.

Extra Tasks (not assessed)

Experiment with different k values and different parameters in KNeighborsClassifier from
scikit-learn, and see their effects on the recognition accuracy and efficiency.
Refer to the scikit-learn documentation for available parameters:
https://scikitlearn.org/stable/modules/generated/sklearn.neighbors.KNeighborsClassifier.html

Background
K – Nearest Neighbours

The KNN algorithm is very simple and very effective. The model representation for KNN is
the entire training dataset. Predictions are made for a new data point by searching through
the entire training set for the K most similar instances (the neighbours) and summarizing the
output variable for those K instances.

For regression problems, this might be the mean
output variable, for classification problems this might be the mode (or most common) class
value. The trick is in how to determine the similarity between the data instances.

Similarity

In order to make predictions we need to calculate the similarity between any two given data
instances. This is needed so that we can locate the k most similar data instances in the
training dataset for a given member of the test dataset and in turn make a prediction.

Given that all four flower measurements are numeric and have the same units, we can
directly use the Euclidean distance measure. This is defined as the square root of the sum of
the squared differences between the two arrays of numbers (read that again a few times
and let it sink in).

References:

https://en.wikipedia.org/wiki/K-nearest_neighbors_algorithm
https://opencv-pythontutroals.readthedocs.io/en/latest/py_tutorials/py_ml/py_knn/py_knn_index.html