How to write a confusion matrix in Python?

Question

I wrote a confusion matrix calculation code in Python:

def conf_mat(prob_arr, input_arr):
        # confusion matrix
        conf_arr = [[0, 0], [0, 0]]

            


        

Answer 1

If you don't want scikit-learn to do the work for you...

    import numpy
    actual = numpy.array(actual)
    predicted = numpy.array(predicted)

    # calculate the confusion matrix; labels is numpy array of classification labels
    cm = numpy.zeros((len(labels), len(labels)))
    for a, p in zip(actual, predicted):
        cm[a][p] += 1

    # also get the accuracy easily with numpy
    accuracy = (actual == predicted).sum() / float(len(actual))

Or take a look at a more complete implementation here in NLTK.