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import os
import random

def estimate_density(DATA_PATH, feature_size):
    """sample 10 times of a size of 1000 for estimating the density of the sparse dataset"""
    if not os.path.exists(DATA_PATH):
        raise Exception("Data is not there!")
    density = []
    P = 0.01
    for _ in xrange(10):
        num_non_zero = 0
        num_sample = 0
        with open(DATA_PATH) as f:
            for line in f:
                if (random.random() < P):
                    num_non_zero += len(line.split(" ")) - 1
                    num_sample += 1
        density.append(num_non_zero * 1.0 / (feature_size * num_sample))
    return sum(density) / len(density)