Friday, November 28, 2014

Minimize complexity by Spark

There is always a trade-off between time complexity and space complexity for computer programs. Deceasing the time cost will increase space cost, and vice versa, The ideal solution to parallelize the program to multiple cores if there is a multiple-core computer, or even scale it out to multiple machines across a cluster, which would eventually reduce both time complexity and space complexity.
Spark is currently the hottest platform for cluster computing on top of Hadoop, and its Python interface provides map, reduce and many other methods, which allow a mapRecdue job in a straightforward way, and therefore easily migrate an algorithm from a single machine to a cluster of many machines.
  • Minimize space complexity
There is a question to look for the only single number from a mostly paired-number array.
Single Number
  Given an array of integers, every element appears twice except for one.
    Find that single one.
    Note:
    Your algorithm should have a linear runtime complexity.
    Could you implement it without using extra memory? 
The optimal space complexity for this question is O(1) by using the bit manipulator xor. For a cluster, since Spark aggregates memory acrosss machines, the space complexity may become O(1/k), where k is the number of the machines in the cluster.
# Space.py
import pyspark
from random import shuffle
from operator import xor
sc = pyspark.Spark.Context()

# Create the test case and the target is 99
testCase = range(0, 99) + range(0, 100)
shuffle(testCase)
# Run the testing with Spark
result = sc.parallelize(testCase).reduce(xor)
# Show the result
print result
sc.stop()
  • Minimize time complexity
There is a question to implement the function (or a method) that returns the square root of an integer.
Sqrt(x)
Implement int sqrt(int x).
Compute and return the square root of x.
The optimal solution could achieve the time complexity of O(lgN) by using binary search. If we pass the sqrt function to Spark, then the time complexity will decreased to O(lgN/k), where k is the number of the machines in the cluster.
# Time.py
import pyspark
sc = pyspark.Spark.Context()
# Implement binary search for square root function
def sqrt(x):
    if x < 0 or not isinstance(x, int):
        raise ValueError
    hi, lo = x/2 + 1, 0
    while hi >= lo:
        mid = (hi + lo) / 2
        if mid * mid > x:
            hi = mid - 1
        else:
            lo = mid + 1
    return int(hi)

# Test the square root algorithm
testCase = sc.parallelize(xrange(1, 100))
result = testCase.map(lambda x: sqrt(x))
# Show the result
for x in result.collect():
    print x
sc.stop()
  • Find the worst rating by accounts
There is a question to find the worst one among a few rating letters for each of the account numbers.
Want to find the worst rating for each account number.
sample.txt is below
Account_number    Rating
1            A
1            B
2            A
2            B
2            C
3            A
3            C
the desired result should be like
1            B
2            C
3            C
The question is essentially one of the grouping questons. Spark’s pair RDD, which reflects the key-value relationship for groups, supplies a one-line solution for it.
import pyspark
sc = pyspark.SparkContext()

# Squeeze the letters by keys
rdd = sc.textFile('sample.txt')
result = rdd.map(lambda x: x.split()).filter(x: x[0].isdigit()).reduceByKey(max) 
# Show the result
for x in result.collect(): 
    print x
sc.stop()
In a conclusion, Spark significantly changes the way we think about data analysis.

Good math, bad engineering

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