In Scala how to compare current values with previous values while processing a stream? -

i new scala not sure how approach problem ? trying find moving average crossovers stream of quotes. not sure how previous values compare them current values ?

if ( fastmovingaverage(n-1) > slowmovingaverage(n-1) && fastmovingaverage(n) < slowmovingaverage(n) ) action    package com.example.csv  import scala.io.source   object fileparser {   val timestamp_location = 3   val bid_location = 4   val offer_location = 5   val fast_window_size = 5   val slow_window_size = 10    def main(args: array[string]) = {     val records = source.fromfile("sample.csv")     .getlines()     .drop(1)     .map(_.split(","))     .takewhile( _ != null)     .sliding(slow_window_size , 1)     .foreach(x => movingaverage(x))   }    def movingaverage(numbers: seq[array[string]]) = {     val listofbids = numbers.map(x => x(bid_location).todouble)     val slowaverage = listofbids.reduceleft(_ + _)/numbers.length     val fastlistofbids = listofbids.takeright(fast_window_size)     val fastaverage = fastlistofbids.reduceleft(_ + _)/fastlistofbids.length     println("slow average " + slowaverage + " fast average " + fastaverage)   }  } 

the short answer use zip operation on slowaverage , fastaverage combine lists , find difference of zipped elements. when difference changes negative value positive value indicates fast average has crossed above(greater) slow average.

here's data used , longer example:

price      fast average(2)  slow average(4) diff 9            8           8.5      7           7.5      6           6.5             7.5              -1 5           5.5             6.5              -1 5           5               5.75             -0.75 6           5.5             5.5               0 7           6.5             5.75              0.75 8           7.5             6.5               1 9           8.5             7.5               1 

google docs link: https://docs.google.com/spreadsheet/ccc?key=0alfb-wgy-ztddhdwu2sts0u5zuxtn2cwdwfoewnpzfe&usp=sharing

most recent price last.

let's see in scala:

scala> val prices = list(9,8,7,6,5,5,6,7,8,9) prices: list[int] = list(9, 8, 7, 6, 5, 5, 6, 7, 8, 9)  scala> val fastaverage = prices.sliding(2).tolist.map(xs => xs.sum / 2.0) fastaverage: list[double] = list(8.5, 7.5, 6.5, 5.5, 5.0, 5.5, 6.5, 7.5, 8.5)  scala> val slowaverage = prices.sliding(4).tolist.map(xs => xs.sum / 4.0) slowaverage: list[double] = list(7.5, 6.5, 5.75, 5.5, 5.75, 6.5, 7.5) 

zip fastaverage , slowaverage since different sizes take last 7 of fastaverage takeright.

scala> val zipped = fastaverage.takeright(7) zip slowaverage zipped: list[(double, double)] = list((6.5,7.5), (5.5,6.5), (5.0,5.75), (5.5,5.5), (6.5,5.75), (7.5,6.5), (8.5,7.5)) 

take difference of zipped averages. change negative positive(>=0) indicates fast average greater slow average aka bullish moving average crossover.

scala> zipped.map(x => x._1 - x._2) res44: list[double] = list(-1.0, -1.0, -0.75, 0.0, 0.75, 1.0, 1.0)