Iterate over dates range (the scala way)

Question

Given a start and an end date I would like to iterate on it by day using a foreach, map or similar function. Something like

(DateTime.now to DateTime.now + 5.day         


        

Answer 1

Solution with java.time API using Scala

Necessary import and initialization

import java.time.temporal.ChronoUnit
import java.time.temporal.ChronoField.EPOCH_DAY
import java.time.{LocalDate, Period}

val now = LocalDate.now
val daysTill = 5

Create List of LocalDate for sample duration

(0 to daysTill)
  .map(days => now.plusDays(days))
  .foreach(println)

Iterate over specific dates between start and end using toEpochDay or getLong(ChronoField.EPOCH_DAY)

//Extract the duration
val endDay = now.plusDays(daysTill)
val startDay = now

val duration = endDay.getLong(EPOCH_DAY) - startDay.getLong(EPOCH_DAY)

/* This code does not give desired results as trudolf pointed
val duration = Period
  .between(now, now.plusDays(daysTill))
  .get(ChronoUnit.DAYS)
*/

//Create list for the duration
(0 to duration)
  .map(days => now.plusDays(days))
  .foreach(println)

Answer 2

you can use something like that:

 object Test extends App {
   private val startDate: DateTime = DateTime.now()
   private val endDate: DateTime = DateTime.now().plusDays(5)
   private val interval: Interval = new Interval(startDate, endDate)
   Stream.from(0,1)
         .takeWhile(index => interval.contains(startDate.plusDays(index)))
         .foreach(index => println(startDate.plusDays(index)))
 }

Answer 3

For just iterating by day, I do:

Iterator.iterate(start) { _ + 1.day }.takeWhile(_.isBefore(end))

This has proven to be useful enough that I have a small helper object to provide an implicit and allow for a type transformation:

object IntervalIterators {
  implicit class ImplicitIterator(val interval: Interval) extends AnyVal {
    def iterateBy(step: Period): Iterator[DateTime] = Iterator.iterate(interval.start) { _ + step }
        .takeWhile(_.isBefore(interval.end))

    def iterateBy[A](step: Period, transform: DateTime => A): Iterator[A] = iterateBy(step).map(transform)

    def iterateByDay: Iterator[LocalDate] = iterateBy(1.day, { _.toLocalDate })

    def iterateByHour: Iterator[DateTime] = iterateBy(1.hour)
  }
}

Sample usage:

import IntervalIterators._

(DateTime.now to 5.day.from(DateTime.now)).iterateByDay // Iterator[LocalDate]

(30.minutes.ago to 1.hour.from(DateTime.now)).iterateBy(1.second)  // Iterator[DateTime], broken down by second

Answer 4

This answer fixes the issue of mrsrinivas answer, that .get(ChronoUnits.DAYS) returns only the days part of the duration, and not the total number of days.

Necessary import and initialization

import java.time.temporal.ChronoUnit
import java.time.{LocalDate, Period}

Note how above answer would lead to wrong result (total number of days is 117)

scala> Period.between(start, end)
res6: java.time.Period = P3M26D

scala> Period.between(start, end).get(ChronoUnit.DAYS)
res7: Long = 26

Iterate over specific dates between start and end

val start = LocalDate.of(2018, 1, 5)
val end   = LocalDate.of(2018, 5, 1)

// Create List of `LocalDate` for the period between start and end date

val dates: IndexedSeq[LocalDate] = (0L to (end.toEpochDay - start.toEpochDay))
  .map(days => start.plusDays(days))

dates.foreach(println)

Answer 5

In this case, the Scala way is the Java way:

When running Scala on Java 9+, we can use java.time.LocalDate::datesUntil:

import java.time.LocalDate
import collection.JavaConverters._

// val start = LocalDate.of(2019, 1, 29)
// val end   = LocalDate.of(2018, 2,  2)
start.datesUntil(end).iterator.asScala
// Iterator[java.time.LocalDate] = <iterator> (2019-01-29, 2019-01-30, 2019-01-31, 2019-02-01)

And if the last date is to be included:

start.datesUntil(end.plusDays(1)).iterator.asScala
// 2019-01-29, 2019-01-30, 2019-01-31, 2019-02-01, 2019-02-02

Answer 6

import java.util.{Calendar, Date}
import scala.annotation.tailrec

/** Gets date list between two dates
  *
  * @param startDate  Start date
  * @param endDate    End date
  * @return           List of dates from startDate to endDate
  */
def getDateRange(startDate: Date, endDate: Date): List[Date] = {
  @tailrec
  def addDate(acc: List[Date], startDate: Date, endDate: Date): List[Date] = {
    if (startDate.after(endDate)) acc
    else addDate(endDate :: acc, startDate, addDays(endDate, -1))
  }

  addDate(List(), startDate, endDate)
}

/** Adds a date offset to the given date
  *
  * @param date       ==> Date
  * @param amount     ==> Offset (can be negative)
  * @return           ==> New date
  */
def addDays(date: Date, amount: Int): Date = {
  val cal = Calendar.getInstance()
  cal.setTime(date)
  cal.add(Calendar.DATE, amount)
  cal.getTime
}