TimeSeries supports merging two TimeArrays, and squishing the timestamp
to a longer-term interval while representing values that make sense.
The merge method performs joins between two TimeArrays. The default
behaviour is to perform an inner join, such that the resulting TimeArray
contains only timestamps that both TimeArrays share, and values that
correspond to that timestamp.
The AAPL object from MarketData has 8,336 rows of data from Dec 12, 1980
to Dec 31, 2013. If we merge it with the CAT object, which contains
13,090 rows of data from Jan 2, 1962 to Dec 31, 2013 we might expect the
resulting TimeArray to have 8,336 rows of data, corresponding to the
length of AAPL. This assumes that every day that Apple Computer, Inc.
traded, Caterpillar, Inc likewise traded. It turns out that this isn't
true. CAT did not trade on Sep 27, 1985 because Hurricane Glorio shut
down the New York Stock Exchange. Apple Computer trades on the electronic
NASDAQ and its trading was not halted on that day. The result of the
merge should then be 8,335 rows:
using TimeSeries
using MarketData
AppleCat = merge(AAPL, CAT);
length(AppleCat)
Left, right, and outer joins can also be performed by passing the
corresponding symbol. These joins introduce NaN values when data for a
particular timestamp only exists in one of the series to be merged. For
example:
merge(op[1:3], cl[2:4]; method=:left)
merge(op[1:3], cl[2:4]; method=:right)
merge(op[1:3], cl[2:4]; method=:outer)
The merge method allows users to specify the value for the meta
field of the merged object. When that value is not explicitly provided,
merge will concatenate the meta field values, assuming these values
to be strings. This covers the vast majority of use cases. In edge cases
when users do not provide a meta value and both field values are not
strings, the merged object will take on Void as its meta field
value:
meta(AppleCat)
CatApple = merge(CAT, AAPL; meta=47);
meta(CatApple)
meta(merge(AppleCat, CatApple))
merge
The collapse method allows for compressing data into a larger time
frame. For example, converting daily data into monthly data. When
compressing dates, something rational has to be done with the values
that lived in the more granular time frame. To define what happens, a
function call is made. In our example, we want to compress the daily
cl closing prices from daily to monthly. It makes sense for us to take
the last value known and have that represented with the corresponding
timestamp. A non-exhaustive list of valid time methods is presented
below.
Dates method |
Time length |
|---|---|
Dates.quarter |
quarterly |
day |
daily |
week |
weekly, starting from Monday. |
month |
monthly |
year |
yearly |
hour |
hourly |
minute |
minutely |
second |
secondly |
Showing this code in REPL:
using TimeSeries
using MarketData
collapse(cl, month, last)
We can also supply the function that chooses the timestamp and the function that determines the corresponding value independently:
using Statistics
collapse(cl, month, last, mean)
collapse
The vcat method is used to concatenate time series: if you have two
time series with the same columns, this function can merge them into a single object.
Notably, it can be used to merge data that is split into multiple files.
Its behaviour is quite different from merge,
which does not consider that its arguments are actually the same time series.
This concatenation is vertical (vcat) because it does not create
columns, it extends existing ones (which are represented vertically).
vcat(tas::Vararg{TimeArray,N} where N)
This function allows complete transformation of the data within the time
series, with alteration on both the time stamps and the associated
values. It works exactly like Base.map: the first argument is a binary
function (the time stamp and the values) that returns two values,
respectively the new time stamp and the new vector of values. It does
not perform any kind of compression like collapse, but rather
transformations.
The simplest example is to postpone all time stamps in the given time series, here by one year:
using TimeSeries
using Dates
ta = TimeArray([Date(2015, 10, 01), Date(2015, 11, 01)], [15, 16])
map((timestamp, values) -> (timestamp + Year(1), values), ta)