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<!DOCTYPE html>
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<head>
<title>MAT381E-Week 7: Handling factors and dates in data</title>
<meta charset="utf-8" />
<meta name="author" content="GΓΌl Δ°nan" />
<meta name="date" content="2021-11-14" />
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class: left, middle, my-title, title-slide
# MAT381E-Week 7: Handling factors and dates in data
### GΓΌl Δ°nan
### Department of Mathematics<br/>Istanbul Technical University
### November 14, 2021
---
class: left
# Outline
* Introduction to `lubridate` package.
*01-lubridate_casestudy.Rmd
---
# Overview
- Up to now, we have dealt with: numeric, character, logical, and factor type of vectors.
- In data science projects, we may encounter variables that are **dates** and **times**.
- **Dates** and **times** can be used to quantify when exactly a certain event occurs and can help us understand chronological relationships.
---
class: middle, center
#### What do you see in this graphic?
<img src="images/gtrend.png" width="80%" height="100%" />
---
#### Date-time functions/classes
- Luckily, `R` provides several options for dealing with **date** and **date/time** data.
- Just as numbers and strings have their own class in `R`, **date** and **date/time** objects have their own class too.
- In base `R`, the built-in `as.Date()` function with `Date` class handles dates (without times); and the `as.POSIXct()` function with `POSIXct` class and `as.POSIXlt()` function with `POSIXlt` class allow for dates and times with control for **time zones**.
|Type |Function |Class | Description |
|:--------------|:-------------|:---------|:----------------------------------------------------|
|date |`as.Date()` |`Date` | represent calendar dates. |
|date-time |`as.POSIXct()`|`POSIXct` | stores seconds since epoch (since `January 1, 1970`).|
|date-time |`as.POSIXlt()`|`POSIXlt` | stores a list of day, month, year, hour, minute, second, etc.|
- Note: POSIX stands for `Portable Operating System Interface` and it is a family of standards specified by the `IEEE Computer Society` for maintaining compatibility between different operating systems.
- The POSIX classes are especially useful when time zone manipulation is important.
- `ct` stands for calendar time.
---
- Computer languages usually use `January 1, 1970`
as the _epoch_ which is an origin (reference) day so that the dates can be converted to numbers. Dates older than the origin are stored as negative integers.
- Similarly, in `R`, except for the `POSIXlt` class, dates are stored internally as the number of days or seconds from `January 1, 1970`.
- The `POSIXlt` class stores date/time values as a list of components (day, month, year, etc.) making it easy to extract these parts.
- Unless you need the list nature of the `POSIXlt` class, the `POSIXct` class is the usual choice for storing dates in `R`.
---
- Let's get the current system date and time.
```r
# get system date
Sys.Date()
```
```
#> [1] "2021-11-14"
```
```r
# check the class
class(Sys.Date())
```
```
#> [1] "Date"
```
```r
# get the system time
Sys.time()
```
```
#> [1] "2021-11-14 18:25:35 +03"
```
```r
# check the class
class(Sys.time())
```
```
#> [1] "POSIXct" "POSIXt"
```
---
class: middle, center
<img src="images/date.jpeg" width="60%" />
---
<style type="text/css">
.pull-left {
float: left;
width: 50%;
}
.pull-right {
float: right;
width: 50%;
}
</style>
#### Date format
.pull-left[
- There are **many ways** to write a date and date formats vary **across the world**.
- Having **different formats** could be confusing for people that come from different countries to understand.
- The [ISO 8601](https://www.iso.org/iso-8601-date-and-time-format.html) recommends writing the date as year, then month, then the day: `YYYY-MM-DD` and uses the 24 hour clock system.
- This default format used in `R` is also the `ISO 8601 standard` for date/time.
```r
Sys.time()
```
```
#> [1] "2021-11-14 18:25:35 +03"
```
]
.pull-right[
<img src="images/iso.png" width="100%" height="100%" />
]
---
- We can create date objects through `as.Date()` function in base `R`.
```r
#check
class(2021-10-04)
```
```
#> [1] "numeric"
```
```r
class("2021-10-04")
```
```
#> [1] "character"
```
```r
#create a data object in base R
school_start_date <- as.Date("2021-10-04")
class(school_start_date)
```
```
#> [1] "Date"
```
```r
#number of days since 1970-01-01
unclass(school_start_date)
```
```
#> [1] 18904
```
```r
#How long has been since the school started?
Sys.Date()-school_start_date
```
```
#> Time difference of 41 days
```
---
- Many of the statistical summary functions, like mean, min, max, etc are able to transparently handle date objects.
```r
important_dates <- as.Date(c("2021-11-01","2021-10-01","2021-09-01"))
important_dates
```
```
#> [1] "2021-11-01" "2021-10-01" "2021-09-01"
```
```r
min(important_dates)
```
```
#> [1] "2021-09-01"
```
```r
max(important_dates)
```
```
#> [1] "2021-11-01"
```
- The `by=argument` to the `seq()` function can be specified in any units of time that the function accepts, making it very easy to generate sequences of dates.
```r
seq(from=as.Date('2021-10-04'), to=as.Date('2022-01-01'), by='10 days') #1 day, 10 days, 2 weeks, 3 months
```
```
#> [1] "2021-10-04" "2021-10-14" "2021-10-24" "2021-11-03" "2021-11-13"
#> [6] "2021-11-23" "2021-12-03" "2021-12-13" "2021-12-23"
```
- where unit of time in `by` argument can be: "secs", "mins", "hours", "days", "weeks", and "months".
---
- The `as.Date()` function allows a variety of **input formats** through the `format` input argument.
| Code | Value |
|:-----|:--------------|
| %d | Day of the month (decimal number)|
| %m | Month (decimal number)|
| %b | Month (abbreviated)|
| %B | Month (full name)|
| %y | Year (2 digit)|
| %Y | Year (4 digit)|
```r
#convert it into a data object with YYYY-MM-DD
#format argument tells the input structure, output is always in YYYY-MM-DD
as.Date('15-11-2021', format = '%d-%m-%Y')
```
```
#> [1] "2021-11-15"
```
```r
#convert it into a data object with YYYY-MM-DD
#format argument tells the input structure, output is always in YYYY-MM-DD
as.Date('November 15, 2021', format='%B %d, %Y')
```
```
#> [1] "2021-11-15"
```
---
- You can also change the format of the date into another date form through `format()` function.
```r
Sys.Date()
```
```
#> [1] "2021-11-14"
```
```r
format(Sys.Date(), '%B %d, %Y')
```
```
#> [1] "November 14, 2021"
```
- See YAML part of my slides!..
```r
---
title: "MAT381E-Week 7: Handling dates in data"
subtitle: ""
author: "GΓΌl Δ°nan"
institute: "Department of Mathematics<br/>Istanbul Technical University"
date: "`r format(Sys.Date(), '%B %d, %Y')`"
---
```
---
- The default input format for `POSIX` dates consists of the year, followed by the month and day, separated by dashes; for date/time values, the date may be followed by **white space** and a time in the form `hour:minutes:seconds` or `hour:minutes`; thus, the following are examples of valid `POSIX` date or date/time objects:
```r
#create POSIX objects
#no need format argument since it is already in YYYY-MM-DD format
release_date <- as.POSIXct("2019-12-12")
release_date
```
```
#> [1] "2019-12-12 +03"
```
```r
class(release_date)
```
```
#> [1] "POSIXct" "POSIXt"
```
```r
unclass(release_date)
```
```
#> [1] 1576098000
#> attr(,"tzone")
#> [1] ""
```
```r
#create POSIX objects
release_date <- as.POSIXct("2019-12-12 08:05:03")
class(release_date)
```
```
#> [1] "POSIXct" "POSIXt"
```
```r
release_date
```
```
#> [1] "2019-12-12 08:05:03 +03"
```
---
```r
#create POSIX objects
release_date <- as.POSIXlt("2019-12-12 08:05:03")
class(release_date)
```
```
#> [1] "POSIXlt" "POSIXt"
```
```r
release_date
```
```
#> [1] "2019-12-12 08:05:03 +03"
```
```r
release_date$hour #you can get the components
```
```
#> [1] 8
```
---
- Use:
- `Date` when there is no time component,
- `POSIXct` when dealing with time and time zones, and
- `POSIXlt` when you want to access/extract the different components.
---
#### Example: Google Trends results of Δ°TΓ
- I imported _Google Trend Search_ results for Δ°TΓ between January 1, 2021 and November 1, 2021 via [gtrendsR](https://cran.r-project.org/web/packages/gtrendsR/) package. Note that my query results are very limited and restricted to Turkey location only.
.panelset[
.panel[.panel-name[Data]
```r
google_trends <- readr::read_csv("data/google_trends_itu.csv")
head(google_trends, 3)
```
```
#> # A tibble: 3 Γ 2
#> date Δ°TΓ_TR
#> <date> <dbl>
#> 1 2021-01-03 39
#> 2 2021-01-10 42
#> 3 2021-01-17 39
```
- The variable `date` in `google.trends` data looks like a character. But, actually, it is `date`:
```r
library(tidyverse)
google_trends %>%
str()
```
```
#> spec_tbl_df [44 Γ 2] (S3: spec_tbl_df/tbl_df/tbl/data.frame)
#> $ date : Date[1:44], format: "2021-01-03" "2021-01-10" ...
#> $ Δ°TΓ_TR: num [1:44] 39 42 39 43 42 49 56 69 58 37 ...
#> - attr(*, "spec")=
#> .. cols(
#> .. date = col_date(format = ""),
#> .. Δ°TΓ_TR = col_double()
#> .. )
#> - attr(*, "problems")=<externalptr>
```
```r
class(google_trends$date)
```
```
#> [1] "Date"
```
]
.panel[.panel-name[Code]
```r
ggplot2::ggplot(data = google_trends) +
geom_line(aes(x = date, y = Δ°TΓ_TR)) +
scale_x_date(limits = as.Date(c("2021-01-01","2021-11-01")),
date_breaks = "1 month" )+
geom_vline(aes(xintercept = as.Date("2021-10-03")), color="red") +
ggtitle("Google Trends search results for Δ°TΓ in 2021") +
ylab("Number of Searches") +
xlab("Date") +
theme_bw()
```
]
.panel[.panel-name[Output]
<img src="lubridate_files/figure-html/unnamed-chunk-27-1.png" width="40%" />
]
]
---
- Plotting functions in `ggplot2` are aware of dates which means that, for example, a scatter plot can use the **numeric representation**
to decide on the position of the point, but include the **string in the labels** via `scale_*_date()` function:
.panelset[
.panel[.panel-name[Code]
```r
library(ggplot2)
ggplot(data = google_trends) +
geom_line(aes(x = date, y = Δ°TΓ_TR)) +
* scale_x_date(limits = as.Date(c("2021-01-01","2021-11-01")),
* date_breaks = "1 month", date_labels = "%b-%d") +
geom_vline(aes(xintercept = as.Date("2021-10-03")), color="red") +
ggtitle("Google Trends search results for Δ°TΓ in 2021") +
ylab("Number of Searches") +
xlab("Date") +
theme_bw()
#more: https://www.statworx.com/at/blog/customizing-time-and-date-scales-in-ggplot2/
```
]
.panel[.panel-name[Output]
<img src="lubridate_files/figure-html/unnamed-chunk-29-1.png" width="40%" />
]
]
---
class: middle, center
---
# lubridate package
- The `tidyverse ecosystem` includes [lubridate](https://lubridate.tidyverse.org/) package for dealing with **date-times** and **time-spans**.
- It is fast and user friendly helping with:
- Parsing of date-time data,
- Extraction and updating of components of a date-time, and
- Algebraic manipulation on date-time and time-span objects.
- First, to be able to use the functionality of `lubridate` we have to load the package:
```r
# The easiest way to get lubridate is to load the whole tidyverse:
# library(tidyverse) or
# Alternatively, load just lubridate:
library(lubridate)
```
---
#### Parsing dates and times (Converting strings or numbers to date-times)
- The `lubridate` package has a number of functions to convert **strings** to **date** and **date-time** objects.
- Here, we simply try to **match the characters to the pattern of the date-time** we are trying to parse.
- The goal is to end up with a date-time in the form: `YYYY-MM-DD HH:MM:SS`.
- For parsing the date, in `lubridate` we can use a combination of the letters βdβ, βmβ, βyβ (standing for day, month, year).
- Use the functions below whose name replicates the order.
|Function |Description |
|----------|-------------------------------------------|
|`ymd()` |converts characters into YYYY-MM-DD format.|
|`ydm()` |converts characters into YYYY-MM-DD format.|
|`mdy()` |converts characters into YYYY-MM-DD format.|
|`dmy()` |converts characters into YYYY-MM-DD format.|
|`hms()` |converts characters into HH:MM:SS format. |
|`hm()` |converts characters into HH:MM format. |
|`h()` |converts characters into HH format. |
---
```r
# When is International Women's day?
library(lubridate)
#pattern is year, month, day
ymd("20210308")
```
```
#> [1] "2021-03-08"
```
```r
#pattern is year, day, month
ydm("20210803")
```
```
#> [1] "2021-03-08"
```
```r
#pattern is month, day, year
mdy("03-08-2021")
```
```
#> [1] "2021-03-08"
```
```r
#pattern is day, month, year
dmy("08/03/2021")
```
```
#> [1] "2021-03-08"
```
```r
#all of them are converted to the YYYY-MM-DD format.
```
---
```r
#if we thing that these strings are in year-month-day format,
#then put all of them in a standard year-month-day format.
x <- c(20090101, "2009-01-02", "2009 01 03", "2009-1-4",
"2009-1, 5", "Created on 2009 1 6", "200901 !!! 07")
ymd(x)
```
```
#> [1] "2009-01-01" "2009-01-02" "2009-01-03" "2009-01-04" "2009-01-05"
#> [6] "2009-01-06" "2009-01-07"
```
---
- A very similar technique can be used to parse times:
```r
#there is no other functions such as smh() etc.
#other versions are hm(), h()
hms("20:30:10")
```
```
#> [1] "20H 30M 10S"
```
- Now if we come across a variable that includes a date and time we can join relevant functions together with an β_β to parse a date-time.
```r
ymd_hms("2006/03/19 20:30:10")
```
```
#> [1] "2006-03-19 20:30:10 UTC"
```
```r
ydm_hms("2006/19/03 20:30:10")
```
```
#> [1] "2006-03-19 20:30:10 UTC"
```
```r
mdy_hms("03/19/2006 20:30:10")
```
```
#> [1] "2006-03-19 20:30:10 UTC"
```
```r
dmy_hms("19/03/2006 20:30:10")
```
```
#> [1] "2006-03-19 20:30:10 UTC"
```
```r
#all of them are converted to the YYYY-MM-DD hh:mm:ss format.
```
- The phrase **Coordinated Universal Time (UTC)** in the output represents the time-zone and by default should be UTC, this stands for universal time coordinated.
---
#### GET components of date-times
- We can use helper functions to get a relevant component in `data-time` objects.
|Function |Description |
|-----------------|-------------------|
|`date()` |Date component |
|`year()` |Year component |
|`month(, label)` |Month component |
|`day()` |Day of month |
|`week()` |Week of the year |
|`wday(, label)` |Day of week |
|`yday()` |Day of year |
|`hour()` |Hour |
|`minute()` |Minute |
|`second()` |Second |
---
```r
womensday <- ymd("2021/03/08")
#replaces "/" with "-".
year(womensday)
```
```
#> [1] 2021
```
```r
month(womensday)
```
```
#> [1] 3
```
```r
week(womensday)
```
```
#> [1] 10
```
```r
#note that Sunday is accepted as the first day of the week
wday(womensday)
```
```
#> [1] 2
```
```r
wday(womensday, label=TRUE)
```
```
#> [1] Mon
#> Levels: Sun < Mon < Tue < Wed < Thu < Fri < Sat
```
```r
yday(womensday)
```
```
#> [1] 67
```
---
#### SET components of date-time
- `lubridate` can be used to not only extract but also **change** parts of date-time objects.
```r
month(womensday) <- 12
womensday
second(womensday) <- 99
womensday
# Use update function to change multiple values
womensday_up <- update(womensday, year = 1976, day = 16, month = 6, hour = 8)
womensday_up
```
---
- In `R`, we can get the current time by typing `Sys.time()`.
```r
Sys.time()
```
```
#> [1] "2021-11-14 18:25:39 +03"
```
```r
class(Sys.time())
```
```
#> [1] "POSIXct" "POSIXt"
```
- The `lubridate` package provides a slightly more advanced function, `now`, that permits you define the time zone:
```r
now()
```
```
#> [1] "2021-11-14 18:25:39 +03"
```
```r
#time in Turkey now
now("Turkey")
```
```
#> [1] "2021-11-14 18:25:39 +03"
```
```r
#time in US-Eastern cost now
now("US/Eastern")
```
```
#> [1] "2021-11-14 10:25:39 EST"
```
- You can see all the available times zones with the `OlsonNames()` function.
---
- The `lubridate` also has a function to extract hours, minutes and seconds:
```r
now() %>% hour()
```
```
#> [1] 18
```
```r
now() %>% minute()
```
```
#> [1] 25
```
```r
now() %>% second()
```
```
#> [1] 39.20986
```
---
- **Hands-on example:** Visit `01-lubridate_casestudy.Rmd` file.
<img src="images/garmin.png" width="80%" height="100%" />
---
class: middle, center
```
#> ππππππππππππππππππππππππππππππ
```
---
- More at:
- [Scale_date](https://ggplot2.tidyverse.org/reference/scale_date.html)
- [Lubridate](https://rstudio-pubs-static.s3.amazonaws.com/422499_dcc2ed60c639458ba349b21e22059ac9.html#chapter-2---parsing-dates-with-lubridate).
- [Dates and Times in R](https://www.stat.berkeley.edu/~s133/dates.html#:~:text=Dates%20stored%20in%20the%20POSIX,more%20accurate%20representation%20of%20times).
- [Missing Dates](https://blog.exploratory.io/populating-missing-dates-with-complete-and-fill-functions-in-r-and-exploratory-79f2a321e6b5).
- [Lubridate](https://wrangle-r.rsquaredacademy.com/lubridate.html).
---
# Attributions
- All images used in this slide are taken from the web.
- This lecture note is mainly developed by following sources:
- [Source 1](https://github.com/datasciencelabs/2020/blob/master/03_wrangling/05_dates-and-times.Rmd),
- [Source 2](https://eddatascienceees.github.io/tutorial-sian-phillips/), and
- [Source 3](https://github.com/MeganBeckett/lubridate-heatmaps_tutorial/blob/master/code/lubridate_tutorial.Rmd).
</textarea>
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