A New Week!

We aren’t computer scientists and that’s okay!

We make lots of mistakes. Mistakes are funny. You can laugh with us.

Let’s go, Simba, Pumbaa, and Timon!

1 More data type: date-time

It’s about time! Lubridate makes working with dates much easier.

We can find how much time has elapsed, add or subtract days, and find seasonal and day of a week averages.

1.1 Load the lubridate package

library(lubridate)

1.2 Convert character to a date

#Conver Month-Day-Year
x <- "08-18-2020" # 08/18/2020

x_date <- mdy(x)

#Conver Day-Month-Year
x <- "18-09-2020"

x_date <- dmy(x)

#convert Year-Month-day, guess which function to use?

# convert Year-Month-day Hour:Minutes

x <- "2020-08-24 8:35 AM"

x_date <- ymd_hm(x)

# convert Year-Month-day Hour:Minutes:Seconds

x <- "2020-08-24 8:35:22 AM"

x_date <- ymd_hms(x)

1.3 Make date

Sometimes we’ll have years, months, days speaded across multiple columns. We can use make_date or make_datetime to create date or datetime.

library(tidyverse)

library(janitor)

aq <- airquality %>% 
  mutate(year = 1973) %>% 
  clean_names()%>% # call clean_names from the janitor package 
  rename(solar = solar_r)  # use rename function to modify the column name

head(aq)

aq <- aq %>% 
  mutate(date = make_date(year, month, day))

# with the date column, we can visualize ozone concentration across May - Sep

aq %>% 
  ggplot(aes(x=date, y=ozone)) +
  geom_point() +
  geom_line()

1.4 Get date parts

# get year of a date object

year(x_date)

#get month

month(x_date)

#day of the month

mday(x_date)

#week of the year

week(x_date)

#day of the week as 1, 2,3 or Sunday, Monday, Tue

wday(x_date, label = T)

2 Join tables

2.1 left_join

left_join(scrap, convert, by = c(“columns to join by”))

left_join() works like a zipper to combine 2 tables based on one or more variables. It’s called “left”-join because the entire table on the left side is retained.

Anything that matches from the right table gets to join the party, but any rows that don’t have a matching ID will be ignored.

Remember our porg friends? We haven’t share their names.

The joined result:

band_members

band_instruments

band_members %>% 
  left_join(band_instruments, by = "name")

# when the join vaiables have different names

band_instruments2

band_members %>% 
  left_join(band_instruments2, by = c("name" = "artist" ))

2.2 Other joins

right_join(): includes all rows in right table.

inner_join(): includes all rows in both tables.

full_join(): includes all rows in left or right table.

band_members %>% 
  inner_join(band_instruments, by = "name")

band_members %>% 
  full_join(band_instruments, by = "name")

Exercise

Install and load the nycflights13 package.

Create a table that includes all flights and weather of their destination airports.

3 Recap

date-time
join tables