Data wrangling - II

Lecture 4

Dr. Greg Chism

University of Arizona
INFO 526 - Fall 2024

Announcements

  • Discussion: 1 project instead of 2.

  • RQ 2 due Wednesday (Sept 25):

    • Should already be live

    • Covers everything since the previous quiz

  • Make sure to review the feedback on your HW 01 repo and close the issue once you’re done reviewing.

  • Once you review, close the issue

  • HW 02 won’t be graded for those who have open issues from the previously assigned homework assignment.

HW 1 lessons learned

Highlights

  • Review HW 1 issues, and show us you reviewed them by closing the issue.

  • DO NOT hard code paths! Use the here package to help with relative paths, if you need.

  • Make sure that all packages are installed/loaded. I suggest the pacman workflow we saw before:

if (!require("pacman")) 
  install.packages("pacman")

pacman::p_load(package1, package2, ...)
pacman::p_load_gh("GitHubPackage1")

HW 1 lessons learned cont…

  1. Start early. No late work exceptions will be made in the future
  2. Ask your peers. I have been monopolized by a few individuals, which is not fair to the rest.
    1. Peers will likely have the answer
    2. Peers will likely get to the question before I will.
  3. Ask descriptive questions. See this page on asking effective questions.
  4. Please respect my work hours. I will no longer reply to messages after 5pm on work days and at all on weekends.

Setup

# load packages
if(!require(pacman))
  install.packages("pacman")

pacman::p_load(countdown,
               tidyverse,
               glue,
               scales,
               ggthemes)

# set theme for ggplot2
ggplot2::theme_set(ggplot2::theme_minimal(base_size = 14))

# set width of code output
options(width = 65)

# set figure parameters for knitr
knitr::opts_chunk$set(
  fig.width = 7,        # 7" width
  fig.asp = 0.618,      # the golden ratio
  fig.retina = 3,       # dpi multiplier for displaying HTML output on retina
  fig.align = "center", # center align figures
  dpi = 300             # higher dpi, sharper image
)

Transforming and reshaping a single data frame (cont.)

Data: Hotel bookings

  • Data from two hotels: one resort and one city hotel
  • Observations: Each row represents a hotel booking
hotels <- read_csv("https://raw.githubusercontent.com/rfordatascience/tidytuesday/master/data/2020/2020-02-11/hotels.csv")

Scenario 1

We…

have a single data frame

want to slice it, and dice it, and juice it, and process it, so we can plot it

Monthly bookings

Come up with a plan for making the following visualization and write the pseudocode.

Livecoding

Reveal below for code developed during live coding session.

Code
#| label: monthly-bookings
#| fig-asp: 0.5
#| echo: false

hotels <- hotels |>
  mutate(
    arrival_date_month = fct_relevel(arrival_date_month, month.name),
    season = case_when(
      arrival_date_month %in% c("December", "January", "February") ~ "Winter",
      arrival_date_month %in% c("March", "April", "May") ~ "Spring",
      arrival_date_month %in% c("June", "July", "August") ~ "Summer",
      TRUE ~ "Fall"
    ),
    season = fct_relevel(season, "Winter", "Spring", "Summer", "Fall")
  )

hotels |>
  count(season, hotel, arrival_date_month) |>
  ggplot(aes(x = arrival_date_month, y = n, group = hotel, linetype = hotel)) +
  geom_line(linewidth = 0.8, color = "cornsilk4") +
  geom_point(aes(shape = season, color = season), size = 4, show.legend = FALSE) +
  scale_x_discrete(labels = month.abb) +
  scale_color_colorblind() +
  scale_shape_manual(values = c("circle", "square", "diamond", "triangle")) +
  labs(
    x = "Arrival month", y = "Number of bookings", linetype = NULL,
    title = "Number of monthly bookings",
    subtitle = "July 2015 to August 2017",
    caption = "Source: Antonio, Almeida and Nunes (2019) | TidyTuesday"
  ) +
  coord_cartesian(clip = "off") +
  theme(
    legend.position = c(0.12, 0.9),
    legend.box.background = element_rect(fill = "white", color = "white"),
    plot.subtitle = element_text(color = "cornsilk4"),
    plot.caption = element_text(color = "cornsilk4")
  )

A few takeaways

  • forcats::fct_relevel() in a mutate() is useful for custom ordering of levels of a factor variable

  • summarize() after group_by() with multiple variables results in a message about the grouping structure of the resulting data frame – the message can be supressed by defining .groups (e.g., .groups = "drop" or .groups = "keep")

  • summarize() also lets you get away with being sloppy and not naming your new column, but that’s not recommended!

Rowwise operations

We want to calculate the total number of guests for each booking. Why does the following not work?

hotels |>
  select(adults, children, babies) |>
  mutate(guests = sum(c(adults, children, babies)))
# A tibble: 119,390 × 4
   adults children babies guests
    <dbl>    <dbl>  <dbl>  <dbl>
 1      2        0      0     NA
 2      2        0      0     NA
 3      1        0      0     NA
 4      1        0      0     NA
 5      2        0      0     NA
 6      2        0      0     NA
 7      2        0      0     NA
 8      2        0      0     NA
 9      2        0      0     NA
10      2        0      0     NA
# ℹ 119,380 more rows

Rowwise operations

hotels |>
  select(adults, children, babies) |>
  rowwise() |> 
  mutate(guests = sum(c(adults, children, babies))) |>
  filter(adults > 0, children > 0, babies > 0) # to show sum works
# A tibble: 172 × 4
# Rowwise: 
   adults children babies guests
    <dbl>    <dbl>  <dbl>  <dbl>
 1      2        1      1      4
 2      2        1      1      4
 3      2        1      1      4
 4      2        1      1      4
 5      2        1      1      4
 6      2        1      1      4
 7      2        1      1      4
 8      2        2      1      5
 9      2        2      1      5
10      1        2      1      4
# ℹ 162 more rows

Columnwise operations

Use across() combined with summarise() to calculate summary statistics for multiple columns at once:

hotels |>
  summarise(across(.cols = starts_with("stays"),  mean))
# A tibble: 1 × 2
  stays_in_weekend_nights stays_in_week_nights
                    <dbl>                <dbl>
1                   0.928                 2.50
hotels |>
  summarise(across(.cols = starts_with("stays"),  list(mean, sd)))
# A tibble: 1 × 4
  stays_in_weekend_nights_1 stays_in_weekend_nights_2
                      <dbl>                     <dbl>
1                     0.928                     0.999
# ℹ 2 more variables: stays_in_week_nights_1 <dbl>,
#   stays_in_week_nights_2 <dbl>

Select helpers

  • starts_with(): Starts with a prefix
  • ends_with(): Ends with a suffix
  • contains(): Contains a literal string
  • num_range(): Matches a numerical range like x01, x02, x03
  • one_of(): Matches variable names in a character vector
  • everything(): Matches all variables
  • last_col(): Select last variable, possibly with an offset
  • matches(): Matches a regular expression (a sequence of symbols/characters expressing a string/pattern to be searched for within text)

Columnwise operations

hotels |>
  group_by(hotel, is_canceled) |>
  summarise(
    across(.cols = starts_with("stays"),  list(mean = mean, sd = sd), .names = "{.fn}_{.col}") 
    )
# A tibble: 4 × 6
# Groups:   hotel [2]
  hotel is_canceled mean_stays_in_weeken…¹ sd_stays_in_weekend_…²
  <chr>       <dbl>                  <dbl>                  <dbl>
1 City…           0                  0.801                  0.862
2 City…           1                  0.788                  0.917
3 Reso…           0                  1.13                   1.14 
4 Reso…           1                  1.34                   1.14 
# ℹ abbreviated names: ¹​mean_stays_in_weekend_nights,
#   ²​sd_stays_in_weekend_nights
# ℹ 2 more variables: mean_stays_in_week_nights <dbl>,
#   sd_stays_in_week_nights <dbl>

Columnwise operations

hotels |>
  group_by(hotel, is_canceled) |>
  summarise(
    across(.cols = starts_with("stays"),  list(mean = mean, sd = sd), .names = "{.fn}_{.col}"),
    .groups = "drop"
    )
# A tibble: 4 × 6
  hotel is_canceled mean_stays_in_weeken…¹ sd_stays_in_weekend_…²
  <chr>       <dbl>                  <dbl>                  <dbl>
1 City…           0                  0.801                  0.862
2 City…           1                  0.788                  0.917
3 Reso…           0                  1.13                   1.14 
4 Reso…           1                  1.34                   1.14 
# ℹ abbreviated names: ¹​mean_stays_in_weekend_nights,
#   ²​sd_stays_in_weekend_nights
# ℹ 2 more variables: mean_stays_in_week_nights <dbl>,
#   sd_stays_in_week_nights <dbl>

Setup for next example: hotel_summary 

hotels_summary <- hotels |>
  group_by(hotel, is_canceled) |>
  summarise(
    across(
      .cols = starts_with("stays"),
      list(mean = mean),
      .names = "{.fn}_{.col}"
    ),
    .groups = "drop"
  )

hotels_summary
# A tibble: 4 × 4
  hotel is_canceled mean_stays_in_weeken…¹ mean_stays_in_week_n…²
  <chr>       <dbl>                  <dbl>                  <dbl>
1 City…           0                  0.801                   2.12
2 City…           1                  0.788                   2.27
3 Reso…           0                  1.13                    3.01
4 Reso…           1                  1.34                    3.44
# ℹ abbreviated names: ¹​mean_stays_in_weekend_nights,
#   ²​mean_stays_in_week_nights

Which variables are plotted in the following visualization? Which aesthetics are they mapped to?

Livecoding

Reveal below for code developed during live coding session.

Code
hotels_summary |>
  mutate(is_canceled = if_else(is_canceled == 0, "Not canceled", "Canceled")) |>
  pivot_longer(cols = starts_with("mean"),
               names_to = "day_type",
               values_to = "mean_stays",
               names_prefix = "mean_stays_in_") |>
  mutate(
    day_type = if_else(str_detect(day_type, "weekend"), "Weekend", "Weekday")
    ) |>
  ggplot(aes(x = str_wrap(is_canceled, 10), y = mean_stays, 
             group = hotel, color = hotel)) +
  geom_point(show.legend = FALSE) +
  geom_line(aes(linetype = hotel), linewidth = 1) +
  facet_wrap(~day_type) +
  labs(
    x = "Booking status",
    y = "Mean number of\nnights of stay",
    color = NULL, linetype = NULL,
    title = "Mean number of stays",
    subtitle = "By hotel type and booking status",
    caption = "Source: Antonio, Almeida and Nunes (2019) | TidyTuesday"
  ) +
  scale_color_manual(values = c("cornsilk4", "deepskyblue3")) +
  scale_y_continuous(limits = c(0, 4), breaks = 0:4) +
  theme(legend.position = "bottom")

pivot_wider() and pivot_longer()

  • From tidyr
  • Incredibly useful for reshaping for plotting
  • Lots of extra arguments to help with reshaping pain!
  • Refer to pivoting vignette when needed

Animation showing what pivoting longer and wider look like

Stats

Stats < > geoms

  • Statistical transformation (stat) transforms the data, typically by summarizing
  • Many of ggplot2’s stats are used behind the scenes to generate many important geoms
stat geom
stat_bin() geom_bar(), geom_freqpoly(), geom_histogram()
stat_bin2d() geom_bin2d()
stat_bindot() geom_dotplot()
stat_binhex() geom_hex()
stat_boxplot() geom_boxplot()
stat_contour() geom_contour()
stat_quantile() geom_quantile()
stat_smooth() geom_smooth()
stat_sum() geom_count()

Layering with stats

hotels |>
  filter(adr < 4000) |>
  ggplot(aes(x = arrival_date_month, y = adr)) +
  geom_point(alpha = 0.5) +
  stat_summary(
    geom = "point", fun = "median", 
    colour = "red", size = 5, pch = 4, stroke = 2
  ) +
  facet_wrap(~ hotel, ncol = 1)

Alternate: layering with stats

hotels |>
  filter(adr < 4000) |>
  ggplot(aes(x = arrival_date_month, y = adr)) +
  geom_point(alpha = 0.5) +
  geom_point(
    stat = "summary", fun = "median", 
    colour = "red", size = 5, pch = 4, stroke = 2
  ) +
  facet_wrap(~ hotel, ncol = 1)

Statistical transformations

What can you say about the distribution of price from the following QQ plot?

hotels |>
  filter(adr < 4000) |>
  ggplot(aes(sample = adr)) +
  stat_qq() +
  stat_qq_line() +
  labs(y = "adr")

🔗 datavizaz.org