A typical data analysis will often involve one or more conditional operations. In Section 3.1¹ we described the ifelse function, which we will use extensively in this book. In this section we present two dplyr functions that provide further functionality for performing conditional operations.

case_when

The case_when function is useful for vectorizing conditional statements. It is similar to ifelse but can output any number of values, as opposed to just TRUE or FALSE. Here is an example splitting numbers into negative, positive, and 0:

x <- c(-2, -1, 0, 1, 2)
case_when(x < 0 ~ "Negative", 
          x > 0 ~ "Positive", 
          TRUE  ~ "Zero")
#> [1] "Negative" "Negative" "Zero"     "Positive" "Positive"

A common use for this function is to define categorical variables based on existing variables. For example, suppose we want to compare the murder rates in four groups of states: New England, West Coast, South, and other. For each state, we need to ask if it is in New England, if it is not we ask if it is in the West Coast, if not we ask if it is in the South, and if not we assign other. Here is how we use case_when to do this:

murders %>% 
  mutate(group = case_when(
    abb %in% c("ME", "NH", "VT", "MA", "RI", "CT") ~ "New England",
    abb %in% c("WA", "OR", "CA") ~ "West Coast",
    region == "South" ~ "South",
    TRUE ~ "Other")) %>%
  group_by(group) %>%
  summarize(rate = sum(total) / sum(population) * 10^5) 
#> `summarise()` ungrouping output (override with `.groups` argument)
#> # A tibble: 4 x 2
#>   group        rate
#>   <chr>       <dbl>
#> 1 New England  1.72
#> 2 Other        2.71
#> 3 South        3.63
#> 4 West Coast   2.90

between

A common operation in data analysis is to determine if a value falls inside an interval. We can check this using conditionals. For example, to check if the elements of a vector x are between a and b we can type

x >= a & x <= b

However, this can become cumbersome, especially within the tidyverse approach. The between function performs the same operation.

between(x, a, b)