get number of elements after filtering in dplyr using length()

Question

Using mtcars dataset, i want to count how many cars have at least 6 cylinders (cyl).

I use length() after filtering, and get a result of 11

library(dplyr)
mtcars %>%
  filter(
    cyl > 6
  ) %>%
  length()

However the code provided by tutorial is like this, and returns result of 14

library(dplyr)
mtcars %>%
  filter(cyl > 6) %>%
  summarise(n())

by viewing the result directly after filtering, it should also be 14

Now i have learnt that summarise(n()) is better Count number of rows by group using dplyr, and there are more better methods counting after filtering, but i am still confused why my code returns a different result and where the 11 comes from.

Thanks

Answer 1

When applied to a dataframe, length() returns number of columns not rows. But you can pull() variables to see their length(). So, your code counted number of columns instead of rows.

library(dplyr)


# Counts columns
mtcars %>%
        filter(
                cyl > 6
        ) %>%
        length()
#> [1] 11

# Counts rows
mtcars %>%
        filter(cyl > 6) %>%
        summarise(n())
#>   n()
#> 1  14

# Counts rows
mtcars %>%
        filter(
                cyl > 6
        ) %>% 
        pull() %>%
        length()
#> [1] 14

^{Created on 2024-05-12 with reprex v2.1.0}

Or you could have just used count() instead of length()