ask a dataframe with a logical variable?

[Tung]

planets_df
     name               type diameter rotation rings
1 Mercury Terrestrial planet    0.382    58.64 FALSE
2   Venus Terrestrial planet    0.949  -243.02 FALSE
3   Earth Terrestrial planet    1.000     1.00 FALSE
4    Mars Terrestrial planet    0.532     1.03 FALSE
5 Jupiter          Gas giant   11.209     0.41  TRUE
6  Saturn          Gas giant    9.449     0.43  TRUE
7  Uranus          Gas giant    4.007    -0.72  TRUE
8 Neptune          Gas giant    3.883     0.67  TRUE

rings_vector <- planets_df$rings

# to use rings_vector to select the data for the four planets with rings
planets_df[rings_vector,]

I feel surprised that the above command just gives me the desired output. Here it seems R automatically filter "TRUE" value of "rings_vector". Can anyone help me explain it?

[mikeR]

What's your problem? You just tell the data frame which rows you want to keep and which ones you want to throw away

[jrmuirhead]

Paraphrasing documentation for ?Extract: For objects such as
matrices, or data.frames x[i, j] that are indexed by rows i and columns j,
you can extract elements by passing \textbf i or \textbf j as logical vectors or expressions that evaluate as a logical vectors.

[Tung]

Yes, I understand x[i,j] and x[i,].

But here is not i, instead it is rings_vector, a logical vector.I'm confused.

[jrmuirhead]

Hi Tung,
To tie together the help documentation and your example, \textbf i is a logical vector of which your rings_vector is the same class, and \textbf X is a matrix or data.frame of which planets_df is the same class. It doesn't matter so much what you name your objects but there are limits.

i <- c(TRUE, FALSE, TRUE)
X <- data.frame(a = letters[1:3], b = letters[4:6])

X[i, ] # returns rows 1 and 3 corresponding where i is TRUE
#>   a b
#> 1 a d
#> 3 c f

apples <- c(TRUE, FALSE, TRUE)

X[apples, ]
#>   a b
#> 1 a d
#> 3 c f

^{Created on 2020-07-24 by the reprex package (v0.3.0)}