can anybody explain me why subsetting in R in the third example does not give the same result as in the first and second ones? I thought the first 3 subsetting cases were equal.
I can subset mtcars["cyl">=6] but why I cannot subset mtcars["cyl"==6]?
Thanks a lot!
data("mtcars")
mtcars[mtcars[,2]==6,]
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
#> Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
#> Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
#> Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
#> Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4
#> Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4
#> Ferrari Dino 19.7 6 145.0 175 3.62 2.770 15.50 0 1 5 6
mtcars[mtcars$cyl==6,]
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
#> Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
#> Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
#> Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
#> Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4
#> Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4
#> Ferrari Dino 19.7 6 145.0 175 3.62 2.770 15.50 0 1 5 6
mtcars["cyl"==6,]
#> [1] mpg cyl disp hp drat wt qsec vs am gear carb
#> <0 rows> (or 0-length row.names)
# but
mtcars["cyl">=6,]
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
#> Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
#> Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
....
Created on 2020-07-31 by the reprex package (v0.3.0)
In the third case, you are getting zero rows because you are comparing the string "cyl" (not the variable cyl) to a number, 6. Since you are comparing a character string to a number, R coerces the two types to be equal (if possible): in this case, R transforms the number 6 to a string "6".
The condition you are now trying to satisfy is "cyl" == "6", which is always false; therefore you get zero rows.
Regarding the fourth case, we can see that
> "cyl" >= 6
[1] TRUE
This is true because, since the two objects are not of the same type ("cyl" is character and 6 is numeric), R coerces the number to a character "6".
Now, the condition "cyl" >= "6" is true because, alphanumerically speaking, the numbers such as "6" come before the letters.