Assigning the variables inside the bracket

April 26, 2022

I have a small dataframe as below.

sad <- structure(list(a = c(1, 2), b = c(3, 4)), class = "data.frame", row.names = c(NA, -2L), class = c("tbl_df", "tbl", 
           "data.frame"))

Can anyone please help me as to what is the below operation doing?

 sad[, a := sad[, "Ma"] == "Y"]
# A tibble: 2 x 2
      a     b
  <dbl> <dbl>
1     1     3
2     2     4

When I execute the above operation, i get the same output, but not sure what is the intent here. Can anyone help me?

>Solution :

Let’s consider this: Breaking down what you have on the right side from the comma in closed brackets which operates on columns. On the other side the left side from comma would operate on rows:

And also consider as @Onyambu noted in the comments:
Note that sad is a tibble eg sad[, a = anything_here] will still produce a result. No error thrown

# will not work no column a (a is a object)

sad[, a]

# All rows and the column named "a" -> this will work
sad[, 'a']

# will not work because you want to get all rows and column "Ma" -> there is no colum Ma in sad
sad[, "Ma"]

# here you assign nothing to object a
a = sad[, "Ma"]

# consecutevily you compare a = sad[, "Ma"] (e.g. nothing) to string 'Y'
# will not work because 'Ma' column don't exist.
sad[, "Ma"] == "Y"