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Thank you for the link. I still end up confused. How does selecting the 3rd column, dt[, 3] jive with >`DT[i, j, by, ...]` which means: “Take DT, subset

Thank you for the link. I still end up confused. How does selecting the 3rd column, dt[, 3] jive with >`DT[i, j, by, ...]` which means: “Take DT, subset rows using i, then calculate j ... I don’t understand how column selection and calculating an expression is the same thing. So now I can’t figure out if DT[, sum(V1)] Will return the column indexed at the sum of V1 or if it will return the sum of the column called V1. How can I tell, from the syntax?

nojito 7y ago
j is always for columns. So for your first example it grabs the third column. The calculation you’re performing is a column selection. For your second example, you’re returning all the rows, selecting column V1, and summing it. I really prefer to name the operations dt[rows, columns, groups]