Log difference in a dataframe column

Just learning so excuse me if I'm being too basic here. But I'm wondering how
should I know that as.ts would be needed for lag ? Is there a thought
process or way to inspect that I should have gone through to know that log
would work on y[,5] but lag would not work on [,5] ? 

Is the general rule that lag is not in the base package and therefore would
not work ?

Thanks for the comments

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