FAQ: Modifying Data Frames in R - Adding a Column

This community-built FAQ covers the “Adding a Column” exercise from the lesson “Modifying Data Frames in R”.

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Just a quick question to check my understanding of mutate().

The following code snippet is provided in the example:

df %>%
  mutate(sales_tax = price * 0.075)

The example then continues: “Now the inventory table has a column called sales_tax , where the value is 0.075 * price:”

Is this statement totally true? I.e., is this modifying the inventory table in place, so to speak, or is mutate() actually returning a new data frame?

This would explain why we need

dogs <- dogs %>%

rather than

dogs %>%

which seems to be implied above.

the first ‘dogs’ is your variable name for the df.

dogs <- dogs %>%

You’re actually overwriting your existing dataframe ‘dogs’ with one that is the same as the existing + the avg_height column.

Is there a reason you need to state the variable/data frame twice though? I thought usually you could do it without having to call it first in other languages - you just redefine it with whatever you’re overwriting it with. (I don’t know if I’m saying this right sorry!)