Create column based on another dataframe

Question

I have what seems a simple requirement, but I can't seem to achieve the desired result. My dataframes are rather large, so I will just give a screenshot: enter image description here

What I want is: if ps2c$ps == present2$pn then ps2c$sf == present2$sf.

If it is not clear, for every ps2c$ps there is a present2$pn (present2 is just the mean data).

A screenshot? Are you kidding me? ;-) Seriously, just paste the output from head(ps2c) and head(present2) into your Q, select the pasted code/output, and click the code button in the toolbar (the one that looks like braces {}) or press Ctrl+k to have your code formatted appropriately. — Gavin Simpson
– Gavin Simpson, Commented Apr 11, 2011 at 9:52
My data frame is massive, but I will bear your suggestion in mind. — Frank Zafka
– Frank Zafka, Commented Apr 11, 2011 at 10:02
but head() only shows the first few lines - my point was, if you can show a screenshot you can show the code and output from head(). — Gavin Simpson
– Gavin Simpson, Commented Apr 11, 2011 at 10:45
dput(head(ps2c)) gives you a representation of the 6 first lines of the data frame that we can run without editing anything. Alternatively, you can construct a toy dataset to show what you want. — Joris Meys
– Joris Meys, Commented Apr 11, 2011 at 10:52
@Joris - that is a great observation, one which helps facilitate providing small snippets of data for illustrative purposes. — Gavin Simpson
– Gavin Simpson, Commented Apr 11, 2011 at 11:08

Andrie · Accepted Answer · 2011-04-11 09:24:29Z

10

You don't supply test data, but something like the following might work:

ps2c$sf <- present2$sf[match(ps2c$ps, present2$pn)]

answered Apr 11, 2011 at 9:24

Andrie

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1 Comment

Vivi Over a year ago

I am working with a dataset with 72 million observations, and I was trying to do this task using a not-straightforward routine which involved the foreach function. I tested it for about 1/1000 of my dataset and it took more than 20 minutes. With your solution it took a second for the whole dataset. I can't thank you enough. R really rocks, I can't wait to be good at it!

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