CODEWITHSUNDEEP
rdata.table
Jay khan
Jay khan

Reputation: 745

Perform operation in Data table based on more than one constraint in R

Normally when I perform operations on data table its on one constraint e.g

 setDT(df)[, mean.price := mean(Price), by = Id]

this command get the mean of price based on unique Id. My question is that if there is a way to mention two constraints in the command e.g 

setDT(df)[, mean.price := mean(Price), by = Id & Year]

So I can get the mean of rows with same Id belonging to same Year.

Suppose I have following data frame

Id Price Year
1  5     2003
1  10    2003
1  4     2003
1  6     2004
1  6     2004
1  10    2004
2  7     2003
2  10    2003
2  3     2003
2  2     2004
2  3     2004
2  10    2004

so the resultant data frame will look like 

Id Price Year  Mean.Price
1  5     2003  6.33
1  10    2003  6.33
1  4     2003  6.33
1  6     2004  7.33
1  6     2004  7.33
1  10    2004  7.33
2  7     2003  7
2  10    2003  7
2  4     2003  7
2  2     2004  5
2  3     2004  5
2  10    2004  5

Upvotes: 0

Views: 54

Answers (1)

Benjamin Schlegel
Benjamin Schlegel

Reputation: 527

You can either write it like akrun wrote it:

setDT(df)[, mean.price := mean(Price), by = .(Id,Year)]

or with the names as vector:

setDT(df)[, mean.price := mean(Price), by = c("Id","Year")]

Upvotes: 1

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