r - assign value to a variable rather than using if statement

Question

Right now, I have dataset consisting of variables Gbcode and ncnty

> str(dt)
'data.frame':   840 obs. of  8 variables:
 $ Gbcode                     : Factor w/ 28 levels "11","12","13",..: 21 22 23 24 25 26 27 28 16 17 ...
 $ ncounty                    : num  0 0 0 0 0 0 0 0 0 0 ...

I want to do the following thing:

if a data record is with Gbcode equal to 11, then assign 20 to its ncnty

Gbcode : 11, 12, 13, 14, 15, 21, 22, 23, 31, 32, 33
Corresponding ncnty: 20, 19, 198, 131, 112, 102, 60, 145, 22, 115, 95

I am wondering whether there is any better solution rather than write an if statement, which would be with many lines in this case, maybe less than 20 lines of code.

Answer 1

This is a merge operation as far as I can tell. Make a little lookup table with your Gbcode/ncnty data, and then merge it in.

# lookup table
lkup <- data.frame(Gbcode=c(11,12,13),ncnty=c(20,19,198))

#example data
dt <- data.frame(Gbcode=c(11,13,12,11,13,12,12))
dt
#  Gbcode
#1     11
#2     13
#3     12
#4     11
#5     13
#6     12
#7     12

Merge:

merge(dt, lkup, by="Gbcode", all.x=TRUE)
#  Gbcode ncnty
#1     11    20
#2     11    20
#3     12    19
#4     12    19
#5     12    19
#6     13   198
#7     13   198

It is sometimes preferable to use match for this sort of thing too:

dt$ncnty <- lkup$ncnty[match(dt$Gbcode,lkup$Gbcode)]