[R] joining "one-to-many"
hadley wickham
h.wickham at gmail.com
Tue Feb 17 16:05:02 CET 2009
On Tue, Feb 17, 2009 at 8:33 AM, Monica Pisica <pisicandru at hotmail.com> wrote:
>
> Hello list,
>
> I am wondering if a joining "one-to-many" can be done a little bit easier. I tried merge function but I was not able to do it, so I end up using for and if.
>
> Suppose you have a table with locations, each location repeated several times, and some attributes at that location. The second table has the same locations, but only once with a different set of attributes. I would like to add the second set of attributes to the first table.
>
> Example:
>
> set.seed <- 123
> loc <- c(rep("L1", 3), rep("L2", 5), rep("L3", 2))
> val1 <- round(rnorm(10),2)
> val2 <- c("a", "b", "c", "a", "b", "d", "f", "e", "b", "e")
> t1 <- data.frame(loc, val1, val2)
> t2 <- data.frame(loc=c("L1","L2","L3"), val3 = c("m", "n", "p"), val4 = c(25, 67, 48))
>
> # join one-to-many
>
> n <- nrow(t1)
> m <- nrow(t2)
> t1$val3 <- rep(1, n)
> t1$val4 <- rep(1, n)
>
> for (i in 1:n) {
> for (j in 1:m){
> if (t1$loc[i]==t2$loc[j]) {
> t1$val3[i] <- as.character(t2$val3[j])
> t1$val4[i] <- t2$val4[j]
> }
> }
> }
>
> Desired result:
>
> t1
> loc val1 val2 val3 val4
> 1 L1 -0.41 a m 25
> 2 L1 -0.69 b m 25
> 3 L1 0.36 c m 25
> 4 L2 1.11 a n 67
> 5 L2 0.15 b n 67
> 6 L2 -0.80 d n 67
> 7 L2 -0.08 f n 67
> 8 L2 -1.01 e n 67
> 9 L3 -1.01 b p 48
> 10 L3 -2.50 e p 48
>
>
> This code works OK but it is slow if the data frames are actually bigger than my little example. I hope somebody knows of a better way of doing these type of things.
merge(t1, t2)
Hadley
--
http://had.co.nz/
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