[R] joining "one-to-many"
Monica Pisica
pisicandru at hotmail.com
Tue Feb 17 15:33:26 CET 2009
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.
Thanks,
Monica
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