I have two data frames
columns df1= a, b, c, d
columns df2= e, f, c, d
I want to say that if df1$a == df2$e and at the same time df1$b == df2$f I want to print in a new column df1$x the values of the corresponding df2$c.
So I wrote this command df1$x <- ifelse(df1$a %in% df2$e & df1$b %in% df2$f, df2$c, ifelse (NULL))
But the values in df1x are not correct. I think I have to do a while loop, but I have no idea from where to start.
Nicoletta
To merge two data frames by multiple columns you can use merge and corresponding by.x and by.y. Both of those accept vector of column names. Parameter all.x means all data from first df1 will be preserved.
df1 <- merge(df1, df2[,c("e","f","c")], by.x=c("a","b"), by.y=c("e","f"), all.x=T)
colnames(df1) <- c("a","b","c","d","x")
For more information see ?merge. For more complex operation you can check out package dplyr and its function left_join.
Result on random data
df1 <- data.frame(a = sample(1:5), b= sample(1:5), c=runif(5), d = runif(5))
df2 <- data.frame(e = df1$a, f = df1$b, c = runif(5), d = runif(5))
df2$e[3] <- 7
Output
> df1
a b c d
1 5 4 0.76677063 0.92123552
2 4 1 0.93524320 0.09275425
3 3 2 0.01121468 0.12035981
4 1 5 0.72992427 0.87711572
5 2 3 0.11680937 0.93696597
> df2$e[3] <- 7
> df2
e f c d
1 5 4 0.6251662 0.1549575
2 4 1 0.8464672 0.8702837
3 7 2 0.5394273 0.4290171
4 1 5 0.4061817 0.9072905
5 2 3 0.3376456 0.4291463
> merge(df1, df2[,c("e","f","c")], by.x=c("a","b"), by.y=c("e","f"), all.x=T)
a b c d x
1 1 5 0.72992427 0.87711572 0.4061817
2 2 3 0.11680937 0.93696597 0.3376456
3 3 2 0.01121468 0.12035981 NA
4 4 1 0.93524320 0.09275425 0.8464672
5 5 4 0.76677063 0.92123552 0.6251662