Use a column in a data.frame to select another - r

Question

How do I use one variable in a data.frame to refer to another?
say I have:
col col1 col2
"col1" 1 5
"col2" 2 6
"col1" 3 7
"col2" 4 8
and I want:
col col1 col2 answer
"col1" 1 5 1
"col2" 2 6 6
"col1" 3 7 3
"col2" 4 8 8
,
df$answer = df[,df$col]
isn't working, and a for loop is taking forever.

Answer 1

I know it's already answered, but I thought another approach might be useful:
read.table(text='col col1 col2
"col1" 1 5
"col2" 2 6
"col1" 3 7
"col2" 4 8',h=T)->df
df$answer <- as.integer(df[ cbind(c(1:nrow(df)), match(df$col, names(df))) ])
df
# col col1 col2 answer
# 1 col1 1 5 1
# 2 col2 2 6 6
# 3 col1 3 7 3
# 4 col2 4 8 8

Answer 2

This doesn't look very hard, but the solution I found isn't very elegant, there are probably better ways. But you can use match and then subset according to the match:
dat <- read.table(text="col col1 col2
col1 1 5
col2 2 6
col1 3 7
col2 4 8", header = T, stringsAsFactors = FALSE)
cols <- unique(dat$col)
matches <- match(dat$col, cols)
dat$answer <- sapply(seq_along(matches), function (i) {
dat[i,cols[matches[i]]]
})
And the result:
> dat
col col1 col2 answer
1 col1 1 5 1
2 col2 2 6 6
3 col1 3 7 3
4 col2 4 8 8
Edit
Actually, here's an already much better approach:
dat$answer <- sapply(1:nrow(dat), function(r) {
dat[r,dat$col[r]]
})
This is apparently what you have tried, but using sapply instead of unlist(lapply, so yeah, not sure if this helps.

Answer 3

In this case with only 2 columns ifelsemight be the fastest and most straightforward solution.
df$answer <- ifelse(df[,1] == "col1",df[,"col1"],df[,"col2”])
col col1 col2 answer
1 col1 1 5 1
2 col2 2 6 6
3 col1 3 7 3
4 col2 4 8 8
Addition as N8TRO asked in his comment for a more general solution.
A simple switch might be all that is needed:
for(i in 1:nrow(df)) df$ans[i] <- switch(df[i,1],df[i,df[i,1]])
or without a "for" loop:
df$ans <- sapply(1:nrow(df),function(i) switch(df[i,1],df[i,df[i,1]]))
example:
df <- data.frame(col=sample(paste0('col',1:5),10,replace=T),col1=1:10,col2=11:20,col3=21:30,col4=31:40,col5=41:50,stringsAsFactors = F)
select the elements:
df$ans <- sapply(1:nrow(df),function(i) switch(df[i,1],df[i,df[i,1]]))
df
col col1 col2 col3 col4 col5 ans
1 col1 1 11 21 31 41 1
2 col1 2 12 22 32 42 2
3 col5 3 13 23 33 43 43
4 col2 4 14 24 34 44 14
5 col3 5 15 25 35 45 25
6 col4 6 16 26 36 46 36
7 col5 7 17 27 37 47 47
8 col3 8 18 28 38 48 28
9 col1 9 19 29 39 49 9
10 col5 10 20 30 40 50 50