Select multiple values from a dataframe using a matrix with indices

Question

I have a for loop, which returns me values from one dataframe using which, which.min and which.max giving me the "coordinates". Here an example

df <- as.data.frame(matrix(rnorm(11284), nrow=403, ncol=28))
row <- matrix(data=c(1:403),nrow=403, ncol=1)
col <- matrix(rnorm(403,14,3), nrow=403, ncol=1)
col <- round(col, 0)
coord <- cbind(row, col)

Coord holds the coordinates for a criterium that I have defined before. I now want to extract the respective values according to those coordinates from df with a for loop

for (i in 1:nrow(coord)) {
print(df[coord[i,1], coord[i,2]])
}

When I use

output <- df[coord[i,1], coord[i,2]])

it only gives me the last expression of the loop. My simple question is now: How do I store not only the last expression from this loop, but the whole vector that is given me by print?

Answer 1

You're surprised? You store every loop only that one value of the loop in output. So after the last loop it's only the last value. The correct loop solution would be :

output <- vector(length=nrow(coord))
for (i in 1:nrow(coord)){
    output[i] <- df[coord[i,1],coord[i,2]]
}

But you don't have to use a loop. Given you have the coordinates in a dataframe or matrix, this can be done a whole lot simpler anyway :

output <- Df[coord]

No loop needed.