Select multiple values from a dataframe using a matrix with indices

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 c开发者_开发问答oordinates 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?

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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.

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This sounds a bit like OP used SAS before, in R variables have no history. Instead of line-by-line operation in R people try to do vector-based calculation and use apply to apply a function to every entry in a vector or matrix. In more conditional cases you can use a for-loop and store the return-values as entries within it and then return or print them all at once:

my <- function() {
    outputs <- rep(0,20)
    for(i in 1:20) {
        outputs[i] <- i*i
    }
return(outputs)
}