R: Get index names while looping through df elements - r

Question

Say, I have a data frame and I need to do something with its cells and remember what cells I have changed. One way is to loop through indices with two for-loops. But is there a way to do this with one loop?
Perfectly I need something like this:
changes = data.frame(Row = character(), Col = character())
for (cell in df){
if (!(is.na(df))){
cell = do.smt(cell)
temp = list(Row = get.row(cell), Col = get.col(cell))
changes = rbind(changes,temp)
}
}
Example of what I need:
df = data.frame(A = c(1,2,3), B = c(4,5,6), C = c(7,8,9))
rownames(df) = c('a','b','c')
changes = data.frame(Row = NA, Col = NA)
for (i in rownames(df)){
for (j in colnames(df)) {
if (df[i,j] > 5) {
df[i,j] = 0
temp = list(Row = i, Col = j)
changes = rbind(changes, temp)
}
}
}

Answer 1

This gets rid of both loops
df = data.frame(A = c(1,2,3), B = c(4,5,6), C = c(7,8,9))
rownames(df) = c('a','b','c')
changes <- which(df > 5, arr.ind=TRUE)
df[changes] <- 0
If you want the format exactly as specified you can sort that out with
changes <- data.frame(changes,row.names=NULL)
changes$row <- rownames(df)[changes$row]
changes$col <- colnames(df)[changes$col]
and its a simple matter of sorting if you're concerned that the order of the rows matches your example output