Jack - 1 year ago 58
R Question

# How to save row names when selecting each column independently instead of row number?

I asked this question a couple days ago and @Juan Bosco helped me and suggested the code which works perfectly and selects top n values from each column. But turns out I need the names of each selected row for each column, so in the list: "Selectedrows", I need something like "t9", "t8", "t7" instead of row number.

`````` names<- c("t1","t10","t11","t2","t3","t4","t5","t6","t7","t8","t9")
values1 <- c(2,3.1,4.5,5.1,6.5,7.1,8.5,9.11,10.1,11.8,12.3)
values2 <- c(1,3.1,3,5.1,6.5,7.1,8.5,9.11,10.1,12,12)
mydf<- data.frame(names,values1,values2)

Selectedrows<- lapply(2:3, function(col_index) {
max_values <- sort(mydf[[col_index]], decreasing = T)[1:3]
max_rows <- sapply(max_values, function(one_value){
as.numeric(rownames(mydf[mydf[[col_index]] == one_value, ]))
})

unique(unlist(max_rows))[1:3]

})
``````

Thanks

In this case, you can use `order`, which returns the index which can be used to sort an array, the first three elements from the `order` results corresponds to the index of the top 3 values if you specify the array to be decreasing; With the index, you can subset the `names` column and get the corresponding values:

``````lapply(2:3, function(col_index) {
mydf[["names"]][order(mydf[[col_index]], decreasing = T)[1:3]]
})

#[[1]]
#[1] "t9" "t8" "t7"

#[[2]]
#[1] "t8" "t9" "t7"
``````

Notice this assumes your names column is character, if not. Do `mydf\$names <- as.character(mydf\$names)` firstly if you want the result to be character instead of factor.

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