当该行的行号等于R中重复的另一列的值时，如何选择一行？

Question

I have a data frame as follows -

df <- cbind(c(1,1,1,2,2,2,3,3,3,3), c(6,12,18,3,9,12,4,8,12,16),c(3,3,3,2,2,2,4,4,4,4))
colnames(df) <- c("ID","value","index")

I want to get the following result -

df1 <- cbind(c(1,2,3), c(18,9,16),c(3,2,4))

So I basically want to extract (for each ID) the row whose row number is equal to the index for that ID. For example, the 3rd row for ID 1, 2nd row for ID 2 and 4th row for ID 4.

I tried the following code

df1 <- df%>%group_by(ID)%>%filter(index==index)

But it is not working. Please help me to solve this problem.

Answer 1

Use slice to select the index row for each ID .

library(dplyr)
df %>% group_by(ID) %>% slice(first(index)) %>% ungroup

#     ID value index
#  <dbl> <dbl> <dbl>
#1     1    18     3
#2     2     9     2
#3     3    16     4

This can be written in data.table and base R as:

library(data.table)
setDT(df)[, .SD[first(index)], ID]

#Base R
subset(df, index == ave(value, ID, FUN = seq_along))

data

df <- data.frame(ID = c(1,1,1,2,2,2,3,3,3,3), 
                 value = c(6,12,18,3,9,12,4,8,12,16),
                 index = c(3,3,3,2,2,2,4,4,4,4))

Answer 2

Just adding to the Ronak Shah's answer, I guess one of the straightforward codes to do what you want is the following:

library(dplyr)
df <- 
    data.frame(ID = c(1,1,1,2,2,2,3,3,3,3), value = c(6,12,18,3,9,12,4,8,12,16), index = c(3,3,3,2,2,2,4,4,4,4))

df %>% group_by(ID) %>% filter(row_number() == index) %>% ungroup