Selecting different numbers of columns on each row of a data frame

Question

This question is about selecting a different number of columns on every row of a data frame. I have a data frame:

df = data.frame(
    START=sample(1:2, 10, repace=T), END=sample(2:4, 10, replace=T),
    X1=rnorm(10), X2=rnorm(10), X3=rnorm(10), X4=rnorm(10)
)

I would like to have a way without loops to select columns (START[i]:END[i])+2 on row i for all rows of my data frame.

What is it you are trying to do? Ie, what will you do w/ the selected columns? Eg, would you want the sums of the values w/i those columns? Something else? — gung - Reinstate Monica, Nov 16 '13 at 01:42
Something like this? `apply(df, 1, function(x)df[,seq(x[1]+2, x[2]+2)])` — user1981275, Nov 16 '13 at 02:03
The goal is to add the ith element of a list to each selected cell on the ith row. — user2998362, Nov 18 '13 at 19:53
Idealy I would like to get a matrix of 10 rows by 6 columns with each cell being a T or F. — user2998362, Nov 18 '13 at 20:05

cryo111 · Accepted Answer · 2013-11-18T21:54:01.320

0

Base R solution

lapply(split(df,1:nrow(df)),function(row) row[(row$START+2):(row$END+2)])

Or something similar as given in the comment above (I would store the output in a list)

library(plyr)
alply(df,1,function(row) row[(row$START+2):(row$END+2)])

Edit per request of OP:

To get a TRUE/FALSE index matrix, use the following R base solution

idx_matrix=col(df)>=df$START+2&col(df)<=df$END+2
df[idx_matrix]

Note, however, that you lose some information here (compared to the list based solution).

edited Nov 18 '13 at 21:54

answered Nov 16 '13 at 02:38

cryo111

To be more clear by select I ment the array locations. Ideally in some sort of array of Trues and Falses. MY goal is to add the ith element of a list to each selected element in the ith row. – user2998362 Nov 18 '13 at 20:15
I have edited the answer above. This gives you a TRUE/FALSE index matrix. – cryo111 Nov 18 '13 at 21:54
This works great, thanks! So it turns out I was really asking for the functions col and row! Good to know – user2998362 Nov 18 '13 at 22:09
You are welcome (since you seem to be new to Stackoverflow: you can accept answers by checking the tick below the up/down voting arrows) – cryo111 Nov 18 '13 at 22:21

1 Answers1