CODEWITHSUNDEEP
rindexingdataframe
DeltaIV
DeltaIV

Reputation: 5646

select and extract a vector of indices from a data frame

I have a data frame and a vector:

parameters <- data.frame(index = c(3:12, 15:18, 23:25), variable = c("Pin", 
                      "Pout", "Tin", "speed", "D", "L", "Cin_h", "Cout_h", "VdA", 
                      "preswirl", "mu", "mol_weight", "Cp", "Z", "ffactor_N", 
                      "ffactor_M", "nfreqs"),
                      value = c(65, 4, 16.85, 7900, 110, 60, 0.1975, .1875, 2.31,
                                0.2, 0.0011877, 22.0, 1.4, 1.0, 0.0785, -0.1101,
                                30))
temp <- runif(100)

I want to read a vector of indices from parameters, and use them as an index vector to read into temp. For example, let's assume I want the index corresponding to the variable nfreqs and Pout:

library(dplyr)
index_vector <- parameters %>% filter(variable %in% c("Pout","nfreqs")) %>% select(index)

The problem is that now index_vector is not a vector, but a dataframe, thus I can't use it to read into temp:

> temp[index_vector]
Error in temp[index_vector] : invalid subscript type 'list'

temp[index_vector$index] works, of course. But I was wondering if I could directly extract the vector I'm interested in, during the dplyr call, i.e., by suitably modifying 

index_vector <- parameters %>% filter(variable %in% c("Pout","nfreqs")) %>% select(index)

Upvotes: 1

Views: 1180

Answers (1)

akrun
akrun

Reputation: 886938

We need to extract the column

temp[index_vector$index]
#[1] 0.2784451 0.1061800

But, if we need to do this within the %>% 

library(magrittr)
parameters %>% 
    filter(variable %in% c("Pout","nfreqs")) %>%
    .$index %>%
    extract(temp, .)
#[1] 0.2784451 0.1061800

Upvotes: 1

Related Questions