conditionally assign first nth element in vector as H and rests as L

Question

I have a vector of randomly sampled numbers. For example,

vec1 <- sample(1:574)

I would like to assign first 25 percent value in this sample

Answer 1

I think you mean this:

vec1 <- sample(1:574)
L = vec1[1:143]
H = vec1[144:length(vec1)]

Anyway by percentage you can do this:

vec1 <- sample(1:574)
num_to_25 <- floor(0.25*length(vec1))
L = vec1[1:num_to_25]
H = vec1[(num_to_25+1):length(vec1)]

With function you can Use:

assigner = function (input_vactor,percentage_of_L=25){
   num_to_25 <- floor(percentage_of_L*0.01*length(input_vactor))
   L = input_vactor[1:num_to_25]
   H = input_vactor[(num_to_25+1):length(input_vactor)]
   return (list(L=L,H=H))
}

And use it like this:

vec1 <- sample(1:574)
h = assigner(vec1)$H
l = assigner(vec1)$L

Edit for Edit: For your edited question change second function to this:

sensitivity.rand <- function(vector, threshold){
  num_to_thres <- floor(threshold*0.01*length(vector))
  l = length (vector)
  score = c(rep("L",num_to_thres),rep("H",l-num_to_thres))
  return(score)
} 

Answer 2

caretpackage includes createDataPartition function, that will do the job:

library("caret")

SAMPLE_SIZE <- 0.25
SIZE_VECT <- 574

# Set seed for reproducibility
set.seed(pi)

vec1 <- sample(1:SIZE_VECT)

in_sample <- createDataPartition(y = vec1, p = SAMPLE_SIZE, list = FALSE)
H <- vec1[in_sample]
L <- vec1[-in_sample]
# Actual sample size
100 * length(H) / (length(L) + length(H))
#  [1] 25.08711