writing an R function

Question

I want to write a R function that give me the value on 10th percentile of observations. I want to use this function with sapply . Like for mean , sapply is

sapply(1 : n, function(i) mean(a1))

Say eg I have 100 values and 10th percentile of 100 is 10 i want that the value that is on 10th line get printed.

X
45
80
70
56
78
78
56
90
35
190

.......... up till 100 values

Desired output: print value on 10th line ie 190 in the above column

I want the function to calculate first 10th percentile of my 100 observations and later just print the value that come on that position.

Answer 1

The desired function will look like:

give_quant_value <- function(vec, quantile) {
    return(vec[quantile(1:length(vec),p=quantile,type=1)])
}

where
- vec is vector of interest
- quantile is quantile of interest

Proof:

set.seed(42)
a1 <- rnorm(100)
give_quant_value(a1, 0.1)
-0.062714099052421

You may read more about quantile function while typing ?quantile in your terminal

Answer 2

Here is a simple function I wrote: It takes on data as a data.frame object.

select_percentile<-function(df,n){
  df<-df
  nth<-(n/nrow(df))*100
  return(df[nth,])
}

Using data from another answer:

 set.seed(42)
   a1 <- rnorm(100)
    df1<-as.data.frame(a1)
    select_percentile(df1,10)

Result:

#[1] -0.0627141