R：将 which() 与 quantile() 的输出一起使用？

Question

So, I'm trying to get the value in a column a from the data frame df, which is the 90th percentile of column b. I used the following code to get the value at quantile:

p = quantile(df$b, c(0.9))

After this, I wanted to use this to get the row number of the value, so that I could use that to get the corresponding value in column a:

which(df$b == p)

but for some reason it just gives an output of

integer(0)

I substituted the variable, p, with the actual value, which was 1.68, and it worked, and creating another variable with just 1.68 works too, but using it with the result of the quantile never gives the right value.

I've tried using as.numeric , using p[[1]] and as.double . No change to the result whatsoever. Any help is appreciated in understanding why this happens and if there's another way to go about this.

Edit: to clarify, there are entries exactly at the 90th percentile, and using a hard coded value which was returned by the quantile function will return these. The problem occurs in the which function when we use the output of quantile.

Answer 1

'quantile' will give you the the 90% percentile of your data which may or may not be a value in your data therefore if you want the value closest to the 90% percentile you will need to match it a little differently:

Your data:

df <- data.frame(b = runif(50))

The 90th percentile

p = quantile(df$b, c(0.9))

The index the is closest to the 90th percentile:

index <- which(abs(df$b - p) == (min(abs(df$b - p), na.rm = TRUE)))
index

Get the value(s) from the dataframe using the index:

df$b[index]