How do one find the index number of 1st quartile, median and 2nd quartile?

Question

How do I find the index number of the 1st quartile, median and 2nd quartile? I have a data set containing 1006 observations. I could easily find the position of the minimum and maximum value by using

match(min(sp500_logreturns), as.numeric(sp500_logreturns))

But it just spits out NA if i use quantile(sp500_logreturns, 0.25) and 0.5 and 0.75 instead of min(sp500_logreturns) in the match -function... How could I else find the index number?

Answer 1

Possibly even simpler than using match together with a calculation of quantile is to simply select the element at the (fractional) position you want after ordering:

Test data:

sp500_logreturns=sample(1:100,size=10006,replace=T)

For first quartile (0.25):

firstq_index=order(sp500_logreturns)[length(sp500_logreturns)*0.25]
# check:
mean(sp500_logreturns<sp500_logreturns[firstq_index])
# [1] 0.2467519