Return the 90th percentile values in R

For example, I have a dataset of 30-years air temperature of a city, the dataset looks like:

Year  Julian_date  temperature
1991    1             2.1
1991    2             2.2
...     ...           ...
1991    365           2.3
1992    1             2.1
...     ...           ...
1992    365           2.5
...     ...           ...
2020    366           2.5

I would like to calculate the 90th percentile value of each Julian date (from different years), and returen the results, like:

Julian_date        value(the 90th percentile)
1                  2.4
2                  2.6
...                ...
365                2.5

How should I write the code in r?

>Solution :

You can first group by Julian_date, then use the quantile function to set the probability inside summarise.

library(tidyverse)

df %>% 
  group_by(Julian_date) %>% 
  summarise("value (the 90th percentile)" = quantile(temperature, probs=0.9, na.rm=TRUE))

Output

  Julian_date `value (the 90th percentile)`
        <int>                         <dbl>
1           1                           2.1
2           2                           2.2
3         365                           2.5

Data

df <- structure(list(Year = c(1991L, 1991L, 1991L, 1992L, 1992L, 2020L
), Julian_date = c(1L, 2L, 365L, 1L, 365L, 365L), temperature = c(2.1, 
2.2, 2.3, 2.1, 2.5, 2.5)), class = "data.frame", row.names = c(NA, 
-6L))