How to save quartiles in separate groups in R-CodePudding

I have a list of the grades of my students from several courses. I want to select the 1st quartile (saving in a group with the name A) and the 3rd quartile (saving in a group with the name B) and the rest of the students saving in a group with the name C. My question is how can I save their grades in that separate group? and how can I find the students in each quartile?

I used summary to see quartiles

Mark=SS(summary)

 HIS              BIO              CHE                
 Min.   :-6.800   Min.   :-23.30   Min.   :-25.20     
 1st Qu.: 1.500   1st Qu.: 12.10   1st Qu.: 17.70   
 Median : 2.200   Median : 16.70   Median : 23.90   
 Mean   : 2.126   Mean   : 16.71   Mean   : 23.91   
 3rd Qu.: 3.800   3rd Qu.: 22.40   3rd Qu.: 31.10    
 Max.   :20.200   Max.   : 67.30   Max.   : 77.20     

quantile(SS$HIS, 0.25)
1.500

Kind regards

CodePudding user response：

One option is use the quantile() function to obtain the values of the quartiles and then use the cut() function to define the groups.

#create test data
x <- as.integer(runif(50, 0, 100))

#find the bottom 25% and top 25% grades (and min & max)
qunats <- quantile(x, c(0, .25, .75, 1))
qunats

#now cut the data into section
grade <- cut(x, qunats, include.lowest = TRUE, labels=c("C", "B", "A"))

data.frame(x, grade)