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What code to use in R 3.5.1 to count numbers of unique var2 for each unique var1?

I have a data with 2 columns. Var1 - school classes, Var2 - names of students in the class. What command can I use to make a new matrix that will show how many times are repeated unique names in var2 for each unique var1?

Var1 Var2
9 Sarah
9 John
12 Sarah
11 Veronica
10 John
10 John
11 Veronica
12 John
12 Veronica
11 Veronica
10 Sarah
9 Veronica
9 John


What command can I use to make a new matrix that will show how many times are repeated unique names in var2 for each unique var1?

_____Sarah____Veronica__John
9______1_______1___________2
10_____1_______0___________1
11_____0_______3___________0
12_____1_______1___________1


Thank you in advance!

2 Answers
2

You need the table function

var1 <- c(9, 9, 12, 11, 10, 10, 11, 12, 12, 11, 10, 9, 9)
var2 <- c("Sarah", "John", "Sarah", "Veronica", "John",
"John", "Veronica", "John" ,"Veronica",
"Veronica", "Sarah", "Veronica", "John")
table(var1, var2)


You want a contigency table:

df <- data.frame(Var1, Var2)
xtabs(~Var1 + Var2, df)
# Var2
#Var1 John Sarah Veronica
# 9 2 1 1
# 10 2 1 0
# 11 0 0 3
# 12 1 1 1


To count the number of unique combinations, you can use subset to remove the duplicates, and then calculate the column sums:

z <- xtabs(~Var1 + Var2, df, subset=!duplicated(df))
colSums(z)


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