使用R将函数应用于数据框中两列的每个组合

Question

I want to apply a function myfoo to every possible combination of two columns in a dataframe mydf and get the result in a matrix format myoutput .

Consider the following dataframe,

# Example dataframe
mydf <- data.frame(var1 = 1:10, var2 = 11:20, var3 = 21:30)
head(mydf)

# var1 var2 var3
# 1    11   21
# 2    12   22
# 3    13   23
# 4    14   24
# 5    15   25

to which I want to apply the following function to every possible combination of two columns,

# Example function
myfoo <- function(varA, varB) sum(varA * varB)

myfoo(var1, var2)
# [1] 935

in order to get this output.

# Desired output
myoutput <- matrix(c(0, 935, 1485, 935, 0, 4035, 1485, 4035, 0), 3, dimnames = list(names(mydf), names(mydf)))
myoutput

#      var1 var2 var3
# var1    0  935 1485
# var2  935    0 4035
# var3 1485 4035    0

Answer 1

In your case I would convert to a matrix (no reason to keep it a data.frame when all columns are of numeric class) and just run the compiled crossprod function which does a matrix cross product.

m <- as.matrix(mydf)
res <- crossprod(m, m)
diag(res) <- 0 # You can probably skip that part
res
#      var1 var2 var3
# var1    0  935 1485
# var2  935    0 4035
# var3 1485 4035    0