Efficient ways to multiply all columns in data frame with each other

Question

Assuming I have a dataframe consisting of three columns

set.seed(24)
df1 <- data.frame(a=runif(10),b=runif(10),c=runif(10))

And want to have one with six columns of all interactions:

a*a, a*b, a*c, b*c, b*b, c*c

The solution I'm looking for should work for any number of columns, not just three

Answer 1

Here is another option with combn where do the combination of column names taking two at a time, multiply the columns after subsetting and cbind with square of the original dataset.

res <- cbind(df1^2, do.call(cbind,combn(colnames(df1), 2, 
               FUN= function(x) list(df1[x[1]]*df1[x[2]]))))
colnames(res)[-(seq_len(ncol(df1)))] <-  combn(colnames(df1), 2, 
                 FUN = paste, collapse=":")
res
#            a           b           c        a:b        a:c         b:c
#1  0.08559952 0.365890531 0.008823729 0.17697473 0.02748285 0.056820059
#2  0.05057603 0.137444401 0.304984209 0.08337501 0.12419698 0.204739766
#3  0.49592997 0.451167798 0.525871254 0.47301970 0.51068123 0.487089495
#4  0.26925425 0.452905189 0.019023202 0.34920860 0.07156869 0.092820832
#5  0.43906475 0.102675746 0.049713853 0.21232357 0.14774167 0.071445132
#6  0.84721676 0.817486693 0.472890881 0.83221898 0.63296215 0.621757189
#7  0.07825199 0.039249934 0.005850588 0.05542008 0.02139673 0.015153719
#8  0.58342170 0.001953909 0.359676293 0.03376319 0.45808619 0.026509902
#9  0.64261164 0.250923183 0.397086073 0.40155468 0.50514566 0.315655035
#10 0.06488487 0.019260683 0.002174826 0.03535148 0.01187911 0.006472142