Multiply unique pairs of values in a vector and sum the result
I want to multiply and then sum the unique pairs of a vector, excluding pairs from the same element, such that for c(1:4)
:
(1*2) + (1*3) + (1*4) + (2*3) + (2*4) + (3*4) == 35
The following code works for the above example:
x <- c(1:4)
bar <- NULL
for( i in 1:length(x)) { bar <- c( bar, i * c((i+1) : length(x)))}
sum(bar[ 1 : (length(bar) - 2)])
However, my actual data is a vector of rational numbers, not integers, so the loop part (i+1)
won't work. Is there a way to look at the next set item after i
eg. j
so what could I write i * c((j : length(x))
?
I understand that loops for
are generally not the most efficient approach, but I couldn't think of how to do this through apply
etc. Greetings are also examples. Thanks for your help.
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An alternative to looping would be to use combn
and multiply combinations using an argument FUN
. Then the sum
result is:
sum(combn(x = 1:4, m = 2, FUN = function(x) x[1] * x[2]))
# [1] 35
Better yet, use prod
in FUN
as suggested by @bgoldst:
sum(combn(x = 1:4, m = 2, FUN = prod))
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