What does the double percentage sign mean

Question

What is the purpose of the double percent (percent percent) in R?

It appears that when you use it, it divides the number in front by the number in back as many times as it can and returns the remainder. Is that accurate?

When would this be useful, just out of curiosity?

Answer 1

According to the "Arithmetic operators" help page (accessible via?" percent percent "),

'x mod y' is indicated by 'percent percent'.

which is only helpful if you've done enough programming to know that this is referring to modular division, i.e. integer-divide x by y and return the remainder. This is applicable in a wide range of situations. For example (from @GavinSimpson in comments), %% is useful if you are running a loop and want to print some kind of progress indicator to the screen every nth iteration (e.g. use if (i %% 10 == 0) { #do something} to do something every 10th iteration).

Since %% also works for floating-point numbers in R, I've just dug up an example where if (any(wts %% 1 != 0)) is used to test where any of the wts values are non-integer.