version 1.0.0 cummean() function is providing weird results

[nsingh-phd]

For testing, I have this small vector x where the long(er) way is giving me the results as expected version 1.0.0 cummean() is not.

library(tidyverse)
x <- 1:5

# long(er) way
cumsum(x) / seq_along(x)
#> [1] 1.0 1.5 2.0 2.5 3.0

# dplyr 0.8.5 cummean()
cummean(x)
#> [1] 1.0 1.5 2.0 2.5 3.0

# dplyr 1.0.0 cummean()
cummean(x)
#> [1] 1.000000 1.000000 1.333333 1.750000 2.200000

^{Created on 2020-05-31 by the reprex package (v0.3.0)}

[brianrice2]

It looks like the indexing is off by one for dplyr_cummean in /src/funs.cpp, causing the first index to be repeated twice (and the last index to be dropped). I'll submit a pull request with a slight change which I think makes it work as intended.

[nsingh-phd]

Thanks @brianrice2 for requesting a quick fix.