setnames() accepts function for old= and new=

[shrektan]

Closes #3703

library(data.table)
DT = data.table(a=1:3, b=4:6, c=7:9)
setnames(DT, base::toupper)
names(DT)
#> [1] "A" "B" "C"
setnames(DT, c('B','C'), function(x) sprintf('W_%s_W', x))
names(DT)
#> [1] "A"     "W_B_W" "W_C_W"
DT = data.table(a=1:3, b=4:6, c=7:9)
# support numeric old as well
setnames(DT, 1, toupper)
names(DT)
#> [1] "A" "b" "c"
setnames(DT, -1, toupper)
names(DT)
#> [1] "A" "B" "C"

^{Created on 2019-09-12 by the reprex package (v0.3.0)}

[codecov]

Codecov Report

Merging #3860 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3860      +/-   ##
==========================================
+ Coverage   99.42%   99.42%   +<.01%     
==========================================
  Files          71       71              
  Lines       13423    13425       +2     
==========================================
+ Hits        13346    13348       +2     
  Misses         77       77

Impacted Files	Coverage Δ
R/data.table.R	100% <100%> (ø)	⬆️

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