Use merge() to update a data frame with values from a second data frame

by

The optimal solution using data.table

library(data.table)
setDT(foo)
setDT(bar)
foo[bar, on="index", value:=i.value]
foo
#   index value
#1:     a   100
#2:     b   101
#3:     c   200
#4:     d   201

first argument in [ data.table method is named i thus we can refer to column from table in i argument using i. prefix.

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