r - How to sample a dataframe column within a group without reordering of rows? -


i have r dataframe d2 2 columns i.e., "class" , "entry". "class" categorical variable , "entry" continuous given by

d2     class entry 1      1     1 2      2     2 3      3     3 4      4     4 5      1     5 6      2     6 7      3     7 8      4     8 9      1     9 10     2    10 11     3    11 12     4    12 13     1    13 14     2    14 15     3    15 16     4    16 17     1    17 18     2    18 19     3    19 20     4    20

i want sample "entry" column within group given as

ddply(d2,.(class),function(x) x[sample(nrow(x)),])     class entry 1      1     9 2      1     1 3      1     5 4      1    13 5      1    17 6      2    14 7      2    10 8      2     2 9      2     6 10     2    18 11     3    15 12     3    19 13     3     7 14     3     3 15     3    11 16     4    16 17     4    20 18     4    12 19     4     8 20     4     4

it visible desired results obtained order of "class" variable changed compared original data d2. want same output without reordering rows of "class" variable of d2. thanks

here 1 option data.table

library(data.table) setdt(d2)[, entry := sample(entry), = class]

or use ave base r

d2$entry <- with(d2, ave(entry, class, fun = sample))

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