from data table, randomly select one row per group

by

OP provided only a single column in the example. Assuming that there are multiple columns in the original dataset, we group by ‘z’, sample 1 row from the sequence of rows per group, get the row index (.I), extract the column with the row index ($V1) and use that to subset the rows of ‘dt’.

dt[dt[ , .I[sample(.N,1)] , by = z]$V1]

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