from data table, randomly select one row per group

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By : Tuncai

Date : December 01 2020, 05:00 PM

around this issue 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'.

code :

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

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Randomly select a group of doubles

By : benyamin .p.m

Date : March 29 2020, 07:55 AM

this will help I'd use a Collection of Double, say an ArrayList rather than an array of double, and then call Collections.shuffle(myArrayList) on the ArrayList. Bingo, randomized. To get 10 of these values, simply call remove(0) on the list (after checking to make sure that the size is not 0).
This is not too different from shuffling a deck of cards and selecting 10 random cards.

randomly select a fixed number of rows in each group in SQL server table

By : Murat Ipek

Date : March 29 2020, 07:55 AM

like below fixes the issue You can use ROW_NUMBER and use NEWID() to generate a random ORDER:
EDIT: I replaced CHECKSUM(NEWID()) with NEWID() since I cannot prove which is faster and NEWID() is I think the most used.

code :

WITH CTE AS(
    SELECT *,
        RN = ROW_NUMBER() OVER(PARTITION BY id ORDER BY NEWID())
    FROM tbl
)
SELECT
    id, value1, value2
FROM Cte
WHERE RN <= 2

INSERT INTO yourNewTable(id, value1, value2)
    SELECT
        id, value1, value2
    FROM (
        SELECT *,
            RN = ROW_NUMBER() OVER(PARTITION BY id ORDER BY NEWID())
        FROM tbl
    )t
    WHERE RN <= 2

Flag randomly selected N rows by group in data.table

By : toli

Date : March 29 2020, 07:55 AM

like below fixes the issue At the data.table in column C3 I want to flag N randomly selected rows by each group (C1). There are several similar questions have already been asked on SO here, here and here. But based on the answers still cannot figure out how to find a solution for my task.

code :

dt[, C3 := 1:.N %in% sample(.N, min(.N, 2)), by = C1]

dt[, C3 := 1:.N %in% head(sample(.N), 2) , by = C1]

flagsz <- c(2, 1, 2, 3)
dt[, C3 := 1:.N %in% sample(.N, min(.N, flagsz[.GRP])), by = C1]

randomly ordering across groups (not within group) in data.table

By : wayne

Date : March 29 2020, 07:55 AM

may help you . Let's say I want to order the iris dataset (as a data.table) by Species, keeping observations grouped by species and randomly ordering across species. , Alternatively you could do:

code :

e <- d[, .N, Species]
e[, g2 := runif(.N)]
d <- e[, .(Species, g2)][d, on = 'Species']

Randomly select a row from each group using pandas

By : user1840460

Date : March 29 2020, 07:55 AM

it helps some times I have a pandas dataframe df which appears as following: , Use groupby with apply to select a row at random per group.

code :

np.random.seed(0)
df.groupby(['Month', 'Day'])['mnthShape'].apply(np.random.choice).reset_index()

   Month  Day  mnthShape
0      1    1   1.016754
1      1    2   0.963912
2      1    3   1.099451

np.random.seed(0)
(df.groupby(['Month', 'Day'])['mnthShape']
   .apply(pd.Series.sample, n=1)
   .reset_index(level=[0, 1]))

   Month  Day  mnthShape
2      1    1   0.963912
3      1    2   1.016754
6      1    3   1.016754