CODEWITHSUNDEEP
rdata.table
Daoudi Karim
Daoudi Karim

Reputation: 109

Get elements randomly retrieve from another table B with data table

I have a table A of data.table type which contains a set of elements, I would like to add a new mass column to this table, randomly retrieve from another table B. I would like to do this with the data.table package, but I don't know how to optimze it.

Table A

element
----------
silver
chlorine
silver
chlorine
chlorine
chlorine
silver

Table B

mass    element
------ 
0.3     silver
0.5     silver
1.6     silver
1.2     chlorine
5.3     chlorine
0.1     chlorine

Code to build tables:

tableA <- data.table(
  element = c("silver","chlorine","silver", "chlorine","chlorine","chlorine","silver")
)

tableB <- data.table(

  mass = c(0.3,0.5,1.6,1.2,5.3,0.1),
  element = c("silver","silver","silver", "chlorine","chlorine","chlorine")
)

The volume of data being very important I would like to use on the data.table package

Expected result:

    element     mass
-----------------------
    silver      1.6
    chlorine    5.3
    silver      0.3
    chlorine    1.2
    chlorine    1.2
    chlorine    0.1
    silver      1.6

This code returns me an error that can't fix, but do you think the approach is good and optimized?

tableA[, mass := tableB[sample(mass), on = .(element)]$mass]

Upvotes: 2

Views: 39

Answers (2)

langtang
langtang

Reputation: 24722

Here is an alternative data.table approach, if you are interested:

tableA[, mass:=sample(tableB[element==.BY$element,mass], .N, replace=T), by=element]

Output:

    element mass
1:   silver  0.3
2: chlorine  1.2
3:   silver  1.6
4: chlorine  5.3
5: chlorine  5.3
6: chlorine  5.3
7:   silver  0.5

Upvotes: 2

r2evans
r2evans

Reputation: 160447

Here's one way,

set.seed(42)
tableA[tableB[, list(mass = list(mass)), by = element], mass := sapply(i.mass, sample, size = 1), on = .(element)]
tableA
#     element  mass
#      <char> <num>
# 1:   silver   0.3
# 2: chlorine   1.2
# 3:   silver   0.3
# 4: chlorine   5.3
# 5: chlorine   5.3
# 6: chlorine   5.3
# 7:   silver   0.3

Upvotes: 4

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