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Asked: 2026-06-17T19:32:10+00:00

I have two data.tables, main and metrics , both keyed by cid I want

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I have two data.tables, main and metrics, both keyed by cid
I want to add to table main the average of each of several values located in metrics.

However, I would like to filter by code, only averaging those rows in metrics with a given code. 

> metrics
    cid code  DZ value1 value2
1: 1001    A 101      8     21
2: 1001    B 102     11     26
3: 1001    A 103     17     25
4: 1002    A 104     25     39
5: 1002    B 105      6     30
6: 1002    A 106     23     40
7: 1003    A 107     27     32
8: 1003    B 108     16     37
9: 1003    A 109     14     42

# DESIRED OUTPUT
> main
    cid  A.avg.val1   A.avg.val2    B.avg.val1      B.avg.val2    
1: 1001    12.5         23.0            11              26                      
2: 1002    24.0         39.5             6              30            
3: 1003    20.5         37.0            16              37            



#  SAMPLE DATA
set.seed(1)
main <- data.table(cid=1e3+1:3, key="cid")
metrics <- data.table(cid=rep(1e3+1:3, each=3), code=rep(c("A", "B", "A"), 3), DZ=101:109, value1=sample(30, 9), value2=sample(20:50, 9), key="cid")
code.filters <- c("A", "B")

These lines get the desired output, but I am having difficulty assigning the new col back into main. (also, doing it programatically would be preferred). 

main[metrics[code==code.filters[[1]]]][,  list(mean(c(value1))), by=cid]
main[metrics[code==code.filters[[1]]]][,  list(mean(c(value2))), by=cid]
main[metrics[code==code.filters[[2]]]][,  list(mean(c(value1))), by=cid]
main[metrics[code==code.filters[[1]]]][,  list(mean(c(value2))), by=cid]

Additionally, can someone explain why the following line only takes the last value in each group? 

main[metrics[ code=="A"],  A.avg.val1 := mean(c(value1))]
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1 Answer

  1. Editorial Team

    working off of @Arun’s answer, the following gets the desired results: 

    invisible( 
sapply(code.filters, function(cf)
    main[metrics[code==cf, list(avgv1 = mean(value1), avgv2 = mean(value2)), by=cid],
      paste0(cf, c(".avg.val1", ".avg.val2")) :=list(avgv1, avgv2)]
))

> main
    cid A.avg.val1 A.avg.val2 B.avg.val1 B.avg.val2
1: 1001       12.5       23.0         11         26
2: 1002       24.0       39.5          6         30
3: 1003       20.5       37.0         16         37
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