Warning message: in rbindlist(allargs) : NAs introduced by coercion: possible bug in data.table?

Question

While analysing some data, I came across the warning message, which I suspect to be a bug as it is a pretty straightforward command that I have worked with many times.

Answer 1

UPDATE : Now fixed in v1.8.9 by Ricardo

o rbind'ing data.tables containing duplicate, "" or NA column names now works, #2726 & #2384. Thanks to Garrett See and Arun Srinivasan for reporting. This also affected the printing of data.tables with duplicate column names since the head and tail are rbind-ed together internally.

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Yes, bug. Seems to be in the print method of data.tables with duplicated names.

ans = dt[, list(sum(V2), mean(V3)),by=V1]
head(ans)
           V1  V1       V2     # notice the duplicated V1
1: acgmqyuwpe 140 78.07692
2: adcltygwsq 191 76.93333
3: adftozibnh 153 73.82143
4: aeuowtlskr 122 73.04348
5: ahfoqclkpg 143 75.83333
6: ahtczyuipw 135 73.54167
tail(ans)
           V1  V1       V2
1: zugrnehpmq 189 72.63889
2: zuqegoxkpi 150 76.03333
3: zwpserimgf 180 74.81818
4: zxkpdrlcsf 115 72.57895
5: zxvoaeflhq 157 76.53571
6: zyiwcsanlm 145 72.79167
print(ans)
Error in rbindlist(allargs) : 
    (converted from warning) NAs introduced by coercion
rbind(head(ans),tail(ans))
Error in rbindlist(allargs) : 
    (converted from warning) NAs introduced by coercion

As a work around, don't create data.table with column names V1, V2 etc.

It's arising due to this known bug :

#2384 rbind of tables containing duplicate column names doesn't bind correctly

and I've added a link there back to this question.

Thanks!