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.

Warning message:
In rbindlist(allargs) : NAs introduced by coercion

I was able to reproduce the error. Here's a code with which you should be able to reproduce the error.

# unique random names for column V1
set.seed(45)
n <- sapply(1:500, function(x) {
    paste(sample(c(letters[1:26]), 10), collapse="")
})
# generate some values for V2 and V3
dt <- data.table(V1 = sample(n, 30*500, replace = TRUE), 
                 V2 = sample(1:10, 30*500, replace = TRUE), 
                 V3 = sample(50:100, 30*500, replace = TRUE))
setkey(dt, "V1")

# No warning when providing column names (and right results)
dt[, list(s = sum(V2), m = mean(V3)),by=V1]

#              V1   s        m
#   1: acgmqyuwpe 238 74.97778
#   2: adcltygwsq 204 79.94118
#   3: adftozibnh 165 75.51515
#   4: aeuowtlskr 164 75.70968
#   5: ahfoqclkpg 192 73.20000
#  ---                        
# 496: zuqegoxkpi  93 77.95000
# 497: zwpserimgf 178 72.62963
# 498: zxkpdrlcsf 154 78.04167
# 499: zxvoaeflhq 121 75.34615
# 500: zyiwcsanlm 180 76.61290

# Warning message and results with NA
dt[, list(sum(V2), mean(V3)),by=V1]

#              V1  V1       V2
#   1: acgmqyuwpe 238 74.97778
#   2: adcltygwsq 204 79.94118
#   3: adftozibnh 165 75.51515
#   4: aeuowtlskr 164 75.70968
#   5: ahfoqclkpg 192 73.20000
#  ---                        
# 496: zuqegoxkpi  NA 77.95000
# 497: zwpserimgf  NA 72.62963
# 498: zxkpdrlcsf  NA 78.04167
# 499: zxvoaeflhq  NA 75.34615
# 500: zyiwcsanlm  NA 76.61290

Warning message:
In rbindlist(allargs) : NAs introduced by coercion

  • 1) It seems that this happens if you don't provide the column names.

  • 2) Even then, in particular, when V1 (or the column you use in by=) has a lot of unique

    r data.table
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