Why does rbindlist not respect column names?

Question

I just discovered this bug, only to find that some people are calling it a \"feature\". This makes rbindlist NOT like do.call(\"rbind\",l) as

Answer 1

This feature is now implemented in commit 1266 of v1.9.3. From NEWS:

o  'rbindlist' gains 'use.names' and 'fill' arguments and is now implemented 
   entirely in C. Closes #5249    
  -> use.names by default is FALSE for backwards compatibility (doesn't bind by 
     names by default)
  -> rbind(...) now just calls rbindlist() internally, except that 'use.names' 
     is TRUE by default, for compatibility with base (and backwards compatibility).
  -> fill by default is FALSE. If fill is TRUE, use.names has to be TRUE.
  -> At least one item of the input list has to have non-null column names.
  -> Duplicate columns are bound in the order of occurrence, like base.
  -> Attributes that might exist in individual items would be lost in the bound result.
  -> Columns are coerced to the highest SEXPTYPE, if they are different, if/when possible.
  -> And incredibly fast ;).
  -> Documentation updated in much detail. Closes DR #5158.

With this, you can set use.names=TRUE to bind by names. It's set to FALSE by default for backwards compatibility. Alternatively, you can use rbind(..) where use.names=TRUE, again for backwards compatibility.

See this post for more examples and this post for benchmarks.

Examples:

1) Just set use.names=TRUE

DT1 <- data.table(x=1, y=2)
DT2 <- data.table(y=1, x=2)

rbindlist(list(DT1,DT2), use.names=TRUE, fill=FALSE)
#    x y
# 1: 1 2
# 2: 2 1

DT1 <- data.table(x=1, y=2)
DT2 <- data.table(z=2, y=1)

# returns error when fill=FALSE but can't be bound without fill=TRUE
rbindlist(list(DT1, DT2), use.names=TRUE, fill=FALSE)
# Error in rbindlist(list(DT1, DT2), use.names = TRUE, fill = FALSE) : 
    # Answer requires 3 columns whereas one or more item(s) in the input 
    # list has only 2 columns. ...

---

2) Also binds duplicate column names in the order of occurrence:

DT1 <- data.table(x=1, x=2, y=10, y=20, y=30)
DT2 <- data.table(y=-10, x=-2, y=-20, x=-1, y=-30)

rbindlist(list(DT1,DT2), use.names=TRUE)

#     x  x   y   y   y
# 1:  1  2  10  20  30
# 2: -2 -1 -10 -20 -30

---

3) use fill=TRUE if you want to bind by names and fill missing columns

DT1 <- data.table(x=1, y=2)
DT2 <- data.table(y=2, z=-1)

rbindlist(list(DT1, DT2), fill=TRUE)
#     x y  z
# 1:  1 2 NA
# 2: NA 2 -1

HTH