rbindlist data.tables with different number of columns

Question

I am wondering how do I rbindlist data tables with different number of columns, and filling up empty rows with NAs like rbind.fill

 DT1 <- data.table(A = 1:3)
 DT2 <- data.table(A  =4:5, B = letters[4:5])
 l <- list(DT1, DT2)
 rbindlist(l)
 #  Error in rbindlist(l) : 
 #   Item 2 has 2 columns, inconsistent with item 1 which has 1 columns

What I want to get is

   A B
1: 1 NA
2: 2 NA
3: 3 NA
4: 4 d
5: 5 e

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.

Check this post for benchmarks.

---

Examples:

1) Using fill argument of rbindlist:

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

Note that when fill=TRUE, use.names should be TRUE.

---

2) Binding tables with duplicate names appropriately:

DT1 <- data.table(x=1, x=2, y=1, y=2)
DT2 <- data.table(y=3, y=-1, y=-2)

rbindlist(list(DT1, DT2), fill=TRUE)
#     x  x y  y  y
# 1:  1  2 1  2 NA
# 2: NA NA 3 -1 -2

---

3) It's not limited to just data.tables, but works on data.frames and lists as well:

DT1 <- data.table(x=1, y=2)
DT2 <- data.frame(y=2, z=-1)
DT3 <- list(z=10)

rbindlist(list(DT1,DT2,DT3), fill=TRUE)

#     x  y  z
# 1:  1  2 NA
# 2: NA  2 -1
# 3: NA NA 10

---

4) If you would like to bind just by names, you can set just use.names=TRUE, but not fill:

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. ...

---

5) The default is the same for backwards compatibility (use.names=FALSE, fill=FALSE):

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

rbindlist(list(DT1, DT2))

#    x y
# 1: 1 2
# 2: 1 2

HTH