Merging multiple text files in R with a constraint

Question

I have 10 text files containing 1000\'s of rows.

Example: First File:

V1  V2
1   2
2   3
10 20
1   4    
.....

Second file:

<

Answer 1

Here's one way to do it. I'll assume that you've already read your files into a list of data.frames, e.g. with L <- lapply(filenames, read.table). I'll simulate this L below.

# Create dummy data
L <- replicate(5, expand.grid(1:10, 1:10)[sample(100, 10), ], simplify=FALSE)

# Add a column to each data.frame in L. 
# This will indicate presence of the pair when we merge.
L <- lapply(seq_along(L), function(i) { 
  L[[i]][, paste0('DF', i)] <-  1
  L[[i]] 
})

# Merge all the things 
# (hat-tip to @Charles - https://stackoverflow.com/a/8097519/489704)
L.merged <- Reduce(function(...) merge(..., all=T), L)

head(L.merged)

#   Var1 Var2 DF1 DF2 DF3 DF4 DF5
# 1    1    2  NA  NA  NA   1  NA
# 2    1    5   1  NA  NA  NA   1
# 3    1    9  NA  NA  NA   1  NA
# 4    1   10  NA  NA   1   1  NA
# 5    2    5  NA  NA   1  NA  NA
# 6    2    6  NA   1  NA   1  NA

Easy enough to convert the NA to 0 if you want, e.g. with:

L.merged[is.na(L.merged)] <- 0

Relevant post: Merge multiple data frames in a list simultaneously