Loop for merging multiple dataframes from list of dataframes in R

Question

I have multiple dataframes and there are couple of dfs having similar IDs. I want to merge those dfs if they have similar row count.

df1_ID1
ID b c d
1 x y z
1 y z x
1 z x y

df2_ID1
ID b c d
1 x y z
1 y z x
1 z x y

df3_ID2
ID b c d
2 x y z
2 y z x
2 z x y

what I want is

ID1
ID b c d b c d
1 x y z x y z
1 y z x y z x
1 z x y z x y

I'm trying something like this code:

for (i in 1:length(df)) {
  columnnames<-df[[i]][["b"]]
  if (df[[i]][["ID"]] == df[[i++]][["ID"]])
  merge.data.frame(df[[i]],df[[i++]], by = "ID")
}

Answer 1

I'm not a fan of how this ends up with multiple columns with the same name, but that's what you wanted.
You aren't really asking for a merge because that would give 3 x 3 = 9 rows, so I used cbind.
(I changed the name of the list of data.frames to df_list to avoid confusion)

df_list <- list(
  data.frame(ID = 1, b = c('x', 'y', 'z'), c = c('y', 'z', 'x'), d = c('z', 'x', 'y')),
  data.frame(ID = 1, b = c('x', 'y', 'z'), c = c('y', 'z', 'x'), d = c('z', 'x', 'y')),
  data.frame(ID = 2, b = c('x', 'y', 'z'), c = c('y', 'z', 'x'), d = c('z', 'x', 'y'))
  )

for (i in 1:(length(df_list) - 1)) {
  if (NROW(df_list[[i]]) == NROW(df_list[[i + 1]]) &&
      all(df_list[[i]]$ID == df_list[[i + 1]]$ID)) {
    df_list[[i]] <- cbind(df_list[[i]], df_list[[i + 1]][, -1])
    df_list[[i + 1]] <- list()
  }
}
df_list <- df_list[!sapply(df_list, function(x) NROW(x) == 0)]

df_list
[[1]]
  ID b c d b c d
1  1 x y z x y z
2  1 y z x y z x
3  1 z x y z x y

[[2]]
  ID b c d
1  2 x y z
2  2 y z x
3  2 z x y