merge/combine columns with same name but incomplete data

Question

I have two data frames that have some columns with the same names and others with different names. The data frames look something like this:

df1
      ID hel         


        

Answer 1

Here's an approach that involves melting your data, merging the molten data, and using dcast to get it back to a wide form. I've added comments to help understand what is going on.

## Required packages
library(data.table)
library(reshape2)

dcast.data.table(
  merge(
    ## melt the first data.frame and set the key as ID and variable
    setkey(melt(as.data.table(df1), id.vars = "ID"), ID, variable), 
    ## melt the second data.frame
    melt(as.data.table(df2), id.vars = "ID"), 
    ## you'll have 2 value columns...
    all = TRUE)[, value := ifelse(
      ## ... combine them into 1 with ifelse
      is.na(value.x), value.y, value.x)], 
  ## This is your reshaping formula
  ID ~ variable, value.var = "value")
#    ID hello world football baseball hockey soccer
# 1:  1     2     3       43        6      7      4
# 2:  2     5     1       24       32      2      5
# 3:  3    10     8        2       23      8     23
# 4:  4     4    17        5       15      5     12
# 5:  5     9     7       12       23      3     43