Question

I need to combine two data frames (df1 and df2) by matching up two site columns of each data frame to produce a third data frame (df3).

df1 = data.frame(Site.1=c("A","A","B"),
                 Site.2=c("B","C","C"),
                 Score1=c(60,70,80))
df1

 Site.1 Site.2 Score1
1      A      B     60
2      A      C     70
3      B      C     80

df2 = data.frame(Site.1=c("B","A","A"),
                 Site.2=c("C","B","C"),
                 Score2=c(10,20,30))
df2

  Site.1 Site.2 Score2
1      B      C     10
2      A      B     20
3      A      C     30

df3 = data.frame(Site.1=c("A","A","B"),
                 Site.2=c("B","C","C"),
                 Score1=c(60,70,80),
                 Score2=c(20,30,10))
df3

  Site.1 Site.2 Score1 Score2
1      A      B     60     20
2      A      C     70     30
3      B      C     80     10

Answer 1

You want the merge function. Since your column names that you want to match on already have the same name you don’t even need to do anything special. If that wasn’t the case you would want to look into the by.x and by.y parameters that merge takes.

df1 = data.frame(Site.1=c("A","A","B"),Site.2=c("B","C","C"),Score1=c(60,70,80))
df2 = data.frame(Site.1=c("B","A","A"),Site.2=c("C","B","C"), Score2=c(10,20,30))
df3 = data.frame(Site.1=c("A","A","B"),Site.2=c("B","C","C"), Score1=c(60,70,80),Score2=c(20,30,10))
df3

# Merge gives you what you want
merge(df1, df2)