= Combining Data Frames =

<<TableOfContents(3)>>

== rbind() ==
   * rbind() can be used to combine two dataframes (or matrices) in the sense of adding rows, the column names and types must be the same for the two objects
{{{#!highlight r
> x <- data.frame(id=1:3,score=rnorm(3))
> y <- data.frame(id=13:15,score=rnorm(3))
> rbind(x,y)
id       score
1  1  0.71121163
2  2 -0.62973249
3  3  1.17737595
4 13 -0.45074940
5 14 -0.01044197
6 15 -1.05217176
}}}
== cbind() ==
   * cbind() can be used to combine two dataframes (or matrices) in the sense of adding columns, the number of rows must be the same for the two objects
{{{#!highlight r
> cbind(x,y)
id      score1      score2     score3
1  1  0.11440705  0.14536778 -1.1773241
2  2 -1.62862651  0.02020604  0.5686415
3  3  0.05335811  0.25462270  0.8844987
4  4 -0.19931734  0.15625511  0.9287316
5  5 -1.15217836 -1.79804503 -0.7550234
}}}
   * it is not recommended to use cbind() to combining data frames
== merge() ==
   * merge() is the command of choice for merging or joining data frames
   * it is the equivalent of join in sql
   * there are four cases
      * inner join
      * left outer join
      * right outer join
      * full outer join
{{{#!highlight r
> (d1 <- data.frame(id=LETTERS[c(1,2,3)],day1=sample(10,3)))
id day1
1  A    3
2  B    4
3  C    5
> (d2 <- data.frame(id=LETTERS[c(1,3,5,6)],day2=sample(10,4)))
id day2
1  A    7
2  C   10
3  E    3
4  F    6
}}}
=== inner join ===
   * inner join means: keep only the cases present in both of the data frames
{{{#!highlight r
> merge(d1,d2)
id day1 day2
1  A    3    7
2  C    5   10
}}}
=== left outer join ===
   * left outer join means: keep all cases of the left data frame no matter if they are present in the right data frame (all.x=T)
{{{#!highlight r
> merge(d1,d2,all.x = T)
id day1 day2
1  A    3    7
2  B    4   NA
3  C    5   10
}}}
=== right outer join ===
   * right outer join means: keep all cases of the right data frame no matter if they are present in the left data frame (all.y=T)
{{{#!highlight r
> merge(d1,d2,all.y = T)
id day1 day2
1  A    3    7
2  C    5   10
3  E   NA    3
4  F   NA    6
}}}
=== full outer join ===
   * full outer join means: keep all cases of both data frames (all=T)
{{{#!highlight r
> merge(d1,d2,all = T)
id day1 day2
1  A    3    7
2  B    4   NA
3  C    5   10
4  E   NA    3
5  F   NA    6
}}}

   * if not stated otherwise R uses the intersect of the names of both data frames, in our case only \textit{id}
   * you can specify these columns directly by \texttt{by=c("colname1","colname2")} if the columns are named identical or
   * using\\ \texttt{by.x=c("colname1.x","colname2.x"),
=== merge() Exercise ===
   * now read in the file personendaten.txt using the appropriate command
   * join the demographics with our pre1 data frame (even though it does not make sense now)
=== merge() Solution ===
{{{#!highlight r
> persdat <- read.table("../session1/session1data/personendaten.txt",
+                       sep="\t",
+                       header=T)
> pre1 <- merge(persdat,pre1,all.y = T)
> head(pre1)
Subject Sex Age_PRETEST Trial Event.Type Code   Time TTime Uncertainty
1  PRE001   f        3.11     7   Response    2 178963 10009           1
2  PRE001   f        3.11    12   Response    1 238680  8342           1
3  PRE001   f        3.11    17   Response    2 297789  8066           1
4  PRE001   f        3.11    22   Response    1 351321 10811           1
5  PRE001   f        3.11    27   Response    2 403607   713           1
6  PRE001   f        3.11    32   Response    1 467793 23709           1
Duration Uncertainty.1 ReqTime ReqDur Stim.Type Pair.Index    Type Event.Code
1    10197             2       0   next incorrect          7 Picture   RO09.jpg
2     8398             2       0   next incorrect         12 Picture   RO20.jpg
3     8198             2       0   next       hit         17 Picture   RS28.jpg
4    10997             2       0   next       hit         22 Picture   AT26.jpg
5      800             2       0   next       hit         27 Picture   RS23.jpg
6    23794             2       0   next       hit         32 Picture   OF04.jpg
}}}