# [R] Build a matrix from another matrix by specifying the indexes

Petr PIKAL petr.pikal at precheza.cz
Wed Feb 3 10:42:40 CET 2010

```Hi

r-help-bounces at r-project.org napsal dne 02.02.2010 21:32:16:

>
> ke te pehea koe Peter?
> I don't see how I could do this with a merge to be honest...

> mat<-matrix(sample(letters[1:3], 24, replace=T),6,4)
> mat
[,1] [,2] [,3] [,4]
[1,] "b"  "c"  "b"  "c"
[2,] "b"  "a"  "a"  "c"
[3,] "c"  "b"  "c"  "a"
[4,] "b"  "b"  "a"  "a"
[5,] "a"  "a"  "a"  "a"
[6,] "a"  "c"  "b"  "c"
> mat2<-matrix(sample(letters[1:5], 24, replace=T),6,4)
> mat2
[,1] [,2] [,3] [,4]
[1,] "c"  "a"  "d"  "c"
[2,] "c"  "b"  "c"  "b"
[3,] "c"  "c"  "a"  "d"
[4,] "c"  "b"  "d"  "c"
[5,] "e"  "c"  "c"  "e"
[6,] "a"  "b"  "d"  "c"
> df1<-data.frame(mat)
> df2<-data.frame(mat2)
> merge(df1, df2, all=T)
X1 X2 X3 X4
1   a  a  a  a
2   a  b  d  c
3   a  c  b  c
4   b  a  a  c
5   b  b  a  a
6   b  c  b  c
7   c  a  d  c
8   c  b  c  a
9   c  b  c  b
10  c  b  d  c
11  c  c  a  d
12  e  c  c  e
> merge(df1, df2, by=c("X1", "X2"), all=T)
X1 X2 X3.x X4.x X3.y X4.y
1   a  a    a    a <NA> <NA>
2   a  b <NA> <NA>    d    c
3   a  c    b    c <NA> <NA>
4   b  a    a    c <NA> <NA>
5   b  b    a    a <NA> <NA>
6   b  c    b    c <NA> <NA>
7   c  a <NA> <NA>    d    c
8   c  b    c    a    c    b
9   c  b    c    a    d    c
10  c  c <NA> <NA>    a    d
11  e  c <NA> <NA>    c    e

Is this what you want?

Regards
Petr

>
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