# [R] creating upper triangular matrix

Carl Witthoft carl at witthoft.com
Mon Nov 18 14:04:56 CET 2013

```OK, I'm pre-coffee, but what's wrong with using upper.tri to create a new
matrix and then multiplying that matrix by the original "dat" matrix (direct
multiplication, not matrix multiply) to get the desired answer?

Bert Gunter wrote
> I believe matrix indexing makes Arun's complex code wholly unnececessary:
>
> Starting with dat1 as above:
>
> m <- matrix(0,4,4)
> m[as.matrix(dat1[,1:2])] <- dat1[,3]
>
> ## yielding:
> m
>
>      [,1] [,2] [,3] [,4]
> [1,]    0    2    1    1
> [2,]    0    1    2    1
> [3,]    0    0    0    2
> [4,]    0    0    0    0
>
> If you want to get rid of any nonzero diagonal entries:
>
> diag(m) <- 0 ## does it.
>
> Cheers,
> Bert
>
>
>
>
>
> On Sun, Nov 17, 2013 at 10:27 AM, arun <

> smartpink111@

> > wrote:
>> Hi,
>> May be this helps:
>>
>> data data freq
>> 1       2     2
>> 1       3     1
>> 1       4     1
>> 2       3     2
>> 2       4     1
>> 2       2     1
>> val<- unique(c(dat1[,1],dat1[,2]))
>> dat2 <-expand.grid(data=val,data.1=val)
>> library(plyr)
>> library(reshape2)
>> res <- dcast(join(dat2,dat1),data~data.1,value.var="freq",fill=0)
>> row.names(res) <- res[,1]
>> res1 <- as.matrix(res[,-1])
>> diag(res1) <-0
>>
>> #or
>>  m1 <- matrix(0,length(val),length(val),dimnames=list(val,val))
>>
>>  indx1 <- outer(colnames(m1),rownames(m1),paste,sep="")
>>  indx2 <- paste0(dat1[,1],dat1[,2])
>> m1[match(indx2,indx1)] <- dat1[,3]
>>  diag(m1) <- 0
>>  m1
>> #  1 2 3 4
>> #1 0 2 1 1
>> #2 0 0 2 1
>> #3 0 0 0 2
>> #4 0 0 0 0
>>
>> A.K.
>>
>>
>> Hello ,
>> I am working on a project ,
>> i need to create an upper triangular matrix from the data in this form;
>> data data freq
>> 1       2     2
>> 1       3     1
>> 1       4     1
>> 2       3     2
>> 2       4     1
>> 2       2     1
>> 3      4       2
>>
>>  to a triangular matrix in the following form :
>>      1   2    3   4
>> 1    0  2    1   1
>> 2    0  0    2   1
>> 3    0  0    0   2
>> 4    0  0    0   0
>>
>>
>>
>> ______________________________________________
>>

> R-help@

>  mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>
>
> --
>
> Bert Gunter
> Genentech Nonclinical Biostatistics
>
> (650) 467-7374
>
> ______________________________________________

> R-help@

>  mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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