# Re: [R] Summing up diagonals w/o for-loop

From: Greg Snow <Greg.Snow_at_imail.org>
Date: Tue, 26 Feb 2008 13:48:10 -0700

Does this do what you want?

> d <- col(mat) - row(mat)
> tapply( mat, d, sum )

Hope this helps,

```--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow_at_imail.org
(801) 408-8111

> -----Original Message-----

> From: r-help-bounces_at_r-project.org
> [mailto:r-help-bounces_at_r-project.org] On Behalf Of Camarda,
> Carlo Giovanni
> Sent: Tuesday, February 26, 2008 1:32 PM
> To: r-help_at_stat.math.ethz.ch
> Subject: [R] Summing up diagonals w/o for-loop
>
> Dear R-users,
>
> is there any way to sum up the elements of the "diagonals" of
> a matrix without using a for-loop? While there is a simple
> way over rows and columns, I don't see a straightforward
> multiplication for the diagonals, am I too demanding? Or,
> more likely, I'm lack some algebra trick? Is there any
> R-function that can deal with this problem w/o loop?
>
> Actually I would need to sum up just the upper diagonals.
>
> Here a simple, but general, example for presenting the problem.
>
> Thanks in advance for any help,
> Carlo Giovanni Camarda
>
> m <- 7
> n <- 5
> mat <- matrix(1:35, n, m)
> ones.r <- rep(1,n)
> ones.c <- rep(1,m)
> # sum over the rows
> sum.r <- mat%*%ones.c
> # sum over the cols
> sum.c <- t(mat)%*%ones.r
> # sum over the diags
> sum.d <- numeric(m+n-1)
> sum.d[1] <- mat[n,1]
> sum.d[m+n-1] <- mat[1,m]
> for(i in 2:n){
>     sum.d[i] <- sum(diag(mat[(n+1-i):n,1:i])) } for(i in 2:(m-1)){
>     sum.d[i+n-1] <- sum(diag(mat[,i:m])) }
>
>
>
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