R help - Mar 2008 - How to remove double for loop?

Hello everyone.

I use double for loops to fill in matrices, but there are surely
better (and computationally faster) ways to perform that task.
Could someone show me, given the following example of a double for
loop, how this could be done? It is much easier to learn by examples.

Val <- matrix(0, nrow=n+1, ncol=n+1)
for( i in 0:n){
    for(j in 0:i){
        Val[j+1, i+1] <- u^j*d^(i-j)
    }
}

Thank you in advance!

-- 
Jonas Malmros

One possibility might be

Val <- outer(0:n, 0:n, function(i, j) ifelse(i <= j, u^i*d^(j-i), 0)) 

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Jonas Malmros
Sent: Wednesday, 19 March 2008 9:24 PM
To: r-help at r-project.org
Subject: [R] How to remove double for loop?

Hello everyone.

I use double for loops to fill in matrices, but there are surely
better (and computationally faster) ways to perform that task.
Could someone show me, given the following example of a double for
loop, how this could be done? It is much easier to learn by examples.

Val <- matrix(0, nrow=n+1, ncol=n+1)
for( i in 0:n){
    for(j in 0:i){
        Val[j+1, i+1] <- u^j*d^(i-j)
    }
}

Thank you in advance!

-- 
Jonas Malmros

______________________________________________
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https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

Jonas Malmros wrote:
> 
> I use double for loops to fill in matrices, but there are surely
> better (and computationally faster) ways to perform that task.
> Could someone show me, given the following example of a double for
> loop, how this could be done? It is much easier to learn by examples.
> 
> Val <- matrix(0, nrow=n+1, ncol=n+1)
> for( i in 0:n){
>     for(j in 0:i){
>         Val[j+1, i+1] <- u^j*d^(i-j)
>     }
> }
> 
You could use outer. Val <- outer(vector1, vector2) does the
tensor product, meaning that each Val[i,j] will be 
vector1[i] * vector2[j].

In this case, instead of the product, you should compute the
expression:

f <- function(j,i) { u^j * d^(i-j) }

but only when j <= i. So, the function is:

f <- function(j,i) { ifelse(j > i, 0, u^j * d^(i-j) }

and the code becomes:

Val <- outer(0:n, 0:n, f)

"Entities should not be needlessly multplied" (Occam's Razor),
so let's do it without the explicit mention of f:

Val <- outer(0:n, 0:n, function(j,i) ifelse(j > i, 0, u^j*d^(i-j)))

(if u and d are positive integers, there might be a much
faster way of filling Val, using matrix multiplication and
rounding down)

Alberto Monteiro

Try this:

V <- diag(n+1); i <- row(V) - 1; j <- col(V) - 1
(j > i) * u ^ i * d ^ (j - i)

On Wed, Mar 19, 2008 at 7:24 AM, Jonas Malmros <jonas.malmros at
gmail.com> wrote:
> Hello everyone.
>
> I use double for loops to fill in matrices, but there are surely
> better (and computationally faster) ways to perform that task.
> Could someone show me, given the following example of a double for
> loop, how this could be done? It is much easier to learn by examples.
>
> Val <- matrix(0, nrow=n+1, ncol=n+1)
> for( i in 0:n){
>    for(j in 0:i){
>        Val[j+1, i+1] <- u^j*d^(i-j)
>    }
> }
>
> Thank you in advance!
>
> --
> Jonas Malmros
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>