[R] case study of efficient R coding

[Huntsinger, Reid]

t(X)%*%X or better crossprod(X) is the sum of outer products
of the rows of X. The (i,j) entry is the sum of X[k,i]*X[k,j]
over k, i.e., the (i,j) entry of the outer product of X[k, ]
with itself.

Reid Huntsinger

-----Original Message-----
From: Jason Liao [mailto:jg_liao at yahoo.com]
Sent: Tuesday, October 15, 2002 12:38 PM
To: r-help at stat.math.ethz.ch
Subject: [R] case study of efficient R coding


[...]
But first, does anyone know a vectorized way of coding (summing the
outter product of rows)

p = 10;
x = rnorm(p*p);
dim(x) = c(p.p);

var = array(0, c(p,p));
for(i in 1:p) var = var + x[i,]%*% t(x[i,]);

=====
Jason G. Liao, Ph.D.
Division of Biometrics
University of Medicine and Dentistry of New Jersey
335 George Street, Suite 2200
New Brunswick, NJ 08903-2688
phone (732) 235-8611, fax (732) 235-9777
http://www.geocities.com/jg_liao

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