[R] extracting one element of correlation matrices from a list poroduced by the 'by' statement

From: Mohammad A. Chaudhary <mchaudha_at_jhsph.edu>
Date: Thu 14 Apr 2005 - 04:28:17 EST


I am producing 2X2 correlation matrices by a class variable. I need to extract a vector of correlation coefficients only. I am doing that in a loop (see below) but I am sure there would be a simpler way. Please help!  

> by(d1[,c(2,3)],d1[,1],cor)

d1[, 1]: 1

          c e

c 1.0000000 0.1972309

e 0.1972309 1.0000000



d1[, 1]: 2

          c e

c 1.0000000 0.2469402

e 0.2469402 1.0000000



d1[, 1]: 3

          c e

c 1.0000000 0.3177058

e 0.3177058 1.0000000



d1[, 1]: 4

          c e

c 1.0000000 0.3492043

e 0.3492043 1.0000000



d1[, 1]: 5

          c e

c 1.0000000 0.3385547

e 0.3385547 1.0000000



d1[, 1]: 6

          c e

c 1.0000000 0.2876410

e 0.2876410 1.0000000



d1[, 1]: 7

          c e

c 1.0000000 0.3374766

e 0.3374766 1.0000000



d1[, 1]: 8

         c e

c 1.000000 0.438019

e 0.438019 1.000000



d1[, 1]: 9

          c e

c 1.0000000 0.3452468

e 0.3452468 1.0000000



d1[, 1]: 10

          c e

c 1.0000000 0.3746597

e 0.3746597 1.0000000


> t<- rep(0,10)

> ind=0

> for(r in 1:10) {

+ ind=ind+1

+ t[ind] <- by(d1[,c(2,3)],d1[,1],cor)[[r]][1,2]

+                } 

> t

[1] 0.1972309 0.2469402 0.3177058 0.3492043 0.3385547 0.2876410 0.3374766 0.4380190 0.3452468 0.3746597

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