[R] Generating Replicate Datasets (using loops or other means)

[VTLT1999]

Hello All,

I have searched many help forums, message boards, etc. and I just can't
apply the comments to what I need my program to do.  I am running R 2.5.1 on
an XP system, and my desire is to produce replicate datasets for a
simulation study I am running.  Essentially, I have sets of parameters (a's,
b's, and c's) that define a function which produces a decimal value.  This
value is compared to a random uniform value, and is coded a 1 if the
function is greater than the uniform value, 0 if it is <= to the uniform
value.  My code thus far works great, but I just need it to run several
times.    Here we go:

library(mvtnorm)
library(sm)
library(ltm)
library(irtoys)

k<- 5000
set.seed(271828)
t <-
rmvnorm(n=k,mean=c(-1,0,1),sigma=matrix(c(1,.8,.5,.8,1,.8,.5,.8,1),3,3)) 
  #Using mv here because of the likely association of ability (theta = t)
across time.

t1<-as.matrix(t[,1])
t2<-as.matrix(t[,2])
t3<-as.matrix(t[,3])

set.seed(271828)

#  Population item parameters (n=54)  from which we will select relevant
items
#  These are the parameters that are used in the function

a <- c(1.18120, 0.92613, 0.96886, 0.80503, 1.12384, 
       0.84073, 0.85544, 0.86801, 1.01054, 0.82278,
       1.10353, 0.78865, 0.98421, 1.76071, 0.89603, 
       0.84671, 0.89737, 0.74775, 0.32190, 0.69730, 
       0.72059, 1.16762, 1.29257, 1.32902, 0.59540, 
       0.51022, 0.59259, 0.93951, 0.68568, 0.55649, 
       0.88084, 0.52940, 0.45735, 0.57560, 1.11779,
       0.96984, 1.19692, 0.99102, 1.25847, 1.62555, 
       0.63049, 1.07807, 1.04897, 1.23138, 1.14014, 
       1.25230, 1.14844, 0.59287, 0.83143, 0.81723,
       0.52141, 0.61980, 0.49945, 1.02749)

b <- c(-2.51737, -1.95897, -1.72667, -0.82988, -0.36093,
        0.72554,  0.91442,  0.78061,  0.06088,  0.75733,
       -0.76371,  0.24552, -0.42050,  0.88232, -0.81761,
        0.06466, -0.43866, -0.46042,  0.21636, -0.73147,
       -1.44086, -1.03718,  0.07275, -0.17197,  1.53796,
       -0.45631, -1.69826, -0.66506,  0.98921,  0.30714,
       -0.62245,  0.97253,  1.95894,  0.21277,  1.96346,
        1.18825,  1.59917, -0.28401, -1.23530, -0.09671,
       -0.31581, -0.66149, -0.81284, -0.35399, -0.07623, 
        1.06442, -0.68559,  1.07591,  0.97458,  0.06436,
        1.25622,  1.73954,  1.75052,  2.34088)

c <- c(0.00000, 0.00000, 0.00000, 0.00000, 0.19648,
       0.31302, 0.26454, 0.19714, 0.06813, 0.21344,
       0.00000, 0.03371, 0.00000, 0.16581, 0.11054, 
       0.08756, 0.07115, 0.26892, 0.00000, 0.06883, 
       0.00000, 0.14815, 0.32389, 0.19616, 0.17597,
       0.00000, 0.00000, 0.04337, 0.19949, 0.20377, 
       0.00000, 0.06243, 0.13639, 0.00000, 0.18166,
       0.15996, 0.20184, 0.08331, 0.24453, 0.26114, 
       0.16434, 0.20750, 0.32658, 0.31870, 0.45227,
       0.35039, 0.31178, 0.17999, 0.22774, 0.21675,
       0.10153, 0.17764, 0.15205, 0.19858)

#  Item parameters for generating 3PL data for all five testing occasions:
#  This selects the relevant parameters for a particular data generation run
#  Only parameters for the first testing occasion are shown to save space

a1 <- as.matrix(a[c(1:5,15:20,22:24,38:44)])
b1 <- as.matrix(b[c(1:5,15:20,22:24,38:44)])
c1 <- as.matrix(c[c(1:5,15:20,22:24,38:44)])

#  Here is where I would like to begin my replications, but don't know how
to make R do it.
#  The code below produces a matrix of 0's and 1's (which will be used by
another program)
#  I would like to nest this in a "do loop" such that, say, 30 replicate
datasets are produced using the 
#    same parameters. 
   
   N <- nrow(t1)     # number of examinees
   n <- nrow(a1)     # number of items
   d <- 1.7
   theta <- t1  
   response <- matrix (0,N,n)  
   uni <- matrix (runif(N*n),nrow = N)

   for (i in 1:N)
   {
     for (j in 1:n) 
     {
      if ( c1[j]+(1-c1[j])/(1+exp(-d*a1[j]*(theta[i]-b1[j]))) > uni[i,j] )
         response[i,j] = 1
       else 
         response[i,j] = 0
     }
   }
write.table(response, file="C:/responses.dat", sep=" ",row.names=FALSE,
col.names=FALSE)  

I tried earlier nesting this in another for loop, but that indexes elements
of matrices and vectors, and doesn't seem to apply to a "global" loop
methodology.  I am attempting to use replicate as we speak, but
documentation is sparse (help("replicate") is nested in lapply information). 
Any guidance is greatly appreciated.  

Thanks in advance,

Jonathan Beard

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