How to simplify nested conditional for loop in R?

Question

I have this for loop for creating a list of variables:

vars <- c( 'beta.o', 'sd.y') 
for (x in c('ghs', 'site', 'trt')) {
 if(model.parms[[x]] == 1) {
 data <- data[, which(names(data) != x)]
 } else {
 data <- data
 if(x!='ghs') {
 vars <- c(vars, paste('sd.', x, sep = ''))
 }
 m <- min(model.parms[[x]], 5)
 for (i in 1:m) {
 if(i == 1 && x == 'site') {
 vars <- c(vars, 'beta.site[1]')
 }
 if (i > 1) {
 vars <- c(vars, paste('beta.', x, '[', i, ']', sep=''))
 }
 }
 }
}

It has been bothering me terribly, and I have failed the last two times I have tried to replace it, although conceptually it should be able to be written in a few lines. Any advice?

Answer 1

This bit:

for (i in 1:m) {
 if(i == 1 && x == 'site') {
 vars <- c(vars, 'beta.site[1]')
 }
 if (i > 1) {
 vars <- c(vars, paste('beta.', x, '[', i, ']', sep=''))
 }
}

Says handle the first pass differently from all the others. So I would replace it with this:

if (x == 'site') {
 vars <- c(vars, 'beta.site[1]')
}
for (i in 2:m) {
 vars <- c(vars, paste('beta.', x, '[', i, ']', sep=''))
}

Answer 2

The big mistake in your code is you are dynamically growing your vectors. For example, compare

x = NULL 
for(i in 1:100000)
 x = c(x, i)

with

x = numeric(100000)
for(i in 1:100000) 
 x[i] = i 

The other key point is that paste function can be vectorised. So,

for (i in 1:m) {
 if(i == 1 && x == 'site') {
 vars <- c(vars, 'beta.site[1]')
 }
 if (i > 1) {
 vars <- c(vars, paste('beta.', x, '[', i, ']', sep=''))
 }
 }

can be replaced with

if(x == 'site') {
 vars <- c(vars, 'beta.site[1]')
 }
vars = c(vars, paste('beta.', x, '[', i, ']', sep=''))