Including a vector element in an if statement

Hi all,

I've searched high and low on this and found nothing of help. I'm  
using
v2.6.2 and trying to write a function that will count how many  
people from a
dataset fall under a poverty line of 50% of the mean income.

a9 is my 100-element vector of incomes. I want pa9 to be my vector  
that
counts how many of these are classed as in poverty.

My problem is that my command if(a9[i] < mean(a9)/2 ) returns an  
"argument
is of length zero" error. But a9[i] is not really of length zero; if I
remove the if() command and simply set pa9[i] = a9[i] in the loop it  
comes
out fine.

I don't think it's a problem with the type of variable either,  
because if I
multiply the vector by 2 I get the "correct" output, so I think it's a
problem with R interpreting my vector as a list of strings or  
anything.

povertyline = function()
{
pa9 = c()
i=0
while(i<=length(a9))
{
   if(a9[i] < mean(a9)/2)
   {
   pa9[i] = a9[i]
   }
i = i+1
}
return(length(pa9))
}

Has anyone any ideas what's going wrong? (Just for clarification:  
yes, this
is for my homework. However I've already done the assignment in  
Excel and
I'm repeating it to improve my R'ing.)