R help - Apr 2009 - 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.)

Thanks a lot,
Enda

	[[alternative HTML version deleted]]

If all you want is a count, then you can construct a logical vector
and count the TRUEs:

sum(a9 < (mean(a9) / 2))

On Sun, Apr 19, 2009 at 5:58 PM, Enda Hargaden <endahargaden at gmail.com>
wrote:
> 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.)
>
> Thanks a lot,
> Enda
>
> ? ? ? ?[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

On Apr 19, 2009, at 5:58 PM, Enda Hargaden wrote:
> 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.
See below. the 'if' control construct is not set up to accept vectors.
 From the help page ?"if"
if(cond) expr
Arguments
cond 	A length-one logical vector that is not NA.
>
>
> 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.)
Jim gave you a perfectly workable solution, but you might find it  
instructive to review the difference between "if" and the function  
"ifelse". Generally "if" will not be useful in situations
where you
are doing comparisons on indexed objects whereas that is precisely  
what "ifelse" is designed to do. (Tthis was also pointed out in a  
response to a question earlier today.)


David Winsemius, MD
Heritage Laboratories
West Hartford, CT