# Re: [R] Help with loops and how R stores data

From: Ted Harding <Ted.Harding_at_manchester.ac.uk>
Date: Mon, 04 Feb 2008 08:18:12 -0000 (GMT)

On 02-Feb-08 21:16:41, R-novice wrote:
>
> I am trying to make an array c(3,8) that contains the averages of what
> is in another array c(9,8). I want to average rows 1:3, 4:6, 7:9 and
> have a loop replace the generic 1:24 values in my array with the
> average of the three rows.
>
> The problem I am having is that R only replaces the last value from the
> loops, and I am not experienced enough with R to know how it stores
> data when looping and how to overcome this problem. By having it print
> the avg, I was expecting to see 24 numbers, but it only gives me one.
>
> Here is my code.
>
> cts.sds<-array(1:24,c(3,8))
> for(i in 3){
> for(j in 8){
> avg<-sum(exprdata[3*i-2:3*i,j]/3)
> cts.sds[i,j]<-avg
> print(avg)
> }
> }
> print(cts.sds)
>
> Any help with this pesky matter will be greatly appreciated.

I think you have made two types of mistake here.

1. You wrote "for(i in 3)" and "for(i in 3)". This means that 'i' will take only one value, namely 3; and 'j' will take only one value, namely 8. If you wanted to loop over i = 1,2,3 and j=1,2,3,4,5,6,7,8 then you should write

for(i in (1:3)){ for(j in (1:8){ ... }}

2. This is more subtle, and you're far from the only person to have tripped over this one.

Because of the order of precedence of the operators, in the epression

3*i-2:3*i

for each value of 'i' it will first evaluate "2:3", namely {2,3}, and then evaluate the remainder. So, for say i=2, you will get two numbers 3*i-2*i = 2 and 3*i-3*i = 0, whereas what you presumably intended was (3*i-2):(3*i) = 4:6 = {4,5,6}.

The way to avoid this trap is to use parantheses to force the order of evaluation you want:

avg<-sum(exprdata[(3*i-2):(3*i,j)]/3)

Example:
i<-2
3*i-2:3*i
# [1] 2 0
(3*i-2):(3*i)
# [1] 4 5 6

The reason is that the ":" operator takes precedence over the binary operators "*", "+" and "-".

Enter

?Syntax

to get a listing of the various operators in order of precedence.

Hoping this helps,
Ted.

E-Mail: (Ted Harding) <Ted.Harding_at_manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861
```Date: 04-Feb-08                                       Time: 08:18:05
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