# Re: [Rd] [R] Help! What does this R command mean?

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Wed 01 Feb 2006 - 00:57:53 GMT

On 1/31/2006 4:55 PM, Gabor Grothendieck wrote: > I think that a pointer to ?formula needs to be added to ?":"

Good suggestion; I'll do it.

Duncan Murdoch

```>
>
> On 1/31/06, Liaw, Andy <andy_liaw@merck.com> wrote:
```

>> ":" in a formula is not the same as ":" otherwise!
>>
>> Andy
>>
>> From: Ionut Florescu
>>> a:b means - all the element in the vector from a to b
>>> a[,-1] means for the matrix a keep all the rows but not the last or
>>> first -can't remember column.
>>> When in doubt do what I do make a small matrix and apply the
>>> command see
>>> what it does.
>>> After this:
>>> > a=matrix(c(1:9),3,3)
>>> > a[,-1]
>>> [,1] [,2]
>>> [1,] 4 7
>>> [2,] 5 8
>>> [3,] 6 9
>>>
>>> you see that -1 eliminates the first column.
>>> I found the R manual useless myself.
>>> The only thing useful is the search function in the html
>>> help. That has
>>> examples.
>>>
>>> Ionut Florescu
>>>
>>> Michael wrote:
>>>> Hi all,
>>>>
>>>> R is so difficult. I am so desperate.
>>>>
>>>> What does the ":" mean in the following statement?
>>>>
>>>> What does the "[, -1]" mean?
>>>>
>>>>
>>>>> # Leaps takes a design matrix as argument: throw away the intercept
>>>>> # column or leaps will complain
>>>>>
>>>>> X <- model.matrix(lm(V ~ I + D + W +G:I + P + N,
>>> election.table))[,-1]
>>>> Thanks a lot!
>>>>
>>>> [[alternative HTML version deleted]]
>>>>
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>>>>
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```>