R help - Oct 2004 - Selecting from a character vector with logical operators

I'm trying to select a subset from character vector using the logical
operator >, but get the following error

6356     85     SOL   1     25     1   38E6
6357     85     SOL   1     27     1   38E6
6910     95     SOL   1     25     1   38E6
7152     98     SOL   1     19     1   38E6
7153     98     SOL   1     22     2   38E6
7154     98     SOL   1     28     1   38E6
> ca11c93SOL1VIIa<-ca11c93SOL1[ca11c93SOL1$RECTAN>"32Z9",]
Warning message: 
">" not meaningful for factors in: Ops.factor(ca11c93SOL1$RECTAN,
"32Z9")

(note RECTAN is the name assigned to the 7 th column)

Do these operators act on chacters vectors as opposed to numeric vectors and if
so what are the rules for this?

Regards,

Robert Brown

With a character vector it works fine, but your ca11c93SOL1$RECTAN 
seems to be a factor.

 > letters[letters > "j"]
  [1] "k" "l" "m" "n" "o"
"p" "q" "r" "s" "t"
"u" "v" "w" "x" "y"
"z"

 > letfact <- as.factor(letters)
 > letfact[letfact > "j"]
  [1] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA>
[16] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA> <NA> <NA>
Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
Warning message:
">" not meaningful for factors in: Ops.factor(letfact,
"j")

Robert Brown FM CEFAS wrote:
> I'm trying to select a subset from character vector using the logical
operator >, but get the following error
> 
> 6356     85     SOL   1     25     1   38E6
> 6357     85     SOL   1     27     1   38E6
> 6910     95     SOL   1     25     1   38E6
> 7152     98     SOL   1     19     1   38E6
> 7153     98     SOL   1     22     2   38E6
> 7154     98     SOL   1     28     1   38E6
> 
>>ca11c93SOL1VIIa<-ca11c93SOL1[ca11c93SOL1$RECTAN>"32Z9",]
> 
> Warning message: 
> ">" not meaningful for factors in:
Ops.factor(ca11c93SOL1$RECTAN, "32Z9")
> 
> (note RECTAN is the name assigned to the 7 th column)
> 
> Do these operators act on chacters vectors as opposed to numeric vectors
and if so what are the rules for this?
> 
> Regards,
> 
> Robert Brown
> 
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-- 
Chuck Cleland, Ph.D.
NDRI, Inc.
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New York, NY 10010
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Chuck Cleland wrote:
>   With a character vector it works fine, but your ca11c93SOL1$RECTAN 
> seems to be a factor.
> 
>  > letters[letters > "j"]
>  [1] "k" "l" "m" "n" "o"
"p" "q" "r" "s" "t"
"u" "v" "w" "x" "y"
"z"
> 
>  > letfact <- as.factor(letters)
>  > letfact[letfact > "j"]
>  [1] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA> <NA> <NA> <NA> <NA>
> <NA> <NA>
> [16] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA> <NA> <NA>
> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
> Warning message:
> ">" not meaningful for factors in: Ops.factor(letfact,
"j")
You need an ordered factor for this to work:

 > letfact <- as.ordered(letters)
 > letfact[letfact > "j"]
  [1] k l m n o p q r s t u v w x y z
26 Levels: a < b < c < d < e < f < g < h < i < j <
k < l < m < n < o <
... < z

Gav

-- 
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Gavin Simpson                     [T] +44 (0)20 7679 5522
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One can, of course, convert the factor to character as: 
> letfac <- factor(letters)
> letfac[as.character(letfac)>"j"]
 [1] k l m n o p q r s t u v w x y z
Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z

	  Alternatively, one can select from the levels:  
> letfac[levels(letfac)[letfac]>"j"]
 [1] k l m n o p q r s t u v w x y z
Levels: a b c d e f g h i j k l m n o p q r s t u v w x y
z
> 
	  hope this helps.  spencer graves 

Gavin Simpson wrote:
> Chuck Cleland wrote:
>
>>   With a character vector it works fine, but your ca11c93SOL1$RECTAN 
>> seems to be a factor.
>>
>>  > letters[letters > "j"]
>>  [1] "k" "l" "m" "n"
"o" "p" "q" "r" "s"
"t" "u" "v" "w" "x"
"y" "z"
>>
>>  > letfact <- as.factor(letters)
>>  > letfact[letfact > "j"]
>>  [1] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA> <NA> <NA> <NA> <NA>
>> <NA> <NA>
>> [16] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA> <NA> <NA>
>> Levels: a b c d e f g h i j k l m n o p q r s t u v w x y z
>> Warning message:
>> ">" not meaningful for factors in: Ops.factor(letfact,
"j")
>
>
> You need an ordered factor for this to work:
>
> > letfact <- as.ordered(letters)
> > letfact[letfact > "j"]
>  [1] k l m n o p q r s t u v w x y z
> 26 Levels: a < b < c < d < e < f < g < h < i < j
< k < l < m < n < o <
> ... < z
>
> Gav
>
-- 
Spencer Graves, PhD, Senior Development Engineer
O:  (408)938-4420;  mobile:  (408)655-4567