R help - Nov 2004 - escaping backslash in a string

How can I get a single backslash in a character string?

My goal is to escape dots in a string that will be used as a regular
expression. I thought I could do it this way:

gsub(".", "\\.", x)

Unfortunately, "\\" does not represent a literal backslash as I
expected, but rather a pair of backslashes:
> "\\."
[1] "\\."
> "\\"
[1] "\\"

Just a backslash and a dot fails too, since that represents an escaped dot:
> "\."
[1] "."

A single backslash works in the middle of strings sometimes,but it
depends on what the character following it is (presumably depending on
whether the pair of characters represents an escape sequence):
> "a\b"
[1] "a\b"
> "x\y"
[1] "xy"

Is there a way to represent "\"? This seems like a design problem in
the interpreter.
> R.version
         _                    
platform i386-redhat-linux-gnu
arch     i386                 
os       linux-gnu            
system   i386, linux-gnu      
status                        
major    2                    
minor    0.1                  
year     2004                 
month    11                   
day      15                   
language R

Dan Lipsitt <danlipsitt at gmail.com> writes:
> How can I get a single backslash in a character string?
> 
> My goal is to escape dots in a string that will be used as a regular
> expression. I thought I could do it this way:
> 
> gsub(".", "\\.", x)
> 
> Unfortunately, "\\" does not represent a literal backslash as I
> expected, but rather a pair of backslashes:
> 
> > "\\."
> [1] "\\."
> > "\\"
> [1] "\\"
Nononononono.... If you want to know what is inside a string, use
cat() not (implicitly) print()
> cat( "\\.")
\.>

The thing is that print() itself escapes "weird" characters, including
the escape character:
> x <- readLines() # ctr-D terminates (on Linux anyway)
\.
> x
[1] "\\."

> Is there a way to represent "\"? This seems like a design problem
in
> the interpreter.
Yes. Not at all.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907

Dear group, 
 I am using justRMA in bioconductor to get expression
values.  Now I computed fold changes.  the fold
changes i get are log2 values.  

How can I convert these to log10 values to see them as
actual fold change values. 

thank you.

Ah, I see. Thanks. ?print.default and ?cat do not mention this.

Srinivas Iyyer wrote:
> Dear group, 
>  I am using justRMA in bioconductor to get expression
> values.  Now I computed fold changes.  the fold
> changes i get are log2 values.  
> 
> How can I convert these to log10 values to see them as
> actual fold change values. 
First, a bit of etiquette. It is considered impolite to post the same 
question to more than one listserv. Second, although this is not a 
BioC-specific question, that is probably a better place to ask.

To answer; log10 values will *not* give you actual fold changes. These 
will be the same as log2, to a multiplicitive constant. You want to 
convert back to the natural scale e.g., 2^FC, where FC = your fold changes.

Also note that you compute fold change on the log scale by subtraction, 
not division.

HTH,

Jim

> 
> thank you.
> 
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-- 
James W. MacDonald
Affymetrix and cDNA Microarray Core
University of Michigan Cancer Center
1500 E. Medical Center Drive
7410 CCGC
Ann Arbor MI 48109