[R] categorical column to numeric column

[Duncan Murdoch]

On 2/19/2007 8:36 AM, Shubha Vishwanath Karanth wrote:
> Hi R,
> 
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> Let 'dd' be a data frame given as:
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> dd=data.frame(aa=c("a","a","b","a","b","b"),bb=c(1,1,1,2,3,4))
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> Now I want to create a column 'g' such that if dd$aa=a then dd$g=1 else
> dd$g= -1 .
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> So, I gave the below syntax:
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> if((dd$aa)=="a") dd$g=1 else dd$g= -1

if() looks at just the first entry; it's designed for flow of control 
rather than vectorized calculations.  You want ifelse():

ifelse( dd$aa == "a", 1, -1)

Duncan Murdoch
>  
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> But I get the error message as:
> 
> Warning message: 
> 
> the condition has length > 1 and only the first element will be used in:
> if ((dd$aa) == "a") dd$g = 1 else dd$g = -1 
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> and dd=
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>> dd
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>   aa bb g
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> 1  a  1 1
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> 2  a  1 1
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> 3  b  1 1
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> 4  a  2 1
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> 5  b  3 1
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> 6  b  4 1
> 
>> 
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>  
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> Please let me know what is the error I am doing?
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