R help - Oct 2009 - ifelse

When I run this code from an R-script:
       ddd = 360 + round ( atan2(-u,-v) / d2r )
       print(class(ddd))
       print(ddd)
       ifelse ( ddd>360, ddd-360, ddd )
       print(ddd)
       
I get this output:       
[1] "numeric"
[1] 461 213 238 249 251
[1] 461 213 238 249 251

Why does ifelse not change the 461 to 101?

I recreated the vector ddd and ran the same ifelse
code and it did work as expected.
  
> ddd <- c(461, 213, 238, 249, 251)  
> print(class(ddd))
[1] "numeric"
> print(ddd)
[1] 461 213 238 249 251
> ifelse ( ddd>360, ddd-360, ddd )
> print(ddd)
[1] 101 213 238 249 251

What am I missing?

Patrick King


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What I think you are missing is that you didn't change ddd. The ifelse
statement does not assign values to the ddd object. To change ddd it would
read:

ddd <- ifelse ( ddd>360, ddd-360, ddd )

So when you enter "print(ddd)" you get the content of the original
object,
which has not changed. 

What is missing from your examples below is the output of the ifelse
statement. When I run your second example (I can't run the first because you
didn't supply the data objects) 
I get the following:
> ddd <- c(461, 213, 238, 249, 251)
>  print(class(ddd))
[1] "numeric"
> 
>  print(ddd)
[1] 461 213 238 249 251
> 
>  ifelse ( ddd>360, ddd-360, ddd )
[1] 101 213 238 249 251
>  print(ddd)
[1] 461 213 238 249 251
> 
Which is perfectly reasonable given that the ifelse does not change the ddd
object. 



pking wrote:
> 
> When I run this code from an R-script:
>        ddd = 360 + round ( atan2(-u,-v) / d2r )
>        print(class(ddd))
>        print(ddd)
>        ifelse ( ddd>360, ddd-360, ddd )
>        print(ddd)
>        
> I get this output:       
> [1] "numeric"
> [1] 461 213 238 249 251
> [1] 461 213 238 249 251
> 
> Why does ifelse not change the 461 to 101?
> 
> I recreated the vector ddd and ran the same ifelse
> code and it did work as expected.
>   
>> ddd <- c(461, 213, 238, 249, 251)  
>> print(class(ddd))
> [1] "numeric"
> 
>> print(ddd)
> [1] 461 213 238 249 251
> 
>> ifelse ( ddd>360, ddd-360, ddd )
>> print(ddd)
> [1] 101 213 238 249 251
> 
> What am I missing?
> 
> Patrick King
> 
> 
> 
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View this message in context:
http://www.nabble.com/ifelse-tp26041165p26041312.html
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pking wrote:
> When I run this code from an R-script:
>        ddd = 360 + round ( atan2(-u,-v) / d2r )
>        print(class(ddd))
>        print(ddd)
>        ifelse ( ddd>360, ddd-360, ddd )
>        print(ddd)
>        
> I get this output:       
> [1] "numeric"
> [1] 461 213 238 249 251
> [1] 461 213 238 249 251
> 
> Why does ifelse not change the 461 to 101?

Because you never changed ddd

You want the last but one line to be

ddd <- ifelse ( ddd>360, ddd-360, ddd )

I guess.


> I recreated the vector ddd and ran the same ifelse
> code and it did work as expected.
>   
>> ddd <- c(461, 213, 238, 249, 251)  
>> print(class(ddd))
> [1] "numeric"
> 
>> print(ddd)
> [1] 461 213 238 249 251
> 
>> ifelse ( ddd>360, ddd-360, ddd )
>> print(ddd)
> [1] 101 213 238 249 251

I doubt.

> What am I missing?
That you made an assignment

ddd <- ifelse ( ddd>360, ddd-360, ddd )

that you have not told us (nor youself?) in your second case.

Best,
Uwe Ligges

> Patrick King
> 
>