[R] If Then Trouble

[Uwe Ligges]

On 24.04.2011 22:10, Sparks, John James wrote:
> Dear R Helpers,
>
> I have another one of those problems involving a very simple step, but due
> to my inexperience I can't find a way to solve it.  I had a look at a
> number of on-line references, but they don't speak to this problem.
>
> I have a variable with 20 values
>
>> table (testY2$redgroups)
>
>      1     2     3     4     5     6     7     8     9    10    11    12
> 13    14    15    16    17    18    19    20
>     69   734  6079 18578 13693  6412  3548  1646   659   323   129    88
> 90    40    57    33    36    17     6    13
>
> Values 18,19 and 20 have small counts.  So, I want to set the value of
> redgroups for these rows to 17 in order to combine groups.  I would think
> that it would be as easy as
>
> if(testY2$redgroups>17) testY2$redgroups<-17
>
> following the syntax that I have seen in the manuals.  However, I get the
> error message
>
> Warning message:
> In if (testY2$redgroups>  17) testY2$redgroups<- 17 :
>    the condition has length>  1 and only the first element will be used
>
> Can someone please tell me the correct syntax for this?  I would really
> appreciate it.

See help("if") and find if(foo) is only designed for scalar values of 
foo. You actually want to use:

if(testY2$redgroups>17) testY2$redgroups<-17

testY2$redgroups[testY2$redgroups > 17] <- 17

or using some less efficient "if"-like statement:

testY2$redgroups <- ifelse(testY2$redgroups > 17, 17, testY2$redgroups)


Uwe Ligges




>
> Appreciatively yours,
> --John J. Sparks, Ph.D.
>
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