[R] Extracting part of a factor

[Ista Zahn]

I guess this thread has gone on long enough, but I haven't seen anyone
yet suggest what to me seems like the obvious thing if you want to do
this with mutate, namely

testdata <- mutate(testdata, place = as.factor(substr(subject, 1, 3)))

Best,
Ista

On Fri, Mar 4, 2016 at 10:45 PM, Boris Steipe <boris.steipe at utoronto.ca> wrote:
> You mean this?
>   test$place <- factor(test$place)
>
> You can create a new column in a data frame by assigning something to it. E.g.
>    test$pollywog <- 1:6
> ... creates that column in "test".
>
> But factor(test$place) was empty, because no such column previously existed, like:
> R > factor(test$barbapapa)
> factor(0)
> Levels:
>
> So the right hand side has 0 rows, but the left hand side needs six. Of course you could create your column directly:
>
> R > str(test)
> 'data.frame':   6 obs. of  6 variables:
>  $ subject: Factor w/ 6 levels "001-002","002-003",..: 1 2 3 4 5 6
>  $ group  : Factor w/ 2 levels "boys","girls": 1 1 1 2 2 2
>  $ wk1    : int  2 7 9 5 2 1
>  $ wk2    : int  3 6 4 7 6 4
>  $ wk3    : int  4 5 6 8 3 7
>  $ wk4    : int  5 4 1 9 8 4
> R > test$place <- factor(substr(test$subject,1,3))    # here's were it gets done
> R > str(test)
> 'data.frame':   6 obs. of  7 variables:
>  $ subject: Factor w/ 6 levels "001-002","002-003",..: 1 2 3 4 5 6
>  $ group  : Factor w/ 2 levels "boys","girls": 1 1 1 2 2 2
>  $ wk1    : int  2 7 9 5 2 1
>  $ wk2    : int  3 6 4 7 6 4
>  $ wk3    : int  4 5 6 8 3 7
>  $ wk4    : int  5 4 1 9 8 4
>  $ place  : Factor w/ 6 levels "001","002","003",..: 1 2 3 4 5 6
>
> ... it's just that you insisted on mutate().
>
>
>
> Cheers,
> Boris
>
>
> On Mar 4, 2016, at 9:31 PM, KMNanus <kmnanus at gmail.com> wrote:
>
>> Boris -
>>
>> Boy, do I feel dumb - that’s exactly what I wanted.  I’ve tried this every way I can think of without assigning the result to the original name of the data frame.  I was trying to assign the result to a variable (test$place).
>>
>> Can u pls explain to me why assigning the result to the new variable was wrong?
>>
>> BTW, really appreciate your help.
>>
>> Ken
>> kmnanus at gmail.com
>> 914-450-0816 (tel)
>> 347-730-4813 (fax)
>>
>> <image001.jpg>
>>
>>> On Mar 4, 2016, at 9:21 PM, Boris Steipe <boris.steipe at utoronto.ca> wrote:
>>>
>>> LOL you still need to assign it though:
>>>
>>>
>>> test <- mutate(test, place = factor(substr(test$subject,1,3)))
>>>
>>> str(test)
>>> 'data.frame':        6 obs. of  7 variables:
>>> $ subject: Factor w/ 6 levels "001-002","002-003",..: 1 2 3 4 5 6
>>> $ group  : Factor w/ 2 levels "boys","girls": 1 1 1 2 2 2
>>> $ wk1    : int  2 7 9 5 2 1
>>> $ wk2    : int  3 6 4 7 6 4
>>> $ wk3    : int  4 5 6 8 3 7
>>> $ wk4    : int  5 4 1 9 8 4
>>> $ place  : Factor w/ 6 levels "001","002","003",..: 1 2 3 4 5 6
>>>
>>>
>>> Without assigning the result, the output only gets printed to console. Remember that R is a functional language - a properly written R functio does not change anything, it only returns its result.
>>>
>>> :-)
>>>
>>>
>>> On Mar 4, 2016, at 4:13 PM, KMNanus <kmnanus at gmail.com> wrote:
>>>
>>>> If I call mutate this way - mutate(test, place = factor(substr(test$subject,1,3))), I get the same output as above but when I call class(test$place), I get NULL and the variable disappears.
>>>
>>
>
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