[R] Extracting part of a factor

[Boris Steipe]

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.
>> 
>