[R] Creating a simple function

[Duncan Murdoch]

On 21/09/2019 7:38 a.m., Jeff Newmiller wrote:
> The dplyr::select function returns a special variety of data.frame called a tibble. 

I don't think that's always true.  The docs say it returns "An object of 
the same class as .data.", and that's what I'm seeing:

 > str(data.frame(a=c(1,1,2,2), b=1:4) %>% subset(a == 1))
'data.frame':	2 obs. of  2 variables:
  $ a: num  1 1
  $ b: int  1 2

But I believe there are other dplyr functions that take dataframes as 
input and return tibbles, I just don't know which ones.

Duncan Murdoch

The tibble has certain features designed to make it behave consistently 
when indexing is used. Specifically, the `[` operator always returns a 
tibble regardless of how many columns are indicated by the column index. 
This is unlike the conventional data frame which returns a vector when 
exactly one column is indicated by the column index, or a data.frame if 
more than one is indicated.
> 
> A syntax that consistently yields a column vector with both tibbles and data.frames is
> 
> dta[[ 1 ]]
> 
> so
> 
> ctab <- function(data) {
>     CrossTable(data[[1]], data[[2]], prop.chisq = FALSE, prop.c = FALSE,
> prop.t = FALSE, format = "SPSS")
> }
> 
> should work.
> 
> On September 20, 2019 10:59:46 AM PDT, Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
>> On 20/09/2019 11:30 a.m., Zachary Lim wrote:
>>> Hi,
>>>
>>> I'm trying to create a simple function that takes a dataframe as its
>> only argument. I've been using gmodels::CrossTable, but it requires a
>> lot of arguments, e.g.:
>>>
>>> #this runs fine
>>> CrossTable(data$col1, data$col2, prop.chisq = FALSE, prop.c = FALSE,
>> prop.t = FALSE, format = "SPSS")
>>>
>>> Moreover, I wanted to make it compatible with piping, so I decided to
>> create the following function:
>>>
>>> ctab <- function(data) {
>>>     CrossTable(data[,1], data[,2], prop.chisq = FALSE, prop.c = FALSE,
>> prop.t = FALSE, format = "SPSS")
>>> }
>>>
>>> When I try to use this function, however, I get the following error:
>>>
>>> #this results in 'Error: Must use a vector in `[`, not an object of
>> class matrix.'
>>> data %>% select(col1, col2) %>% ctab()
>>>
>>> I tried searching online but couldn't find much about that error
>> (except for in specific and unrelated cases). Moreover, when I created
>> a very simple dataset, it turns out there's no problem:
>>>
>>> #this runs fine
>>> data.frame(C1 = c('x','y','x','y'), C2 = c('a','a','b','b')) %>%
>> ctab()
>>>
>>>
>>> Is this a problem with my function or the data? If it's the data, why
>> does directly calling CrossTable work?
>>
>> Presumably  data %>% select(col1, col2)  isn't giving you a dataframe.
>> However, you haven't given us a reproducible example, so I can't tell
>> you what it's doing.  But that's where you should look.
>>
>> Duncan Murdoch
>>
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>