Re: [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.dun...@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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