likert() does not seem to handle tbl_df data

[briandk]

Expected Behavior

If data is of class tbl_df (or inherits from it), likert::likert() should be able to handle it without issue.

Actual Behavior

Handing in data inheriting from class tbl_df produces an error:

Error in likert(test_data) : 
  The items parameter must be a data frame. If trying to subset a data frame to analyze only one column, try: items=mydf[,1, drop=FALSE].
In addition: Warning message:
In if (class(items) != "data.frame") { :
  the condition has length > 1 and only the first element will be used

Steps to Reproduce

# Checkpoint for reproducibility
require(checkpoint)
checkpoint("2016-03-30", R.version = "3.2.4")

require(dplyr)
require(likert)
data("pisaitems")

test_data <- pisaitems %>%
  select(one_of("ST24Q02", "ST24Q01")) %>%
  tbl_df()

# If `test_data` is a `tbl_df`, this call will fail because likert() claims the
#   `items` parameter is not a data frame
likert(test_data)

# Calling `class()` on `test_data` shows it inherits from multiple classes,
#   including `data.frame`. So, I'm not sure what's happening exactly,
#   but I suspect it's related to this source code block: https://github.com/jbryer/likert/blob/4ff9837ffa17085219c5b23f5e8319b0405e2893/R/likert.R#L71-L74
class(test_data)

# Coercing `test_data` back to a data frame fixes the issue,
#   but is there any reason `tbl_df` data should be failing?
as.data.frame(test_data) %>% likert()

Where I suspect the error might be originating

Based on my inspection, the only part of the likert package source code that could produce this error is here:

likert/R/likert.R

Lines 71 to 74 in 4ff9837

	if(!('data.frame' %in% class(items))) {
	stop(paste0('The items parameter must be a data frame. If trying ',
	'to subset a data frame to analyze only one column, try: ',
	'items=mydf[,1, drop=FALSE].'))

Which in turn means this conditional check is returning TRUE :

if(!('data.frame' %in% class(items)))

Session Info

R version 3.2.4 (2016-03-10)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: OS X 10.11.4 (El Capitan)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] likert_1.3.3      xtable_1.8-2      ggplot2_2.1.0     dplyr_0.4.3       checkpoint_0.3.16

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.4      knitr_1.12.3     magrittr_1.5     mnormt_1.5-4     munsell_0.4.3   
 [6] colorspace_1.2-6 R6_2.1.2         stringr_1.0.0    plyr_1.8.3       tools_3.2.4     
[11] parallel_3.2.4   grid_3.2.4       gtable_0.2.0     psych_1.6.4      DBI_0.3.1       
[16] lazyeval_0.1.10  assertthat_0.1   digest_0.6.9     gridExtra_2.2.1  reshape2_1.4.1  
[21] formatR_1.3      labeling_0.3     stringi_1.0-1    scales_0.4.0    

[jbryer]

I would recommend using as.data.frame before calling likert.