Ants, Bees, Genomes & Evolution

@ Queen Mary University London

Easy mistake comparing numbers in R

It shouldn’t really be necessary to share this. But it keeps popping up (in particular when students are learning to program in R).

It is nonsensical to compare text to numbers. But R will let you. For example:

> "a" > 1
[1] TRUE
> "A" > 1
[1] TRUE
> "bob" > 99
[1] TRUE

I suspect that this is a remnant of the desire to be able to compare letters (e.g., for sorting ASCII characters alphabetically “A” < “D”).

Problem comparing numbers and “text as numbers” in R

When new to programming in R, you might ask the user to input a number. For example with

input_number <- readline(prompt="Enter a number: ")

This comes into R as text, not as a number. Unless you run as.numeric() or strtoi() on the input, it will remain text. Comparisons are then equivalent to the following

> 1 == "10"
> 10 == "10"
[1] TRUE
> 100 == "10"
> 11 < "10"
> 1 < "10"
[1] TRUE
> 9 < "10"   # <- uh-oh
> 3 < "10"

Note that some of these comparisons yield the result that you would expect if only comparing numbers. But some of them (e.g., the last 2) give you an incorrect response. This is dangerous.

R does not show an error message; it acts as is everything is ok, and gives you a response that looks reasonable (i.e., TRUE or FALSE). It would thus be easy for this type of mistake to go undetected.

How can this type of number comparison problem in R be prevented or detected?

  • Always check/force the type. If you run as.numeric() or strtoi on something text that is not unambiguously a number, R will show an error message or return NA.
  • Use a testing framework such as testtthat to ensure that even the small pieces of code you write behave as you would expect.

January 3, 2018