Debugging only when needed

Sat, 25 Jan 2020 00:00:00 +0000

One of the worst thing I experienced is having a bug in a nested call of functions. If it is not simple the problems comes to isolate when and why does it occur. Here I explore something that I found once on twitter but haven’t find explained in Advanced R or other blogs.

Function stack

A simple function to debug is this one:

myfunction1 <- function(x, y) {
  if (sum(x > 0) >= 5) {
    x + y
  } else {
    x - y
  }
}

We can use it:

myfunction1(runif(10), 5)
##  [1] 5.703426 5.516683 5.217670 5.943402 5.173702 5.137270 5.768233 5.132304
##  [9] 5.513719 5.404423
myfunction1(runif(4), 5)
## [1] -4.706239 -4.447211 -4.078848 -4.997719

However if our input has a NA we’ll get an error:

x <- runif(10)
x[5] <- NA
myfunction1(x, 5)
## Error in if (sum(x > 0) >= 5) {: missing value where TRUE/FALSE needed

The problem is easy to find but lets assume it is not. The missing value where TRUE/FALSE needed indicates that we get a NA. So to find where we can use browser and the expression argument as this:

myfunction2 <-  function(x, y) {
  browser(expr = any(is.na(x)))
  if (sum(x > 0) >= 5) {
    x + y
  } else {
    x - y
  }
}
myfunction2(x, 5)

Which lead us to this:

debug with expr

If we use other input we can use the function as usual without triggering the debug:

myfunction2(runif(6), 5)
## [1] 5.231687 5.008704 5.614821 5.561843 5.745173 5.026838

So if you are developing a package or functions and find an error on some calculation and you don’t want to press next or skip a loop just use the expr argument of browser to just jump into the problematic call.

Reproducibility

## ─ Session info ───────────────────────────────────────────────────────────────────────────────────────────────────────
##  setting  value                       
##  version  R version 4.0.1 (2020-06-06)
##  os       Ubuntu 20.04.1 LTS          
##  system   x86_64, linux-gnu           
##  ui       X11                         
##  language (EN)                        
##  collate  en_US.UTF-8                 
##  ctype    en_US.UTF-8                 
##  tz       Europe/Madrid               
##  date     2021-01-08                  
## 
## ─ Packages ───────────────────────────────────────────────────────────────────────────────────────────────────────────
##  package     * version date       lib source                           
##  assertthat    0.2.1   2019-03-21 [1] CRAN (R 4.0.1)                   
##  blogdown      0.21.84 2021-01-07 [1] Github (rstudio/blogdown@c4fbb58)
##  bookdown      0.21    2020-10-13 [1] CRAN (R 4.0.1)                   
##  cli           2.2.0   2020-11-20 [1] CRAN (R 4.0.1)                   
##  crayon        1.3.4   2017-09-16 [1] CRAN (R 4.0.1)                   
##  digest        0.6.27  2020-10-24 [1] CRAN (R 4.0.1)                   
##  evaluate      0.14    2019-05-28 [1] CRAN (R 4.0.1)                   
##  fansi         0.4.1   2020-01-08 [1] CRAN (R 4.0.1)                   
##  glue          1.4.2   2020-08-27 [1] CRAN (R 4.0.1)                   
##  htmltools     0.5.0   2020-06-16 [1] CRAN (R 4.0.1)                   
##  knitr         1.30    2020-09-22 [1] CRAN (R 4.0.1)                   
##  magrittr      2.0.1   2020-11-17 [1] CRAN (R 4.0.1)                   
##  rlang         0.4.10  2020-12-30 [1] CRAN (R 4.0.1)                   
##  rmarkdown     2.6     2020-12-14 [1] CRAN (R 4.0.1)                   
##  sessioninfo   1.1.1   2018-11-05 [1] CRAN (R 4.0.1)                   
##  stringi       1.5.3   2020-09-09 [1] CRAN (R 4.0.1)                   
##  stringr       1.4.0   2019-02-10 [1] CRAN (R 4.0.1)                   
##  withr         2.3.0   2020-09-22 [1] CRAN (R 4.0.1)                   
##  xfun          0.20    2021-01-06 [1] CRAN (R 4.0.1)                   
##  yaml          2.2.1   2020-02-01 [1] CRAN (R 4.0.1)                   
## 
## [1] /home/lluis/bin/R/4.0.1/lib/R/library

debug | B101nfo

Debugging only when needed

Function stack

Reproducibility