R: How to make a switch statement fallthrough

Question

In many languages, there is an instruction called break that tells the interpreter to exit the switch after the current statement. If you omit it, the switch fall-through after the current case has been processed:

switch (current_step)
{
  case 1: 
    print("Processing the first step...");
    # [...]
  case 2: 
    print("Processing the second step...");
    # [...]
  case 3: 
    print("Processing the third step...");
    # [...]
    break;
  case 4: 
    print("All steps have already been processed!");
    break;
}

Such a design pattern can be useful if you want to go through a serie of transitive conditions.

I understand that this can cause bugs due to unintentional fallthrough if the programmer forgets to insert the break statement, but several languages are breaking by default, and include a fallthrough keyword (e.g. continuein Perl).

And by design, the R switch also breaks by default at the end of each case:

switch(current_step, 
  {
    print("Processing the first step...")
  },
  {
    print("Processing the second step...")
  },
  {
    print("Processing the third step...")
  },
  {
    print("All steps have already been processed!")
  }
)

In the above code, if current_step is set to 1, the output will only be "Processing the first step...".

Is there any way in R to force a switch case to fall through the following case?

https://stackoverflow.com/a/17113744/1457051 is the best you are going to be able to do. — hrbrmstr, May 05 '18 at 11:17
@hrbrmstr it is close but still not a real fall-through, this solution only allows multiple labels for one case. — Antoine C., May 05 '18 at 12:53
@Lovy - If you don't like the behavior of the R `switch()` statement, you can always code your own version. — Len Greski, May 05 '18 at 14:55
@Lovy I know. I said "the best you are able to do" for a reason. Feel free to [reimplement the primitive](https://github.com/wch/r-source/blob/3f7ba9e344d4fc3688a16006386b6a5019d89f89/src/main/builtin.c#L1026). — hrbrmstr, May 05 '18 at 18:06

Antoine C. · Accepted Answer · 2018-05-06T12:27:08.167

It appears that such a behavior is not possible with switch(). As suggested in the comments, the best option was to implement my own version.

Thus, I pushed an update to my optional package to implement this feature (CRAN).

With this update, you can now use a fallthrough statement in the pattern-matching function match_with.

The design pattern in the question can be reproduced the following way:

library(optional)
a <- 1
match_with(a
    1, fallthrough(function() "Processing the first step..."),
    2, fallthrough(function() "Processing the second step..."),
    3, function() "Processing the third step...",
    4, function() "All steps have already been processed!"
)
## [1] "Processing the first step..." "Processing the second step..." "Processing the third step..."

You can observe that match_with() is quite similar to switch(), but it has extended capabilities. E.g. the patterns can be lists or functional sequences instead of being simple objects to compare:

library("magrittr")
b <- 4
match_with(a,
  . %>% if (. %% 2 == 0)., 
  fallthrough( function() "This number is even" ),
  . %>% if ( sqrt(.) == round(sqrt(.)) ).,  
  function() "This number is a perfect square"
)
## [1] "This number is even" "This number is a perfect square"

R: How to make a switch statement fallthrough

1 Answers1