R：如何制作切换声明

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 (eg continue in 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?

Answer 1

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. Eg 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"