Add "Breaking Loops" section in Chapter 8

[tabad]

Although the global command

do={<commands>}
    on-error={<commands>}

is covered in section Error trapping of Chapter 10, it might be interesting to show in Chapter 8 how it can be used to break :for, :foreach and :while loops. In this post of MikroTik Forum there is an interesting example of that.

:do {
   :foreach i in 1,2,3,4,5,6 do={
      :if ($i = 4) do={
         :put "break?";
         # This is generating a run-time error
         # The '/break' command does not exist
         /break;
      }
   :if ($i = 2) do={
      do {
         :put "break?";
         /break;
      } on-error={
         :put "error handled. do not break.";
      }
   }
   
   :put $i;
   
   }
} on-error={
   :put "break!";
}

This is its output.

1
break?
error handled. do not break.
2
3
break?
break!

[wifinigel]

Thanks for this suggestion, much appreciated.

It is interesting that you raise this, as I had a long debate debate with myself about this topic when writing the book. I couldn't decide whether to include it or not, as it is a valuable method of breaking out of loops.

I "chickened out" of including it, as I was worried about people including it in their code if they did not fully understand the impact if they have syntax errors in the ":do { }" block of their code. The book is aimed at people of all abilities, so I was slightly concerned that people with less experience might see a code example in isolation and use it without understanding the caveats around using it.

Your suggestion has made me think again about this topic. I think in retrospect I should have included it in the defensive coding section.

Leave this one with me and I may add it in to chapter 10, with a forward reference from chapter 8 about the technique. I only have a certain amount of "wiggle room" of what I can add before Amazon decide that my manuscript has to be a new release of the book, but I think I could squeeze this one in. I may also include an article on the web site to cover this so that it is available to a wider audience, including those who have an earlier paper copy.

Thanks again for raising this one.

Regards

Nigel.