Loop exit

[Simon ORiordan from UK]

Goto the cockpit and see what the hold up is. And don't call me Shirley.

[SortaCore]

I've been known to use it for "load variable amounts of stuff from DB as needed" and use goto to get to the cleanup/UI enabling at the end. It probably comes from the habit of preferring:

void someFunc()
{
if (!A)
return;
DoStuffWithA()
DoMoreCrud();
}

rather than

void someFunc()
{
if (A)
{
DoStuffWithA();
DoMoreCrud();
}
}
// TWO close braces with no code between? Surely you jest.

[L Braun]

After a loop, how do you know if you finished it or breaked out of it? This post is not about just leaving a loop, but about knowing how you left it and act according it. We probably all know how to do that in c, but this is about a language that adds syntax elements for that.

[Mark_Wallace]

Looks like a nonsense reason, to me. You can just put a message before the break statement.

I wanna be a eunuchs developer! Pass me a bread knife!

[Mark_Wallace]

Mark_Wallace wrote:

You can just put a message before the break statement.

Goto! Before the goto statement! Damn!

I wanna be a eunuchs developer! Pass me a bread knife!

[Stefan_Lang]

How do you know the difference in Planc if you used exit***? As I said, I don't understand what, exactly, these statements do, and the OP doesn't inidicate they do anything beyond breaking out of the loop. That's what break does, too. Hence my question.

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

[Eduard Matei]

"Are there other languages out there with something similar?" Python has this "syntactic sugar" for alternate loop exit:

for item in iterable:
if condition(item):
break
process(item)
else:
print("No item in iterable meets the condition")

The else signifies that the for loop has finished without "break"-ing.

[Matthew Barnett]

In Python, loops can take an 'else' clause. It's run if you don't break out of the loop. For example:

for item in collection:
if some_test(item):
print('Found one!')
break
else:
print('No match found.')

[L Braun]

After reading the original post again, I wasn't sure I got it right, so I had a look in wikipedia about PLANC. Now I think EXITFOR specifies what to do if the for loop exits normally and the EXITWHILE specifies what to do when a WHILE clause (one of many) becomes true. Both blocks are specified inside the loop. So I see a WHILE in PLANC as a "if() break;" construction in c. And EXITFOR and EXITWHILE would be coded as something like if (got_out_with_break) {// EXITWHILE block} else {// EXITFOR block} but I could be wrong :) As stated below - Python got it as well.

[Mike E Andrews]

Visual Basic.NET supports similar constructs, such as:

Exit For
Exit While
Exit Do

and continuations like:

Continue For
Continue While
Continue Do

[Stefan_Lang]

After reading the python remark below I think I've got it (good thing you pointed that out! :thumbsup:) I'm not quite convinced of the benefits though. It may indeed - as the OP stated - safe you an extra if or flag variable. But the price you pay is readability: the conditional code can be in an entirely different place than the condition, with potentially a lot of code in between. Even worse, after reading over the OP again, it seems like there can be several while statements that can all trigger the same exitwhile, meaning that there can be several conditons that are all in different places, separated from the conditional code and the other conditions as well! How on earth are you supposed to keep track of the flow of control in code like that? :confused: There may be cases where such a language construct may make sense, and even be better readable than the alternatives offered in C and other languages. But I sense a great potential of abuse, and I suspect it takes both experience and sense of responsibility to use it well. Should a programming language support such a feature? If you say "yes", will you also agree that cars should be allowed to use the sidewalks (provided they are wide enough)?. These are the same questions! So, the answer is also the same: we cannot assume that people will use that option responsibly, so we're better off without it!

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

[Member 4608898]

But if you had for (...) { for (...) { I want to exitfor from the outer loop from a condition in the inner loop } } How would you do it? goto was invented for a reason and this is it!!! :-D

[crazedDotNetDev]

In fact I seem to remember an "exit for" in QBASIC. Loop exits go way back in many BASIC compliers. But all you C# coders don't worry; you're still "the best". You'll become better coders as C# becomes more like BASIC. :wtf: LOL, sorry couldn't help myself. :-O

- great coders make code look easy - When humans are doing things computers could be doing instead, the computers get together late at night and laugh at us. - ¿Neal Ford?

[dg6yhw11]

BASIC provides several exit statements such as EXIT LOOP EXIT FOR You can also set the condition in the call to the loop WHILE Not [answer you want is found] ... WEND will exit when the condition is met (of course you have to add another exit test to avoid infinite loops :) There are a bunch of similar constructs. To use this with some OOP type language just write a little function in BASIC (Visual Studio, PowerBasic), compile to a DLL and call it from the object you need to use. Visual Studio would probably let you do it all in the same project but then you wouldn't have the DLL to reuse.

[Frank Malcolm]

Delphi's version of Pascal has Break (quit the loop) and Continue (begin the next iteration).

[kalberts]

That is like break and continue in C, isn't it? Does it provide two alternatative ways out of a loop: One alternative is executed if the break was performed,the other if break was not performed? If it does, some more syntax must be defined.

[kalberts]

CHILL has a very nice solution to this: Every block, whether a procedure, loop, switch or even a linear sequence of statements enclosed with BEGIN and END, could be prefixed with a label. In CHILL, a label does not identify a point in the program, but a block, and consequently label scopes could be nested. So to leave the outer loop, you would write

OuterLoop:
DO FOR (...)

InnerLoop:
DO FOR (...)
...
IF THEN EXIT InnerLoop; FI;
...
IF THEN EXIT OuterLoop; FI
...
OD

OD

(Here I illustrate both leaving the inner loop and the outer loop.) However, CHILL doesn't provide what I asked for in my original post: If the post-loop processing depends on whether you completed the loop or left prematurely by EXIT, you must set some variable to a magic value and test it after the loop, and the post-loop processing would syntactically (e.g. with respect to variable scope) be outside the loop. While we are at CHILL: Another nifty syntactic sugar cube is the keyword EVER:

DO FOR EVER
...
OD

The semantics of EVER is quite obvious. I like this so much that whenever I need to program an inner loop in C, I set up a

#define ever (;;)

to be able to code it as "for ever {...}" in C. Sure, any well seasoned C programmer would prefer "while (1) {...}", but even though I have been writing more lines of C code than in any other language the last thirty years, I still read it as "while one what???"

[kalberts]

But does it provide two ways out? I.e. alternate code blocks executed one but not the other, depending on how you exited the loop?

[kalberts]

Yes, this is in the right direction. As your example illustrate, you do not have a symmetrical handling of the two ways out of the loop: The "breakout handling" is embedded in the loop code. with no syntactical indication that it is anything but ordinary actions within the loop. The 'else' provides half of what I want, and half is far better than nothing :)

[kalberts]

You are right that in the Planc language you may have several 'while' conditions within a loop, and they all jump to the same exitwhile clause. If you need to distinguish between different reasons for leaving the loop prematurely, you must test variables in the exitwhile clause. It is important to note that block-wise (e.g. with respect to variable scopes), exitwhile is a part of the loop, so e.g. a for loop counter is available, as well as all local variables within the loop. We used this quite extensively, with one or more while exits, but very rarely there was a need to run alternate execution paths in the exitwhile clause; the cleanup actions, or reporting actions or whatever, was almost always the same for all early exits (but different for loop completion). I never saw any Planc programmer "abusing" this mechanism (even summer interns who were still students), and I cannot see how that abuse would be. If you need to handle the situation differently if you got to the end or if you did not get to the end, I see no cleaner way to do it in e.g. C constructs.