I'm writing a program for a Schneider PLC using structured text, and I'm trying to do it using object oriented programming.
Being a newbie in PLC programming, I wrote a simple test program such a this:
okFlag:=myObject.aMethod();
IF okFlag THEN
// it's ok, go on
ELSE
// error handling
END_IF
aMethod must perform some operations, wait for the result (there is a "time-out" check to avoid deadlocks) and return TRUE or FALSE
This is what I expected during program execution
1) when the okFlag:=myObject.aMethod(); is reached, the code inside aMethod is executed until a result is returned. When I say "executed" I mean that in the next scan cycle the execution of aMethodcontinues from the point it had reached before.
2) the result of method calling is checked and the main flow of the program is executed
and this is what happens:
1) aMethod is executed but the program flow continues. That is, when it reaches the end of aMethod a value it's returned, even if the events that aMethod should wait for are still executing.
2) on the next cycle, aMethod is called again and restarts from the beginning
This is the first solution I found:
VAR_STATIC
imBusy: BOOL
END_VAR
METHOD aMethod: INT;
IF NOT(imBusy) THEN
imBusy:=FALSE;
aMethod:=-1; // result of method while in progress
ELSE
aMethod:=-1;
<rest of code. If everything is ok, the result is 0, otherwise is 1>
END_IF
imBusy:=aMethod<0;
and the main program:
CASE (myObject.aMethod()) OF
0: // it's ok, go on
1: // error handling
ELSE
// still executing...
END_CASE
and this seems to work, but I don't know if it's the right approach.
There are some libraries from Schneider which use methods that return boolean and seem to work as I expected in my program. That is: when the cycle reaches the call to method for the first time the program flow is "deviated" somehow so that in the next cycle it enters again the method until it's finished. It's there a way to have this behaviour ?
generally OOP isn't the approach that people would take when using IEC61131 languages. Your best bet is probably to implement your code as a state machine. I've used this approach in the past as a way of simplifying a complex sequence so that it is easier for plant maintainers to interpret.
Typically what I would recommend if you are going to take this approach is to try to segregate your state machine itself from your working code; you can implement a state machine of X steps, and then have your working code reference the statemachine step.
A simple example might look like:
stepNo := 0;
IF (start AND stepNo = 0) THEN
StepNo = 1;
END_IF;
(* there's a shortcut unity operation for resetting this array to zeroes which is faster, but I can't remember it off the top of my head... *)
ActiveStepArray := BlankStepArray;
IF stepNo > 0 THEN
IF StepComplete[stepNo] THEN
stepNo := stepNo +1;
END_IF;
ActiveStepArray[stepNo] := true;
END_IF;
Then in other code sections you can put...
IF ActiveStep[1] THEN
(* Do something *)
StepComplete[1] := true;
END_IF;
IF ActiveStep[2] THEN
(* Do Something *)
StepComplete[2] := true;
END_IF;
(* etc *)
The nice thing about this approach is that you can actually put all of the state machine code (including jumps, resets etc) into a DFB, test it and then shelve it, and then just use the active step, step complete, and any other inputs you require.
Your code is still always going to execute an entire section of logic, but if you really want to avoid that then you'll have to use a lot of IF statements, which will impede readability.
Hope that helps.
Why not use SFC it makes your live easier in many cases, since it is state machine language itself. Do subprogram, wait condition do another .. rince and repeat. :)
Don't hang just for ST, the other IEC languages are better in some other tasks and keep thing as clear as possible. There should be not so much "this is my cake" mentality on the industrial PLC programming circles as it is on the many other programming fields, since application timeline can be 40 years and you left the firm 20 years ago to better job and programs are almost always location/customer or atleast hardware specific.
http://www.automation.com/pdf_articles/IEC_Programming_Thayer_L.pdf
I often use this code pattern:
while(true) {
//do something
if(<some condition>) {
break;
}
}
Another programmer told me that this was bad practice and that I should replace it with the more standard:
while(!<some condition>) {
//do something
}
His reasoning was that you could "forget the break" too easily and have an endless loop. I told him that in the second example you could just as easily put in a condition which never returned true and so just as easily have an endless loop, so both are equally valid practices.
Further, I often prefer the former as it makes the code easier to read when you have multiple break points, i.e. multiple conditions which get out of the loop.
Can anyone enrichen this argument by adding evidence for one side or the other?
There is a discrepancy between the two examples. The first will execute the "do something" at least once every time even if the statement is never true. The second will only "do something" when the statement evaluates to true.
I think what you are looking for is a do-while loop. I 100% agree that while (true) is not a good idea because it makes it hard to maintain this code and the way you are escaping the loop is very goto esque which is considered bad practice.
Try:
do {
//do something
} while (!something);
Check your individual language documentation for the exact syntax. But look at this code, it basically does what is in the do, then checks the while portion to see if it should do it again.
To quote that noted developer of days gone by, Wordsworth:
...
In truth the prison, unto which we doom
Ourselves, no prison is; and hence for me,
In sundry moods, 'twas pastime to be bound
Within the Sonnet's scanty plot of ground;
Pleased if some souls (for such their needs must be)
Who have felt the weight of too much liberty,
Should find brief solace there, as I have found.
Wordsworth accepted the strict requirements of the sonnet as a liberating frame, rather than as a straightjacket. I'd suggest that the heart of "structured programming" is about giving up the freedom to build arbitrarily-complex flow graphs in favor of a liberating ease of understanding.
I freely agree that sometimes an early exit is the simplest way to express an action. However, my experience has been that when I force myself to use the simplest possible control structures (and really think about designing within those constraints), I most often find that the result is simpler, clearer code. The drawback with
while (true) {
action0;
if (test0) break;
action1;
}
is that it's easy to let action0 and action1 become larger and larger chunks of code, or to add "just one more" test-break-action sequence, until it becomes difficult to point to a specific line and answer the question, "What conditions do I know hold at this point?" So, without making rules for other programmers, I try to avoid the while (true) {...} idiom in my own code whenever possible.
When you can write your code in the form
while (condition) { ... }
or
while (!condition) { ... }
with no exits (break, continue, or goto) in the body, that form is preferred, because someone can read the code and understand the termination condition just by looking at the header. That's good.
But lots of loops don't fit this model, and the infinite loop with explicit exit(s) in the middle is an honorable model. (Loops with continue are usually harder to understand than loops with break.) If you want some evidence or authority to cite, look no further than Don Knuth's famous paper on Structured Programming with Goto Statements; you will find all the examples, arguments, and explanations you could want.
A minor point of idiom: writing while (true) { ... } brands you as an old Pascal programmer or perhaps these days a Java programmer. If you are writing in C or C++, the preferred idiom is
for (;;) { ... }
There's no good reason for this, but you should write it this way because this is the way C programmers expect to see it.
I prefer
while(!<some condition>) {
//do something
}
but I think it's more a matter of readability, rather than the potential to "forget the break." I think that forgetting the break is a rather weak argument, as that would be a bug and you'd find and fix it right away.
The argument I have against using a break to get out of an endless loop is that you're essentially using the break statement as a goto. I'm not religiously against using goto (if the language supports it, it's fair game), but I do try to replace it if there's a more readable alternative.
In the case of many break points I would replace them with
while( !<some condition> ||
!<some other condition> ||
!<something completely different> ) {
//do something
}
Consolidating all of the stop conditions this way makes it a lot easier to see what's going to end this loop. break statements could be sprinkled around, and that's anything but readable.
while (true) might make sense if you have many statements and you want to stop if any fail
while (true) {
if (!function1() ) return;
if (!function2() ) return;
if (!function3() ) return;
if (!function4() ) return;
}
is better than
while (!fail) {
if (!fail) {
fail = function1()
}
if (!fail) {
fail = function2()
}
........
}
Javier made an interesting comment on my earlier answer (the one quoting Wordsworth):
I think while(true){} is a more 'pure' construct than while(condition){}.
and I couldn't respond adequately in 300 characters (sorry!)
In my teaching and mentoring, I've informally defined "complexity" as "How much of the rest of the code I need to have in my head to be able to understand this single line or expression?" The more stuff I have to bear in mind, the more complex the code is. The more the code tells me explicitly, the less complex.
So, with the goal of reducing complexity, let me reply to Javier in terms of completeness and strength rather than purity.
I think of this code fragment:
while (c1) {
// p1
a1;
// p2
...
// pz
az;
}
as expressing two things simultaneously:
the (entire) body will be repeated as long as c1 remains true, and
at point 1, where a1 is performed, c1 is guaranteed to hold.
The difference is one of perspective; the first of these has to do with the outer, dynamic behavior of the entire loop in general, while the second is useful to understanding the inner, static guarantee which I can count on while thinking about a1 in particular. Of course the net effect of a1 may invalidate c1, requiring that I think harder about what I can count on at point 2, etc.
Let's put a specific (tiny) example in place to think about the condition and first action:
while (index < length(someString)) {
// p1
char c = someString.charAt(index++);
// p2
...
}
The "outer" issue is that the loop is clearly doing something within someString that can only be done as long as index is positioned in the someString. This sets up an expectation that we'll be modifying either index or someString within the body (at a location and manner not known until I examine the body) so that termination eventually occurs. That gives me both context and expectation for thinking about the body.
The "inner" issue is that we're guaranteed that the action following point 1 will be legal, so while reading the code at point 2 I can think about what is being done with a char value I know has been legally obtained. (We can't even evaluate the condition if someString is a null ref, but I'm also assuming we've guarded against that in the context around this example!)
In contrast, a loop of the form:
while (true) {
// p1
a1;
// p2
...
}
lets me down on both issues. At the outer level, I am left wondering whether this means that I really should expect this loop to cycle forever (e.g. the main event dispatch loop of an operating system), or whether there's something else going on. This gives me neither an explicit context for reading the body, nor an expectation of what constitutes progress toward (uncertain) termination.
At the inner level, I have absolutely no explicit guarantee about any circumstances that may hold at point 1. The condition true, which is of course true everywhere, is the weakest possible statement about what we can know at any point in the program. Understanding the preconditions of an action are very valuable information when trying to think about what the action accomplishes!
So, I suggest that the while (true) ... idiom is much more incomplete and weak, and therefore more complex, than while (c1) ... according to the logic I've described above.
The problem is that not every algorithm sticks to the "while(cond){action}" model.
The general loop model is like this :
loop_prepare
loop:
action_A
if(cond) exit_loop
action_B
goto loop
after_loop_code
When there is no action_A you can replace it by :
loop_prepare
while(cond)
action_B
after_loop_code
When there is no action_B you can replace it by :
loop_prepare
do action_A
while(cond)
after_loop_code
In the general case, action_A will be executed n times and action_B will be executed (n-1) times.
A real life example is : print all the elements of a table separated by commas.
We want all the n elements with (n-1) commas.
You always can do some tricks to stick to the while-loop model, but this will always repeat code or check twice the same condition (for every loops) or add a new variable. So you will always be less efficient and less readable than the while-true-break loop model.
Example of (bad) "trick" : add variable and condition
loop_prepare
b=true // one more local variable : more complex code
while(b): // one more condition on every loop : less efficient
action_A
if(cond) b=false // the real condition is here
else action_B
after_loop_code
Example of (bad) "trick" : repeat the code. The repeated code must not be forgotten while modifying one of the two sections.
loop_prepare
action_A
while(cond):
action_B
action_A
after_loop_code
Note : in the last example, the programmer can obfuscate (willingly or not) the code by mixing the "loop_prepare" with the first "action_A", and action_B with the second action_A. So he can have the feeling he is not doing this.
The first is OK if there are many ways to break from the loop, or if the break condition cannot be expressed easily at the top of the loop (for example, the content of the loop needs to run halfway but the other half must not run, on the last iteration).
But if you can avoid it, you should, because programming should be about writing very complex things in the most obvious way possible, while also implementing features correctly and performantly. That's why your friend is, in the general case, correct. Your friend's way of writing loop constructs is much more obvious (assuming the conditions described in the preceding paragraph do not obtain).
There's a substantially identical question already in SO at Is WHILE TRUE…BREAK…END WHILE a good design?. #Glomek answered (in an underrated post):
Sometimes it's very good design. See Structured Programing With Goto Statements by Donald Knuth for some examples. I use this basic idea often for loops that run "n and a half times," especially read/process loops. However, I generally try to have only one break statement. This makes it easier to reason about the state of the program after the loop terminates.
Somewhat later, I responded with the related, and also woefully underrated, comment (in part because I didn't notice Glomek's the first time round, I think):
One fascinating article is Knuth's "Structured Programming with go to Statements" from 1974 (available in his book 'Literate Programming', and probably elsewhere too). It discusses, amongst other things, controlled ways of breaking out of loops, and (not using the term) the loop-and-a-half statement.
Ada also provides looping constructs, including
loopname:
loop
...
exit loopname when ...condition...;
...
end loop loopname;
The original question's code is similar to this in intent.
One difference between the referenced SO item and this is the 'final break'; that is a single-shot loop which uses break to exit the loop early. There have been questions on whether that is a good style too - I don't have the cross-reference at hand.
Sometime you need infinite loop, for example listening on port or waiting for connection.
So while(true)... should not categorized as good or bad, let situation decide what to use
It depends on what you’re trying to do, but in general I prefer putting the conditional in the while.
It’s simpler, since you don't need another test in the code.
It’s easier to read, since you don’t have to go hunting for a break inside the loop.
You’re reinventing the wheel. The whole point of while is to do something as long as a test is true. Why subvert that by putting the break condition somewhere else?
I’d use a while(true) loop if I was writing a daemon or other process that should run until it gets killed.
If there's one (and only one) non-exceptional break condition, putting that condition directly into the control-flow construct (the while) is preferable. Seeing while(true) { ... } makes me as a code-reader think that there's no simple way to enumerate the break conditions and makes me think "look carefully at this and think about carefully about the break conditions (what is set before them in the current loop and what might have been set in the previous loop)"
In short, I'm with your colleague in the simplest case, but while(true){ ... } is not uncommon.
The perfect consultant's answer: it depends. Most cases, the right thing to do is either use a while loop
while (condition is true ) {
// do something
}
or a "repeat until" which is done in a C-like language with
do {
// do something
} while ( condition is true);
If either of these cases works, use them.
Sometimes, like in the inner loop of a server, you really mean that a program should keep going until something external interrupts it. (Consider, eg, an httpd daemon -- it isn't going to stop unless it crashes or it's stopped by a shutdown.)
THEN AND ONLY THEN use a while(1):
while(1) {
accept connection
fork child process
}
Final case is the rare occasion where you want to do some part of the function before terminating. In that case, use:
while(1) { // or for(;;)
// do some stuff
if (condition met) break;
// otherwise do more stuff.
}
I think the benefit of using "while(true)" is probably to let multiple exit condition easier to write especially if these exit condition has to appear in different location within the code block. However, for me, it could be chaotic when I have to dry-run the code to see how the code interacts.
Personally I will try to avoid while(true). The reason is that whenever I look back at the code written previously, I usually find that I need to figure out when it runs/terminates more than what it actually does. Therefore, having to locate the "breaks" first is a bit troublesome for me.
If there is a need for multiple exit condition, I tend to refactor the condition determining logic into a separate function so that the loop block looks clean and easier to understand.
No, that's not bad since you may not always know the exit condition when you setup the loop or may have multiple exit conditions. However it does require more care to prevent an infinite loop.
He is probably correct.
Functionally the two can be identical.
However, for readability and understanding program flow, the while(condition) is better. The break smacks more of a goto of sorts. The while (condition) is very clear on the conditions which continue the loop, etc. That doesn't mean break is wrong, just can be less readable.
A few advantages of using the latter construct that come to my mind:
it's easier to understand what the loop is doing without looking for breaks in the loop's code.
if you don't use other breaks in the loop code, there's only one exit point in your loop and that's the while() condition.
generally ends up being less code, which adds to readability.
I prefer the while(!) approach because it more clearly and immediately conveys the intent of the loop.
There has been much talk about readability here and its very well constructed but as with all loops that are not fixed in size (ie. do while and while) you run at a risk.
His reasoning was that you could "forget the break" too easily and have an endless loop.
Within a while loop you are in fact asking for a process that runs indefinitely unless something happens, and if that something does not happen within a certain parameter, you will get exactly what you wanted... an endless loop.
What your friend recommend is different from what you did. Your own code is more akin to
do{
// do something
}while(!<some condition>);
which always run the loop at least once, regardless of the condition.
But there are times breaks are perfectly okay, as mentioned by others. In response to your friend's worry of "forget the break", I often write in the following form:
while(true){
// do something
if(<some condition>) break;
// continue do something
}
By good indentation, the break point is clear to first time reader of the code, look as structural as codes which break at the beginning or bottom of a loop.
It's not so much the while(true) part that's bad, but the fact that you have to break or goto out of it that is the problem. break and goto are not really acceptable methods of flow control.
I also don't really see the point. Even in something that loops through the entire duration of a program, you can at least have like a boolean called Quit or something that you set to true to get out of the loop properly in a loop like while(!Quit)... Not just calling break at some arbitrary point and jumping out,
using loops like
while(1) { do stuff }
is necessary in some situations. If you do any embedded systems programming (think microcontrollers like PICs, MSP430, and DSP programming) then almost all your code will be in a while(1) loop. When coding for DSPs sometimes you just need a while(1){} and the rest of the code is an interrupt service routine (ISR).
If you loop over an external condition (not being changed inside the loop), you use while(t), where t is the condition. However, if the loop stops when the condition changes inside the loop, it's more convenient to have the exit point explicitly marked with break, instead of waiting for it to happen on the next iteration of the loop:
while (true) {
...
a := a + 1;
if (a > 10) break; // right here!
...
}
As was already mentioned in a few other answers, the less code you have to keep in your head while reading a particular line, the better.