After reading this topic and after experimenting a bit, I am trying to understand how the Lua length operator works when a table contains nil values.
Before I started to investigate, I thought that the length was simply the number of consecutive non-nil elements, starting at index 1:
print(#{nil}) -- 0
print(#{"o"}) -- 1
print(#{"o",nil}) -- 1
print(#{"o","o"}) -- 2
print(#{"o","o",nil}) -- 2
That looks pretty simple, right?
But my headache started when I accidentally added an element after a nil-terminated table:
print(#{"o",nil,"o"})
My guess was that it should probably print 1 because it would stop counting when the first nil is found. Or maybe it should print 2 if the length operator is greedy enough to look for non-nil elements after the first nil. But the above code prints 3.
So I’ve ran several other tests to see what happens:
-- nil before the end
print(#{nil,"o"}) -- 2
print(#{nil,"o","o"}) -- 3
print(#{"o",nil,"o"}) -- 3
-- several nil elements
print(#{"o",nil,nil}) -- 1
print(#{nil,"o",nil}) -- 0
print(#{nil,nil,"o"}) -- 3
I should mention that repl.it currently uses Lua 5.1.5 which is rather old, but if you test with the Lua demo, which currently uses Lua 5.3.5, you’ll get the same results.
By looking at those results and by looking at this answer, I assume that:
if the last element is not nil, the length operator returns the full size of the table, including nil entries if any
if the last element is nil, it counts the number of consecutive non-nil and stops counting at the first nil
Are those assumptions correct?
Can we predict a 100% well-defined behavior when a table contains one or several nil values?
The Lua documentation states that the length of a table is only defined if the table is a sequence. Does that mean that the length operator has undefined behavior for non-sequences?
Apart from the length operator, can nil values cause any trouble in a table?
We can predict some behaviour, but it is not standardised, and as such you should never rely on it. It's quite possible that the behaviour may change within this major version of Lua.
Should you ever need to fill a table with nil values, I suggest wrapping the table and replace holes with a unique placeholder value (eg. NIL={}; if v==nil then t[k]=NIL end, this is quite cheap to test against and safe.).
That said...
As there is even a difference in the result of # depending on how the table is defined, you'll have to distinguish between statically defined (constant) tables and dynamic defined (muted) tables.
Static table definitions:
#{nil,nil,nil,nil,nil, 1} -- 6
#{3, 2, nil, 1} -- 4
#{nil,nil,nil, 1, 1,nil} -- 0
#{nil,nil, 1, 1, 1,nil} -- 5
#{nil, 1, 1, 1, 1,nil} -- 5
#{nil,nil,nil,nil, 1,nil} -- 0
#{nil,nil, 1,nil, 1,nil,nil} -- 5
#{nil,nil,nil, 1,nil,nil, 1,nil} -- 4
Using this kind of definition, as long as the last value is non-nil, you will get a length equal to the position of the last value. If the last value is nil, Lua starts a (non-linear) search from the tail until it finds the first non-nil value.
Dynamic data definition
local x={}; x[5]=1;print(#x) -- 0
local x={}; x[1]=1;x[2]=1;x[3]=1;x[5]=1;print(#x) -- 3
local x={}; x[1]=1;x[2]=1;x[4]=1;x[5]=1;print(#x) -- 5
#{[5]=1} -- 0
local x={nil,nil,nil,1};x[5]=1;print(#x) -- 0
As soon as the table was changed once, the operator works the other way (that includes static definitions with []). If the first element is nil, # always returns 0, but if not it starts a search that I did not investigate further (I guess you can check the sources, though I don't think it's a standard binary search), until it finds a nil value that is preceded by a non-nil value.
As said before, relying on this behaviour is not a good idea, and invites lots of issues down the road. Though if you want to make a nasty unmaintainable program to mess with a colleague, that's a sure way to do it.
When a table is a sequence (all numeric keys start at 1 and there are no nil gaps), # is defined to be precisely the count of those elements.
For non-sequence tables, it is a bit more complicated. Lua 5.2 seems to leave the result as undefined. For 5.1 and 5.3, the result of the operation is a border.
A border in a table is any positive index that contains a non-nil value followed by nil, or 0 if the first element is nil. # is defined to return any value that satifies these conditions.
Looking at it from another perspective, since tables contain an "array" part and a "map" part, Lua has no way of knowing where the "map" indices start. For example, you can create a table with 1000 values and then set the first 999 of them to nil; that could leave you with a table of "size" 1000. However, you can also start with an empty table and set the 1000th element, having a table of "size" 0 but still structurally equivalent to the first one. The result of # is then simply the first valid value the internal algorithm finds.
The length operator produces undefined behaviour for tables that aren't sequences (i.e. tables with nil elements in the middle of the array). This means that even if the Lua implementation always behaves in a certain way, you shouldn't rely on that behaviour, as it may change in future versions of Lua, or in different implementations like LuaJIT.
You can use nils in tables - there is nothing wrong with that - just don't use the length operator on a table which might have nils before non-nil values.
The post you linked to contains more details about how the actual algorithm works. It mentions counting elements with a "binsearch", i.e. a binary search. This is not the same as just counting the elements one by one - if there are nils in the table, then depending on their exact position, the binary search algorithm may treat them as the end of the table, or may just ignore them.
To sum up, the algorithm is harder to predict than you were assuming, and even though it is technically possible to predict what will happen in any given case, you shouldn't rely on that behaviour as it is liable to change.
I am working with a third party device which has some implementation of Lua, and communicates in BACnet. The documentation is pretty janky, not providing any sort of help for any more advanced programming ideas. It's simply, "This is how you set variables...". So, I am trying to just figure it out, and hoping you all can help.
I need to set a long list of variables to certain values. I have a userdata 'ME', with a bunch of variables named MVXX (e.g. - MV21, MV98, MV56, etc).
(This is all kind of background for BACnet.) Variables in BACnet all have 17 'priorities', i.e., every BACnet variable is actually a sort of list of 17 values, with priority 16 being the default. So, typically, if I were to say ME.MV12 = 23, that would set MV12's priority-16 to the desired value of 23.
However, I need to set priority 17. I can do this in the provided Lua implementation, by saying ME.MV12_PV[17] = 23. I can set any of the priorities I want by indexing that PV. (Corollaries - what is PV? What is the underscore? How do I get to these objects? Or are they just interpreted from Lua to some function in C on the backend?)
All this being said, I need to make that variable name dynamic, so that i can set whichever value I need to set, based on some other code. I have made several attempts.
This tells me the object(MV12_PV[17]) does not exist:
x = 12
ME["MV" .. x .. "_PV[17]"] = 23
But this works fine, setting priority 16 to 23:
x = 12
ME["MV" .. x] = 23
I was trying to attempt some sort of what I think is called an evaluation, or eval. But, this just prints out function followed by some random 8 digit number:
x = 12
test = assert(loadstring("MV" .. x .. "_PV[17] = 23"))
print(test)
Any help? Apologies if I am unclear - tbh, I am so far behind the 8-ball I am pretty much grabbing at straws.
Underscores can be part of Lua identifiers (variable and function names). They are just part of the variable name (like letters are) and aren't a special Lua operator like [ and ] are.
In the expression ME.MV12_PV[17] we have ME being an object with a bunch of fields, ME.MV12_PV being an array stored in the "MV12_PV" field of that object and ME.MV12_PV[17] is the 17th slot in that array.
If you want to access fields dynamically, the thing to know is that accessing a field with dot notation in Lua is equivalent to using bracket notation and passing in the field name as a string:
-- The following are all equivalent:
x.foo
x["foo"]
local fieldname = "foo"
x[fieldname]
So in your case you might want to try doing something like this:
local n = 12
ME["MV"..n.."_PV"][17] = 23
BACnet "Commmandable" Objects (e.g. Binary Output, Analog Output, and o[tionally Binary Value, Analog Value and a handful of others) actually have 16 priorities (1-16). The "17th" you are referring to may be the "Relinquish Default", a value that is used if all 16 priorities are set to NULL or "Relinquished".
Perhaps your system will allow you to write to a BACnet Property called "Relinquish Default".
I need a pattern that will grab values for two or more parameters of varying length.
I need to convert a scanf format %i to a lua pattern and it is proving very difficult. I don't need to worry about the type of storage that can be passed in with scanf. Just the %i for integer and if they specify a specific link.
Documentation for scanf, if needed, can be found here.
This is what I have so far:
if(scanfLetter == "i" or scanfLetter == "d" or scanfLetter == "u") then
if(specifiedNum == 0)then
newPattern = "([%+%-]?%d+)"
elseif(specifiedNum >= 2)then
newPattern = "([%+%-%d]?%d^".. string.rep("%d?", specifiedNum-2)..")$"
else
newPattern = "(%d)"
end
which basically just checks to see if they passed in a specific number or not (specifiedNum). If it is 0 that means they didn't.
It all works until there are two length specified %is in a row. For example:
%5i%6i
If they enter in 6 or more characters it works fine because match will return two values: the first one consisting of the first 5 numbers and the 6th goes the next value.
The trouble is if there are fewer than 5. In scanf or printf it will not match %5i%6i if there are 5 or fewer numbers, but in my pattern it will still match under string.match and it returns all the values but the last in the first return and the second value get the last number entered.
More specific example so you don't have to type it all out and see it the pattern ends up looking like.
Given:
([%.%+%-%d]?%d[%.%d]?[%.%d]?[%.%d]?)([%.%+%-%d]?%d[%.%d]?[%.%d]?[%.%d]?[%.%d]?)
If 123456789 is passed to it, the match returns 2 values:
`12345` and `6789`
However, if 1234 is passed in, the match returns 2 values:
`123` and `4`
which is incorrect (it should not be a match).
Is what I seek possible?
(Maybe somebody has already written a scanf format to lua patterns converter?)
I have this Cobol paragraph that will search one table which at this point in my example would have a table counter of 2 which is what the first INDEX loop does. The variable A represents an Occurs that is defined in a file (include) which has 5 occurrences. I can get to the if statement but it returns false. I read the information out of a ParmCard and store that in the table which is Table-B and the ParmCard is correct.
I did get it to find one value when was changing values around (conditional statements) but I know that both of the values that it is looking for in the ParmCard are in the file and should be found and it should find two results. I would have tried Expeditor but the system was down at work.
Is there something wrong with the index or may be I think that the perform's are working one way but they are really working a different way? This Search paragraph gets executed with every read of the ID file thus it will look in the table as many times as the ID file has an ID and ID symbols are unique.
Question: Why would the IF-STATEMENT not be working?
Code:
SEARCH-PARAGRAPH.
PERFORM VARYING SUB FROM 1 BY 1 UNTIL SUB > 2 <--DUPLICATE INDEXER
IF A(TAB) = TABLE-B(SUB) THEN
MOVE 6 TO TAB
MOVE 'TRUE' TO FOUND-IS
PERFORM WRITE-FILE THRU X-WF
PERFORM LOG-RESULT THRU X-LR
END-IF
END-PERFORM
X-SP. EXIT.
SEARCH-INDEX.
PERFORM VARYING I FROM 1 BY 1 UNTIL I > 2
DISPLAY 'INDEX --> ' I
PERFORM VARYING TAB FROM 1 BY 1 UNTIL TAB > 5
DISPLAY 'TAB --> ' TAB
PERFORM SEARCH-PARAGRPAH THRU X-SP
END-PERFORM
END-PERFORM.
X-SEARCH-INDEX. EXIT.
Here is the way that it works now and I do get the results I want. It is difficult to past the company code up because you never know who might have a problem.
New Code:
READ-PROV.
READ P-FILE
AT END
MOVE 'Y' TO EOF2
GO TO X-READ-PROV
NOT AT END
ADD 1 TO T-REC-READ
MOVE P-RECORD TO TEST-RECORD
PERFORM CHECK-MATCH THRU X-CHECK-MATCH
END-READ.
X-READ-PROV. EXIT.
CHECK-MATCH.
PERFORM VARYING SUB FROM 1 BY 1 UNTIL SUB > TABLECOUNTER
IF PID >= FROM(SUB) AND
PID <= THRU(SUB) THEN
IF TODAY < P-END-DTE THEN
IF TOTAL-PD = 0 AND
TOTAL-PD = 0 AND
TOTAL-PD = 0 AND
TOTAL-PD = 0 AND
TOTAL-PD = 0 THEN
IF PBILLIND NOT EQUAL 'Y'
PERFORM VARYING TAB FROM 1 BY 1 UNTIL TAB > 5
IF P-CD(TAB) = TY(SUB) THEN
MOVE 6 TO TAB
DISPLAY('***Found***')
ADD 1 TO T-REC-FOUND
END-IF
END-PERFORM
END-IF
END-IF
END-IF
END-IF
END-PERFORM.
X-CM. EXIT.
We can't tell.
There is nothing "wrong" with your nested PERFORM. The IF test is failing simply because it is never true.
We can't get you further with that without seeing your data-definitions, sample input and expected output.
However... my guess would be that the problem is with your data from the PARM in the JCL. That is the most likely area.
It is of course possible that the problem is with the other definition.
A couple of things whilst waiting.
Please always post the actual code, always. We don't want to look for errors in what you have typed here, we want to see the actual code. You have not shown the actual code, because it will not compile, as INDEX is a Reserved Word in COBOL, so you can't use it for a data-name.
Please always bear in mind that what you think may be wrong may not be the problem, so post everything we are likely to need (data-definitions, data you used, actual results you got with the code (including anything you've added for problem-determination), results which were expected).
Some tips.
A paragraph requires a full-stop/period after the paragraph-name and before the next paragraph. If you put that second full-stop/period on a line of its own, and have no full-stops/periods attached to your PROCEDURE code itself, you'll make things look neater and avoid problems when you want to copy some lines which happen to have a full-stop/period to a place where they cause you a mess.
You are using literal values. This is bad. When the number of entries in one of your tables changes, you have to change those literal values. Say the 2 needs to be changed to 5. You have to look at every occurrence of the literal 2 and decide if it needs to be changed. Then change it to 5. Then you get another request, to change the table which originally had five entries so that it will have six. See how difficult/error-prone life can be?
If instead you have unique and well-named data-names for your maximum number of entries, you only have one place to make a change, and you know it can be changed without reference to the rest of the code (assuming someone clever hasn't seen it has a value they want for something, and use it despite its name, of course...).
The content of those fields you can set automatically:
01 TABLE-1.
05 FILLER OCCURS 2 TIMES.
10 A PIC X(10).
01 TABLE-2.
05 FILLER OCCURS 5 TIMES.
10 TABLE-B PIC X(10).
01 TABLE-1-NO-OF-ENTRIES COMP PIC 9(4).
01 TABLE-2-NO-OF-ENTRIES COMP PIC 9(4).
...
PROCEDURE DIVISION.
...
COMPUTE TABLE-1-NO-OF-ENTRIES = LENGTH OF TABLE-1
/ LENGTH OF A
COMPUTE TABLE-2-NO-OF-ENTRIES = LENGTH OF TABLE-2
/ LENGTH OF TABLE-B
DISPLAY TABLE-1-NO-OF-ENTRIES
DISPLAY TABLE-2-NO-OF-ENTRIES
That gives you the output 2 and 5.
The names I've used are a mixture of yours and some for demonstration purposes only. Make everything meaningful, and by that I don't mean trite, as my example names would be in real life.
If you insist on escaping from within your PERFORM like that (and take note of Bruce Martin's comment), you can calculate your escape value by using new, aptly-named, fields and giving them the value of the above plus one.
To do a nested loop when the outer loop only has two entries is overkill. You don't need to escape out of the loops like you do, if you have a termination condition on the loop.
That'll do for now until we see your definitions, data and results.
I have a VB6 form with a text boxes for minimum and maximum values. The text boxes have a MaxLength of 4, and I have code for the keyPress event to limit it to numeric entry. The code checks to make sure that max > min, however it is behaving very strangely. It seems to be comparing the values in scientific notation or something. For example, it evaluates 30 > 200 = true, and 100 > 20 = false. However if I change the entries to 030 > 200 and 100 > 020, then it gives me the correct answer. Does anyone know why it would be acting this way?
My code is below, I am using control arrays for the minimum and maximum text boxes.
For cnt = 0 To 6
If ParameterMin(cnt) > ParameterMax(cnt) Then
MsgBox ("Default, Min, or Max values out of range. Line not updated.")
Exit Sub
End If
Next cnt
That is how text comparison behaves for numbers represented as variable length text (in general, not just VB6).
Either pad with zeros to a fixed length and continue comparing as text (as you noted)
OR
(preferable) Convert to integers and then compare.
If I understood correctly, you can alter the code to
If Val(ParameterMin(cnt)) > Val(ParameterMax(cnt)) Then
I wish to advise one thing -(IMHO...) if possible, avoid checking data during key_press/key_up/key_down .
Can you change the GUI to contain a "submit" button and check your "form" there ?
Hope I helped...