i want to count the data type of each redis key, I write following code, but run error, how to fix it?
local detail = {}
detail.hash = 0
detail.set = 0
detail.string = 0
local match = redis.call('KEYS','*')
for i,v in ipairs(match) do
local val = redis.call('TYPE',v)
detail.val = detail.val + 1
end
return detail
(error) ERR Error running script (call to f_29ae9e57b4b82e2ae1d5020e418f04fcc98ebef4): #user_script:10: user_script:10: attempt to perform arithmetic on field 'val' (a nil value)
The error tells you that detail.val is nil. That means that there is no table value for key "val". Hence you are not allowed to do any arithmetic operations on it.
Problem a)
detail.val is syntactic sugar for detail["val"]. So if you expect val to be a string the correct way to use it as a table key is detail[val].
Possible problem b)
Doing a quick research I found that this redis call might return a table, not a string. So if detail[val] doesn't work check val's type.
Related
from https://www.lua.org/manual/5.3/manual.html
see section 2.4. Concerning the metamethod operation __newindex states the following quote:
__newindex: The indexing assignment table[key] = value. Like the index event, this event happens when table is not a table or when key is not
present in table. The metamethod is looked up in table.
Like with indexing, the metamethod for this event can be either a
function or a table. If it is a function, it is called with table,
key, and value as arguments. If it is a table, Lua does an indexing
assignment to this table with the same key and value. (This assignment
is regular, not raw, and therefore can trigger another metamethod.)
Whenever there is a __newindex metamethod, Lua does not perform the
primitive assignment. (If necessary, the metamethod itself can call
rawset to do the assignment.)
of that I ask what the follow specifically intends to say
"Lua does not perform the
primitive assignment. (If necessary, the metamethod itself can call
rawset to do the assignment.)"
Does this mean that if the value is a number, which is a primitive, it will not be assigned to the provided table through the metamethod event and we have to use rawget or something? This is very confusing and contradictory to me.
I want to show same examples to help you figure out this confusion.
The primitive assignment example:
local test = {}
test['x'] = 1 -- equal to rawset(test, 'x', 1)
print(test['x']) -- 1
print(rawget(test,'x')) -- 1
the primitive assignment code test['x'] = 1 equal to rawset(test, 'x', 1) when the table test have no __newindexmetamethod.
then the __newindex metamethod example:
local test = {}
setmetatable(test, {__newindex = function(t,key,value) end})
test['x'] = 1
print(test['x']) -- nil
print(rawget(test,'x')) -- nil
the assignment test['x'] = 1 will trigger to call the __newindex function.
if __newindex do nothing, then nothing happens, we will get nil result of test['x'].
If the __newindex function call rawset:
local test = {}
setmetatable(test, {
__newindex = function(t,key,value)
rawset(t,key,value) -- t:test key:'x' value:1
end})
test['x'] = 1
print(test['x']) -- 1
print(rawget(test,'x')) -- 1
the code have same effect as the first example.
So the manual say:
"Lua does not perform the primitive assignment. (If necessary, the metamethod itself can call rawset to do the assignment.)"
Then the problem is, how we can use __newindex?
It can be used to separate the old and new index in table.
local test = {y = 1}
local newtest = {}
setmetatable(test, {
__newindex =
function(t,key,value)
newtest[key] = value
end,
__index = newtest
})
test["x"] = 1
print(test['x']) -- 1
print(test['y']) -- 1
print(rawget(test, 'x')) -- nil
print(rawget(test, 'y')) -- 1
the old index 'x' and new index 'y' can all be accessed by test[key], and can be separated by rawget(test, key)
I am making quite the complex thing and I am trying to use tables as variable names cause I have found that lua works with it, that is:
lua
{[{1,2}]="Meep"}
The issue is it is callable, when I do it and try to call it using the same kind of table, it won't find it.
I have tried looking for it and such but I have no clue why it won't do this.
ua
local c = {[{1,2}]="Meep"}
print(c[{1,2}],c)
Do I expect to become but it does not.
"Meep",{[{1,2}]="Meep"}
but what I get is
nil,{[{1,2}]="Meep"}
If I however try
lua
local m={1,2}
local c = {[m]="Meep"}
print(c[m],c)
it becomes the correct one, is there a way to avoid that middle man? After all m=={1,2} will return true.
The problem you have is that tables in lua are represented as references. If you compare two different talbes you are comparing those references. So the equation only gets true if the given tables are exactly the same.
t = { 1, 2, 3 }
t2 = { 1, 2, 3 }
print(t == t) -- true
print(t2 == t) -- false
print(t2 == t2) -- true
Because of this fact, you can pass them in function per reference.
function f(t)
t[1] = 5
end
t2 = { 1 }
f(t2)
print(t2[1]) -- 5
To bypass this behavior, you could (like suggested in comments) serialize the table before using it as a key.
I'm using mksqlite to create and access an SQL database from matlab, and I want to get the number of rows in a table. I've tried this:
num = mksqlite('SELECT COUNT(*) FROM myTable');
, but the returned value isn't very helpful. If I put a breakpoint in my script and examine the variable, I find that it's a struct with a single field, called 'COUNT(_)', which seems to actually be an invalid name for a field, so I can't access it:
K>> class(num)
ans =
struct
K>> num
num =
COUNT(_): 0
K>> num.COUNT(_)
??? num.COUNT(_)
|
Error: The input character is not valid in MATLAB statements or expressions.
K>> num.COUNT()
??? Reference to non-existent field 'COUNT'.
K>> num.COUNT
??? Reference to non-existent field 'COUNT'.
Even the MATLAB IDE can't access it. If I try to double click the field in the variable editor, this gets spat out:
??? openvar('num.COUNT(_)', num.COUNT(_));
|
Error: The input character is not valid in MATLAB statements or expressions.
So how can I access this field?
You are correct that the problem is that mksqlite somehow manages to create an invalid field name that can't be read. The simplest solution is to add an AS clause to your SQL so that the field has a sensible name:
>> num = mksqlite('SELECT COUNT(*) AS cnt FROM myTable')
num =
cnt: 0
Then to remove the extra layer of indirection you can do:
>> num = num.cnt;
>> num
num =
0
I would like to use a Groovy closure to process data coming from a SQL table. For each new row, the computation would depend on what has been computed previously. However, new rows may become available on further runs of the application, so I would like to be able to reload the closure, initialised with the intermediate state it had when the closure was last executed in the previous run of the application.
For example, a closure intending to compute the moving average over 3 rows would be implemented like this:
def prev2Val = null
def prevVal = null
def prevId = null
Closure c = { row ->
println([ prev2Val, prevVal, prevId])
def latestVal = row['val']
if (prev2Val != null) {
def movMean = (prev2Val + prevVal + latestVal) / 3
sql.execute("INSERT INTO output(id, val) VALUES (?, ?)", [prevId, movMean])
}
sql.execute("UPDATE test_data SET processed=TRUE WHERE id=?", [row['id']])
prev2Val = prevVal
prevVal = latestVal
prevId = row['id']
}
test_data has 3 columns: id (auto-incremented primary key), value and processed. A moving mean is calculated based on the two previous values and inserted into the output table, against the id of the previous row. Processed rows are flagged with processed=TRUE.
If all the data was available from the start, this could be called like this:
sql.eachRow("SELECT id, val FROM test_data WHERE processed=FALSE ORDER BY id", c)
The problem comes when new rows become available after the application has already been run. This can be simulated by processing a small batch each time (e.g. using LIMIT 5 in the previous statement).
I would like to be able to dump the full state of the closure at the end of the execution of eachRow (saving the intermediate data somewhere in the database for example) and re-initialise it again when I re-run the whole application (by loading those intermediate variable from the database).
In this particular example, I can do this manually by storing the values of prev2Val, prevVal and prevId, but I'm looking for a generic solution where knowing exactly which variables are used wouldn't be necessary.
Perhaps something like c.getState() which would return [ prev2Val: 1, prevVal: 2, prevId: 6] (for example), and where I could use c.setState([ prev2Val: 1, prevVal: 2, prevId: 6]) next time the application is executed (if there is a state stored).
I would also need to exclude sql from the list. It seems this can be done using c.#sql=null.
I realise this is unlikely to work in the general case, but I'm looking for something sufficiently generic for most cases. I've tried to dehydrate, serialize and rehydrate the closure, as described in this Groovy issue, but I'm not sure how to save and store all the # fields in a single operation.
Is this possible? Is there a better way to remember state between executions, assuming the list of variables used by the closure isn't necessarily known in advance?
Not sure this will work in the long run, and you might be better returning a list containing the values to pass to the closure to get the next set of data, but you can interrogate the binding of the closure.
Given:
def closure = { row ->
a = 1
b = 2
c = 4
}
If you execute it:
closure( 1 )
You can then compose a function like:
def extractVarsFromClosure( Closure cl ) {
cl.binding.variables.findAll {
!it.key.startsWith( '_' ) && it.key != 'args'
}
}
Which when executed:
println extractVarsFromClosure( closure )
prints:
['a':1, 'b':2, 'c':4]
However, any 'free' variables defined in the local binding (without a def) will be in the closures binding too, so:
fish = 42
println extractVarsFromClosure( closure )
will print:
['a':1, 'b':2, 'c':4, 'fish':42]
But
def fish = 42
println extractVarsFromClosure( closure )
will not print the value fish
I'm using ADODB connection in C. The code works more or less fine but I'm getting errors when there is no result for my query and I try to read it. Relevant code:
__object *con, *rec;
con = __object_create("ADODB.Connection");
if(con!=NULL) con->Open("odbc name");
if (con == NULL || con->State==0)
{
return 0;
}
rec= __object_create("ADODB.RecordSet");
sprintf(query, "SELECT SUM(column) FROM table WHERE %s", constraint);
rec->CursorLocation=3;
rec->Open(query, con, 1, 3);
float result = rec->Fields(0); // <- Error here
rec->Close();
__object_delete(rec);
__object_delete(con);
I'm getting error code 80020005 (Type mismatch). The DB column is type float. When there are records that meet the constraint and I get a result everything works fine. But when it matches no records the DB server returns null and I get the error. Fortunately result is set to 0 which is reasonable but I would like to detect this better.
The standard ways (BOF/EOF, Fields->Count, == NULL, ...) all fail. Most code samples I can find are for VB and not C so they are not really helpful.
rec->Fields(0).Value returns a variant, before assigning to the float variable check if the variant represents a VT_NULL which indicates that the data returned is NULL.