In a Sybase database I am working with result sets are used (misused?) as variables.
For example, one often finds lines such as the following:
select SOMETHING = 'bla'
"SOMETHING" is technically a result set ... and the content of the result set is used by the application accessing the database. Since "SOMETHING" is not a variable, it does not get declared anywhere.
I have never seen this kind of hack before (and colleagues of mine couldn't explain to me the reason why it was done that way) and I have not found anything about it on google.
Is there some reference available that explains why one would want to use such a hack as opposed to "normal" variables?
I think you are not reading this correctly. This query simply means that there is a one-column result set with the column named 'SOMETHING'. This query is equivalent to: SELECT 'bla' AS SOMETHING
Related
What is the difference between:
db.getCollection('booking').find()
and
db.booking.find()
Are they exactly the same, or when should I use which one?
db.getCollection('booking').find({_id:"0J0DR"})
db.booking.find({_id:"0J0DR"})
Yes, they are exactly the same and you can use either.
The first form db.getCollection(collectionName).find() becomes handy when your collection name contains special characters that will otherwise render the other syntax redundant.
Example:
Suppose your collection has a name that begin with _ or matches a database shell method or has a space, then you can use db.getCollection("booking trips").find() or db["booking trips"].find() where doing db.booking trips.find() is impossible.
I prefer using db.collection() to either as it will work on nonexistent collections, which is particularly useful when for example creating the first user in a users collection that doesn't yet exist.
db.collection('users').findOneAndUpdate(...) // Won't throw even if the collection doesn't exist yet
In addition to the previous answers, on the shell, they might be exactly the same but in real IDE (like PyCharm), db.getCollection(collectionName) gives you back the whole doculment even with out the find() method.
I have a process which is repeated on a set of data stored in separate folders. Each time a certain folders data is processed I need new variable names as I need to results separate after the initial processing is finished for more processing.
For example at the start of each new block of the repeated function I declare sets of arrays
Set_1 = zeros(dim, number);
vectors_1 = zeros(dim, number);
For the next set of data I need:
`Set_2 = .........`
and so on. There is going to be alot of these sets so I need a way to automate the creation of these variables, and the use the new variables names in the function whilst maintaining that they are separate once all the functions are completed.
I first tried using strcat('Set_1',int2str(number)) = zeros(dim, number) but this does not work, I believe because it means I would be trying to set an array as a string. I'm sure there must be a way to create one function and have the variables dynamically created but it seems to be beyond me, so it's probably quite obvious, so if anyone can tell me a way that would be great.
I'd not do it like this. It's a bad habit, it's better to use a cell array or a struct to keep multiple sets. There is a small overhead (size-wise) per field, but it'll be a lot easier to maintain later on.
If you really, really want to do that use eval on the string you composed.
The MATLAB function genvarname does what you want. In your case it would look something like:
eval(genvarname('Set_', who)) = zeros(dim, number);
However, I would follow the recommendations of previous answers and use a cell or struct to store the results.
This sort of pattern is considered harmful since it requires the eval function. See one of the following for techniques for avoiding it:
http://www.mathworks.co.uk/support/tech-notes/1100/1103.html
http://blogs.mathworks.com/loren/2005/12/28/evading-eval/
If you insist on using eval, then use something like:
eval(sprintf('Set_1%d = zeros(dim, number);', number))
I wish to expand a structure (bac) with a number of fields from another structure (BT). The names of these fields are contained in the cell array (adds) as strings.
this is what i have now (and obviously doesn't do the job, explaining this post):
for i=1:numel(adds)
eval(genvarname('bac.',adds{i})) = eval(strcat('BT.',adds{i}));
end
I also tried using sprintf, which did not seem to work for me. I feel confident one of you knows how to do it, since I feel it should be rather easy.
The best way of doing this is to use dynamic field names:
for i=1:numel(adds)
bac.(adds{i}) = BT.(adds{i});
end
I'm reasonably new to Matlab, and have been trying to teach myself. I have looked for a similar question, but can't find one that's quite right.
In my workspace I have several structures with similar names. These structures will always start with the same word ('Base'), though the rest of the name will change ('1', '2', '3'), so for example Base1, Base2, Base3... etc. These variables were generated using the data cursor tool in a figure, so contain the fields Target, Position and DataIndex. I am only interested in the value in Base*.Position(1,1). I would like to extract this value from each structure, as many times as there are structures (in one instance there may be 6 structures, another time only 4).
I am considering using the eval function, but it seems to work on exact strings rather than only the first part of a name. Additionally, a lot of documentation seems to advise against using eval.
So far I have:
clearvar except 'Base*'
list_variables=who;
for i=1:length(list_variables)
BaseTS(i) = eval('Base1.Position(1,1)');
end
It's the for loop I'm stuck on, as I don't know how to generalise so it will extract the value .Position(1,1) for each different structure name.
Thanks in advance
Instead of having many structures called Base1, Base2 etc rather put your structure in an array. Then you could rather call Base(1).Position(1,1), Base(2).Position... etc. Your code will be more flexible and manageable this way.
So I suggest when you export using the data cursor, export to a variable called Base_temp and then immediately stick this into the next element of an array:
Base(end+1) = Base_temp
or even:
Position(end+1) = Base_temp.Position(1,1);
Then it's just a case of pressing up and enter after each time you export with the data cursor.
What you have read about avioding eval is correct, it's very rare (if ever) that eval is a good idea. It makes your code hard to read and very hard to debug. But since you're learning, this is how you could fix your loop. (But don't do this way, seriously don't, use arrays rather):
for i=1:length(list_variables)
BaseTS(i) = eval(['Base', num2str(i), '.Position(1,1)']);
end
in other words use string concatenation to build up your string and use the looping variable (i) to get the different numbers. You'll need num2str to convert fromthe number to the string. But don't do it this way. This is a bad way.
Dan's suggestion about avoiding eval is very valid. But if you decide to keep on the structures you have in your workspace, here's something without loops, but again cellfun seems to use loops internally. So, I guess this could be an alternative solution, with the not-so-popular eval -
list1 = who('Base*')
list2 = cellstr(strcat('BaseTS(',num2str([1:numel(list1)]'),')='));%%//'
ev1 = strcat(list2,list1,'.Position(1,1)');%%//'
cellfun(#evalc,ev1,'uni',0)
Ruby has this very interesting functionality in which when you create a class with 'Class.new' and assign it to a constant (uppercase), the language "magically" sets up the name of the class so it matches the constant.
# This is ruby code
MyRubyClass = Class.new(SuperClass)
puts MyRubyClass.name # "MyRubyClass"
It seems ruby "captures" the assignment and inserts sets the name on the anonymous class.
I'd like to know if there's a way to do something similar in Lua.
I've implemented my own class system, but for it to work I've got to specify the same name twice:
-- This is Lua code
MyLuaClass = class('MyLuaClass', SuperClass)
print(MyLuaClass.name) -- MyLuaClass
I'd like to get rid of that 'MyLuaClass' string. Is there any way to do this in Lua?
When assigning to global variables you can set a __newindex metamethod for the table of globals to catch assignments of class variables and do whatever is needed.
You can eliminate one of the mentions of MyLuaClass...
> function class(name,superclass) _G[name] = {superclass=superclass} end
> class('MyLuaClass',33)
> =MyLuaClass
table: 0x10010b900
> =MyLuaClass.superclass
33
>
Not really. Lua is not an object-orientated language. It can behave like one sometimes. But far from every time. Classes are not special values in Lua. A table has the value you put in it, no more. The best you can do is manually set the key in _G from the class function and eliminate having to take the return value.
I guess that if it REALLY, REALLY bothers you, you could use debug.traceback(), get a stack trace, find the calling file, and parse it to find the variable name. Then set that. But that's more than a little overkill.
With respect at least to Lua 5.2: You can capture assignments to A) the global table of a Lua State, as mentioned in a previous reply, and also B) to any other Lua Object whose __index and __newindex metamethods have been substituted (by replacing the metatable), this I can confirm as I'm currently using both these techniques to hook and redirect assignments made by Lua scripts to external C/C++ resource management.
There is a gotcha with regards to reading them back though, the trick is to NOT let the values be set in a Lua State.
As soon as they exist there, your hooks will fail to be called, so if you want to go down this path, you need to capture ALL get/set attempts, and NEVER store the values in a Lua State.