1) Do these generate the same byte code?
2) If not, is there any gain in using one over the other in certain circumstances?
// LINQ select statement
return from item in collection
select item.Property;
// foreach in an iterator block
foreach (item in collection)
yield return item.Property;
They don't generate the same code but boil down to the same thing, you get an object implementing IEnumerable<typeof(Property)>. The difference is that linq provides iterator from its library (in this case most likely by using WhereSelectArrayIterator or WhereSelectListIterator) whereas in the second example you yourself generate an iterator block that dissects a collection. An iterator block method is always, by ways of compiler magic, compiled as a separate class implementing IEnumerable<typeof(yield)> which you don't see but instantiate implicitly when you call iterator block method.
Performance wise, #1 should be slightly (but just slightly) faster for indexable collections because when you loop through the resulting IEnumerable you go directly from your foreach into collection retrieval in an optimized linq iterator. In example #2 you go from foreach into your iterator block's foreach and from there into collection retrieval and your performance depends mostly on how smart the compiler is at optimizing yield logic. In any case I would imagine that for any complex collection mechanism the cost of retrieval marginalizes this difference.
IMHO, I would always go with #1, if nothing else it saves me from having to write a separate method just for iterating.
No they don't generate the same byte code. The first one returns a pre-existing class in the framework. The second one returns a compiler generated state machine that returns the items from the collection. That state machine is very similar to the class that exists in the framework already.
I doubt there's much performance difference between the two. Both are doing very similar things in the end.
Related
I'm designing a interpreter which uses recursive descent and I've got to the point where I'm starting to implement built in methods.
One example of a method I'm implementing is the print() method which outputs to the console, just like python's print() method and Java's System.out.println().
However it has come to my attention that there are multiple ways of implementing these built in methods. I'm sure there is many more but I have determined 2 viable ways of achieving this and I'm trying to establish which way is the best practise. For context below is the different layers I'm using within my interpreter which is loosely based off of https://www.geeksforgeeks.org/introduction-of-compiler-design/ and other tutorials I've come across.
Lexer
Parser
Semantic Analyser
Interpreter / Code Generator
1. Creating a AST Node for each individual built in method.
This method entails programming the parser to generate a node for each individual method. This means that a unique node will exist for each method. For example:
The parser will look to generate a node when a TPRINT token is found in the lexer.
print : TPRINT TLPAREN expr TRPAREN {$$ = new Print($3);}
;
And this is what the print class looks like.
class Print : public Node {
public:
virtual VariableValue visit_Semantic(SemanticAnalyzer* analyzer) override;
virtual VariableValue visit_Interpreter(Interpreter* interpreter) override;
Node* value;
Print(Node* cvalue) {
value = cvalue;
}
}
From there I define the visit_Semantic and visit_interpreter methods, and visit them using recursion from the top node.
I can think of a couple advantages/disadvantages to using this method:
Advantages
When the code generator walks the tree and visits the Print node's visit_interpreter method, it can straight away execute the response directly as it is programmed into it's visit methods.
Disadvantages
I will have to write a lot of copy paste code. I will have to create a node for each separate method and define it's parser grammar.
2. Creating a Generic AST Node for a method call Node and then use a lookup table to determine which method is being called.
This involves creating one generic node MethodCall and grammar to determine if a method has been called, with some unique identifier such as a string for the method it is referring to. Then, when the MethodCall's visit_Interpreter or visit_Semantic method is called, it looks up in a table which code to execute.
methcall : TIDENTIFIER TLPAREN call_params TRPAREN {$$ = new MethodCall($1->c_str(), $3);}
;
MethodCall Node. Here the unique identifier is std::string methodName:
class MethodCall : public Node {
public:
virtual VariableValue visit_Semantic(SemanticAnalyzer* analyzer) override;
virtual VariableValue visit_Interpreter(Interpreter* interpreter) override;
std::string methodName;
ExprList *params;
MethodCall(std::string cmethodName, ExprList *cparams) {
params = cparams;
methodName = cmethodName;
}
};
Advantages:
One generic grammar/node for all method calls. This makes it more readable
Disadvantages:
At some point the unique identifier std::string methodName will have to be compared in a lookup table to determine a response for it. This is not as efficient as directly programming in a response to a Node's visit methods.
Which practise is the best way of handling methods within a compiler/interpreter? Are there different practises all together which are better, or are there any other disadvantages/advantages I am missing?
I'm fairly new to compiler/interpreter design so please ask me to clarify if I have got some terminology wrong.
As far as I see it, you have to split things up into methods somewhere. The question is, do you want to implement this as part of the parser definition (solution 1) or do you want to implement this on the C++ side (solution 2).
Personally, I would prefer to keep the parser definition simple and move this logic to the C++ side, that is solution 2.
From a runtime perspective of solution 2, I wouldn't worry too much about that. But in the end, it depends on how frequently that method is called and how many identifiers you have. Having just a few identifiers is different from comparing say hundreds of strings in an "else if" manner.
You could first implement it the simple straight-forward way, i.e. match the string identifiers in an "else if" manner, and see if you run into runtime issues.
If you experience runtime issues, you could use a hash function. The "hardcore" way would be to implement an optimal hash function yourself and check hash function optimality offline, since you know the set of string identifiers. But for your application that would be probably be rather overkill or for academic purposes, and I would recommend to just use unordered_map from the STL (which uses hashing under the hood, see also How std::unordered_map is implemented) to map your string identifiers to index numbers, so that you can implement your jump table with an efficient switch operation on those index numbers.
You should most definitely use a table lookup. It makes things much easier for you. Also, think about the functions users define! Then you'll definitely need a table.
I have a public method which uses a variable (only in the scope of the public method) I pass as a parameter we will call A, this method calls a private method multiple times which also requires the parameter.
At present I am passing the parameter every time but it looks weird, is it bad practice to make this member variable of the class or would the uncertainty about whether it is initialized out way the advantages of not having to pass it?
Simplified pseudo code:
public_method(parameter a)
do something with a
private_method(string_a, a)
private_method(string_b, a)
private_method(string_c, a)
private_method(String, parameter a)
do something with String and a
Additional information: parameter a is a read only map with over 100 entries and in reality I will be calling private_method about 50 times
I had this same problem myself.
I implemented it differently in 3 different contexts to see hands-on what are result using 3 different strategies, see below.
Note that I am type of programmer that makes many changes to the code always trying to improve it. Thus I settle only for the code that is amenable to changes, readbale, would you call this "flexible" code. I settle only for very clear code.
After experimentation, I came to these results:
Passing a as parameter is perfectly OK if you have one or two - short number - of such values. Passing in parmeters has very good visibility, clarity, clear passing lines, well visible lifetime (initialization points, destruction points), amenable to changes, easy to track.
If number of such values begin to grow to >= 5-6 values, I swithc to approach #3 below.
Passing values through class members -- did not do good to clarity of my code, eventually I got rid of it. It makes for less clear code. Code becomes muddled. I did not like it. It had no advantages.
As alternative to (1) and (2), I adopted Inner class approach, in cases when amount of such values is > 5 (which makes for too long argument list).
I pack those values into small Inner class and pass such object by reference as argument to all internal members.
Public function of a class usually creates an object of Inner class (I call is Impl or Ctx or Args) and passes it down to private functions.
This combines clarity of arg passing with brevity. It's perfect.
Good luck
Edit
Consider preparing array of strings and using a loop rather than writing 50 almost-identical calls. Something like char *strings[] = {...} (C/C++).
This really depends on your use case. Does 'a' represent a state that your application/object care about? Then you might want to make it a member of your object. Evaluate the big picture, think about maintenance, extensibility when designing structures.
If your parameter a is a of a class of your own, you might consider making the private_method a public method for the variable a.
Otherwise, I do not think this looks weird. If you only need a in just 1 function, making it a private variable of your class would be silly (at least to me). However, if you'd need it like 20 times I would do so :P Or even better, just make 'a' an object of your own that has that certain function you need.
A method should ideally not pass more than 7 parameters. Using the number of parameters more than 6-7 usually indicates a problem with the design (do the 7 parameters represent an object of a nested class?).
As for your question, if you want to make the parameter private only for the sake of passing between private methods without the parameter having anything to do with the current state of the object (or some information about the object), then it is not recommended that you do so.
From a performance point of view (memory consumption), reference parameters can be passed around as method parameters without any significant impact on the memory consumption as they are passed by reference rather than by value (i.e. a copy of the data is not created). For small number of parameters that can be grouped together you can use a struct. For example, if the parameters represent x and y coordinates of a point, then pass them in a single Point structure.
Bottomline
Ask yourself this question, does the parameter that you are making as a members represent any information (data) about the object? (data can be state or unique identification information). If the answer to his question is a clear no, then do not include the parameter as a member of the class.
More information
Limit number of parameters per method?
Parameter passing in C#
Suppose I have a List<T> with 1000 items in it.
I'm then passing this to a method that filters this List.
As it drops through the various cases (for example there could be 50), List<T> may have up to 50 various Linq Where() operations performed on it.
I'm interested in this running as quickly as possible. Therefore, I don't want this List<T> filtered each time a Where() is performed on it.
Essentially I need it to defer the actual manipulation of the List<T> until all the filters have been applied.
Is this done natively by the compiler? Or just when I call .ToList() on the IEnumerable that a List<T>.Where() returns, or should I perform the Where() operations on X (Where X = List.AsQueryable())?
Hope this makes sense.
Yes, deferred execution is supported natively. Every time you apply a query or lambda expression on your List the query stores all the expressions that is executed only when you call .ToList() on the Query.
Each call to Where will create a new object which knows about your filter and the sequence it's being called on.
When this new object is asked for a value (and I'm being deliberately fuzzy between an iterator and an iterable here) it will ask the original sequence for the next value, check the filter, and either return the value or iterate back, asking the original sequence for the next value etc.
So if you call Where 50 times (as in list.Where(...).Where(...).Where(...), you end up with something which needs to go up and down the call stack at least 50 times for each item returned. How much performance impact will that have? I don't know: you should measure it.
One possible alternative is to build an expression tree and then compile it down into a delegate at the end, and then call Where. This would certainly be a bit more effort, but it could end up being more efficient. Effectively, it would let you change this:
list.Where(x => x.SomeValue == 1)
.Where(x => x.SomethingElse != null)
.Where(x => x.FinalCondition)
.ToList()
into
list.Where(x => x.SomeValue == 1 && x.SomethingElse != null && x.FinalCondition)
.ToList()
If you know that you're just going to be combining a lot of "where" filters together, this may end up being more efficient than going via IQueryable<T>. As ever, check performance of the simplest possible solution before doing something more complicated though.
There is so much failure in the question and the comments. The answers are good but don't hit hard enough to break through the failure.
Suppose you have a list and a query.
List<T> source = new List<T>(){ /*10 items*/ };
IEnumerable<T> query = source.Where(filter1);
query = query.Where(filter2);
query = query.Where(filter3);
...
query = query.Where(filter10);
Is [lazy evaluation] done natively by the compiler?
No. Lazy evaluation is due to the implementation of Enumerable.Where
This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.
speed penalty there is on calling List.AsQueryable().ToList()
Don't call AsQueryable, you only need to use Enumerable.Where.
thus won't prevent a 50 calls deep call stack
Depth of call stack is much much less important than having a highly effective filter first. If you can reduce the number of elements early, you reduce the number of method calls later.
In C#, you could do something like this:
public IEnumerable<T> GetItems<T>()
{
for (int i=0; i<10000000; i++) {
yield return i;
}
}
This returns an enumerable sequence of 10 million integers without ever allocating a collection in memory of that length.
Is there a way of doing an equivalent thing in Ruby? The specific example I am trying to deal with is the flattening of a rectangular array into a sequence of values to be enumerated. The return value does not have to be an Array or Set, but rather some kind of sequence that can only be iterated/enumerated in order, not by index. Consequently, the entire sequence need not be allocated in memory concurrently. In .NET, this is IEnumerable and IEnumerable<T>.
Any clarification on the terminology used here in the Ruby world would be helpful, as I am more familiar with .NET terminology.
EDIT
Perhaps my original question wasn't really clear enough -- I think the fact that yield has very different meanings in C# and Ruby is the cause of confusion here.
I don't want a solution that requires my method to use a block. I want a solution that has an actual return value. A return value allows convenient processing of the sequence (filtering, projection, concatenation, zipping, etc).
Here's a simple example of how I might use get_items:
things = obj.get_items.select { |i| !i.thing.nil? }.map { |i| i.thing }
In C#, any method returning IEnumerable that uses a yield return causes the compiler to generate a finite state machine behind the scenes that caters for this behaviour. I suspect something similar could be achieved using Ruby's continuations, but I haven't seen an example and am not quite clear myself on how this would be done.
It does indeed seem possible that I might use Enumerable to achieve this. A simple solution would be to us an Array (which includes module Enumerable), but I do not want to create an intermediate collection with N items in memory when it's possible to just provide them lazily and avoid any memory spike at all.
If this still doesn't make sense, then consider the above code example. get_items returns an enumeration, upon which select is called. What is passed to select is an instance that knows how to provide the next item in the sequence whenever it is needed. Importantly, the whole collection of items hasn't been calculated yet. Only when select needs an item will it ask for it, and the latent code in get_items will kick into action and provide it. This laziness carries along the chain, such that select only draws the next item from the sequence when map asks for it. As such, a long chain of operations can be performed on one data item at a time. In fact, code structured in this way can even process an infinite sequence of values without any kinds of memory errors.
So, this kind of laziness is easily coded in C#, and I don't know how to do it in Ruby.
I hope that's clearer (I'll try to avoid writing questions at 3AM in future.)
It's supported by Enumerator since Ruby 1.9 (and back-ported to 1.8.7). See Generator: Ruby.
Cliche example:
fib = Enumerator.new do |y|
y.yield i = 0
y.yield j = 1
while true
k = i + j
y.yield k
i = j
j = k
end
end
100.times { puts fib.next() }
Your specific example is equivalent to 10000000.times, but let's assume for a moment that the times method didn't exist and you wanted to implement it yourself, it'd look like this:
class Integer
def my_times
return enum_for(:my_times) unless block_given?
i=0
while i<self
yield i
i += 1
end
end
end
10000.my_times # Returns an Enumerable which will let
# you iterate of the numbers from 0 to 10000 (exclusive)
Edit: To clarify my answer a bit:
In the above example my_times can be (and is) used without a block and it will return an Enumerable object, which will let you iterate over the numbers from 0 to n. So it is exactly equivalent to your example in C#.
This works using the enum_for method. The enum_for method takes as its argument the name of a method, which will yield some items. It then returns an instance of class Enumerator (which includes the module Enumerable), which when iterated over will execute the given method and give you the items which were yielded by the method. Note that if you only iterate over the first x items of the enumerable, the method will only execute until x items have been yielded (i.e. only as much as necessary of the method will be executed) and if you iterate over the enumerable twice, the method will be executed twice.
In 1.8.7+ it has become to define methods, which yield items, so that when called without a block, they will return an Enumerator which will let the user iterate over those items lazily. This is done by adding the line return enum_for(:name_of_this_method) unless block_given? to the beginning of the method like I did in my example.
Without having much ruby experience, what C# does in yield return is usually known as lazy evaluation or lazy execution: providing answers only as they are needed. It's not about allocating memory, it's about deferring computation until actually needed, expressed in a way similar to simple linear execution (rather than the underlying iterator-with-state-saving).
A quick google turned up a ruby library in beta. See if it's what you want.
C# ripped the 'yield' keyword right out of Ruby- see Implementing Iterators here for more.
As for your actual problem, you have presumably an array of arrays and you want to create a one-way iteration over the complete length of the list? Perhaps worth looking at array.flatten as a starting point - if the performance is alright then you probably don't need to go too much further.
For example:
m_lottTorqueTools = (From t In m_lottTorqueTools _
Where Not t.SlotNumber = toolTuple.SlotNumber _
And Not t.StationIndex = toolTuple.StationIndex).ToList
What algorithm occurs here? Is there a nested for loop going on in the background? Does it construct a hash table for these fields? I'm curious.
Query expressions are translated into extension method calls, usually. (They don't have to be, but 99.9% of queries use IEnumerable<T> or IQueryable<T>.)
The exact algorithm of what that method does varies from method to method. Your sample query wouldn't use any hash tables, but joins or grouping operations do, for example.
The simple Where call translates to something like this in C# (using iterator blocks, which aren't available in VB at the moment as far as I'm aware):
public static IEnumerable<T> Where(this IEnumerable<T> source,
Func<T, bool> predicate)
{
// Argument checking omitted
foreach (T element in source)
{
if (predicate(element))
{
yield return element;
}
}
}
The predicate is provided as a delegate (or an expression tree if you're using IQueryable<T>) and is called on each item in the sequence. The results are streamed and execution is deferred - in other words, nothing happens until you start asking for items from the result, and even then it only does as much as it needs to in order to provide the next result. Some operators aren't deferred (basically the ones which return a single value instead of a sequence) and some buffer the input (e.g. Reverse has to read to the end of the sequence before it can return any results, because the last result it reads is the first one it has to yield).
It's beyond the scope of a single answer to give details of every single LINQ operator I'm afraid, but if you have questions about specific ones I'm sure we can oblige.
I should add that if you're using LINQ to SQL or another provider that's based on IQueryable<T>, things are rather different. The Queryable class builds up the query (with the help of the provider, which implements IQueryable<T> to start with) and then the query is generally translated into a more appropriate form (e.g. SQL) by the provider. The exact details (including buffering, streaming etc) will entirely depend on the provider.
LINQ in general has a lot going on behind the scenes. With any query, it is first translated into an expression tree using an IQueryableProvider and from there the query is typically compiled into CIL code and a delegate is generated pointing to this function, which you are essentially using whenever you call the query. That's an extremely simplified overview - if you want to read a great article on the subject, I recommend you look at How LINQ Works - Creating Queries. Jon Skeet also posted a good answer on this site to this question.
What happens not only depends on the methods, but also depends on the LINQ provider in use. In the case where an IQueryable<T> is being returned, it is the LINQ provider that interprets the expression tree and processes it however it likes.