STL Algorithms

Stephen Hewitt

This is how I'd do it: typedef std::vector<whatever> collection_t; collection_t coll; // Fill 'coll' here... for (collection_t::iterator i=vec.begin(); i!=vec.end(); ++i) { // Do stuff here... } Now you can change the type of the collection by altering one line. For example to use a list instead:typedef std::list<whatever> collection_t;
This is an expression of the general design principle that one design decision should be expressed in only one place, if possible. In this case the choice of collection type only requires a change in one location and not scattered places throughout the code. STL and iterators don't make this happen automatically but they provide the abstractions to make this sort of thing possible. In your first example you'd have to make changes in multiple places including inside the loop as lists don't support indexing.

Steve

Zac Howland

Michael Dunn wrote:

The VC 6 STL docs blow so I haven't read them much and don't know what algorithm functions exist

See, this kind of comment is the kind that surprises me. My undergraduate textbook for my Data Structures and Algorithms class went over all that very well ... and Bjarne's book covers them fairly well, also. Seems to me those would be required reading for any C++ programmer (just like Petzold and Prosise are generally considered to be required reading for doing Windows programming in C++).

Michael Dunn wrote:

The STL naming scheme sucks so it's hard to figure out what some functions are for from just their name (and the docs aren't much help, see 1. I mean, seriously, WTF does "bind2nd" do??)

It does exactly what it says it does: binds the second argument to what you pass in to bind2nd. Namely, when passing a binary function/functor into an algorithm that is expecting a unary function/functor, it allows you to bind the second value of the function call to something outside the algorithm. The syntax is a bit tough to get the hang of at first, but it makes sense once you get use to it.

Michael Dunn wrote:

Most of the time when I do use algorithms, it involves writing a special-case functor for just that usage, which is workable but just feels sloppy. I know about mem_fun_ref and the like, but I haven't taken the time to figure out how they work.

There are some things where functors are useful (a general case Deleter functor, for example), but most of the time, I just write a simple function that isn't a member function to do whatever operation I was intending to do in the loop, and then use for_each or transform. It doesn't have to be a functor to be passed into an algorithm.

Michael Dunn wrote:

Instead of fighting with the docs, I'll just write a for loop and be done with it

So, what you are saying is that you basically ignore principles of good software engineering practices with regard to code reuse because you are too lazy to learn to use the standard library's power?

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Zac Howland

Steve Echols wrote:

This makes much more sense to me (but I'm old school): vector vec; for (int i = 0; i < vec.size(); i++) { } than vector vec; vector::iterator it; for (it = vec.begin(); it != vec.end(); it++) { }

See, but neither is "correct". Assume you are calling DoSomething(const whatever& w) for each iteration of the loop. The entire loop can be written as follows:

vector<whatever> vec;
// fill it somewhere
// DoSomething is declared and implemented somewhere
for_each(vec.begin(), vec.end(), DoSomething);

Using iterators in place of the index doesn't buy you anything in terms of taking advantage of the pre-made algorithms STL gives you.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Zac Howland

Stephen Hewitt wrote:

typedef std::vector collection_t; collection_t coll; // Fill 'coll' here... for (collection_t::iterator i=vec.begin(); i!=vec.end(); ++i) { // Do stuff here... }

That still doesn't use STL's algorithms. Granted, it is reimplementing a for_each or a transform (depending on what you do in the loop), but the fact is it is still reimplementing it needlessly.

Stephen Hewitt wrote:

This is an expression of the general design principle that one design decision should be expressed in only one place, if possible. In this case the choice of collection type only requires a change in one location and not scattered places throughout the code. STL and iterators don't make this happen automatically but they provide the abstractions to make this sort of thing possible.

Take a look at Scott Meyer's "Effective STL" for why this isn't really true.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Kevin McFarlane

Zac Howland wrote:

Why is it that so many people don't use the STL algorithms?

When I first started using STL for some time I just used the basic containers and looped over them. The VC++ 6 docs were good enough to get going with this kind of thing. Algorithms and function objects seemed rather mysterious. But once I found out what they were about then I started using them. The trigger for me was having to maintain code that used STL heavily. Then I was forced to understand it. I also bought Stroustup 3rd ed and Josuttis. I didn't do CS at college. My background is in civil engineering. I guess there are probably some old school C/C++ devs who cannot see any reason to move beyond what they consider to be tried-and-tested. This basically means when you maintain their code you are typically met with a mixture of error-prone C and old-style C++.

Kevin

Kevin McFarlane

Michael Dunn wrote:

The VC 6 STL docs blow so I haven't read them much and don't know what algorithm functions exist

Perhaps. But I found them good enough to get going quite far with STL. I had used STL for some time before buying a book on it.

Kevin

Zac Howland

Kevin McFarlane wrote:

I guess there are probably some old school C/C++ devs who cannot see any reason to move beyond what they consider to be tried-and-tested. This basically means when you maintain their code you are typically met with a mixture of error-prone C and old-style C++.

I can understand that (even though I disagree with the reasoning, "I've always done it this way ..." -- that isn't a reason in my opinion), but many of the questions I've seen are coming from either those that are still in college or those that are fresh out of college. I can almost understand seeing it from someone still in college if they are a first or second year CS or CPE major. But 3rd year+?

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Kevin McFarlane

Zac Howland wrote:

I can understand that (even though I disagree with the reasoning, "I've always done it this way ..." -- that isn't a reason in my opinion)

I fully agree. I was just reporting how these guys think.:)

Zac Howland wrote:

many of the questions I've seen are coming from either those that are still in college or those that are fresh out of college.

Doesn't say much for the state of CS courses does it?

Kevin

Zac Howland

Kevin McFarlane wrote:

Doesn't say much for the state of CS courses does it?

No, actually, it scares me. These guys are making my degree less valuable :sigh: Of course, then there are the colleges that stopped teaching C/C++ as the primary language for CS majors and switch over to Java. I have yet to meet one of them that understands basic (language-independent) design principles because Java doesn't force you to learn them.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Stephen Hewitt

Zac Howland wrote:

That still doesn't use STL's algorithms. Granted, it is reimplementing a for_each or a transform (depending on what you do in the loop), but the fact is it is still reimplementing it needlessly.

No this loop doesn't use algorithms, just iterators. I could have used the for_each algorithm but this would have been, depending on what I'm doing inside the loop, an example of Michael Dunn's objection to STL: having to write a "one off" functor. If what you're doing in the loop is simple or unique it is often better to use a hand written loop instead of using for_each. This is the exact reason why suggestions to include lambda functions in C++ and libraries like Boost.Lambda were written. The point I was trying to get at was that using iterators in preference to direct indexing increases you ability to make alterations to the code. For example is this really necessary or "good form": template <typename T> struct square_plus_one : std::unary_function<T, T> { T operator()(const T &v) const { return (v*v)+1; } }; int main(int argc, char* argv[]) { typedef std::vector<int> collection_t; collection_t coll; // Fill 'coll' here... std::for_each(coll.begin(), coll.end(), square_plus_one()); return 0; } I would argue the following is better and the price we pay (re-implementing the loop) is of no real consequence in this case: typedef std::vector<int> collection_t; collection_t coll; // Fill 'coll' here... for (collection_t::iterator i=vec.begin(); i!=vec.end(); ++i) { *i = ((*i)*(*1))+1; }

Zac Howland wrote:

Take a look at Scott Meyer's "Effective STL" for why this isn't really true.

What's not true? If I want to see if I get better performance or use less memory using a std::list I simply change one line - this is a fact. Not many people will think it's better to have to make multiple scattered changes and run the risk of introducing an error somewhere along the way. If you've got a specific point make it here as opposed to referring to a book but giving no clue to what to mean

Zac Howland

Stephen Hewitt wrote:

Michael Dunn's objection to STL: having to write a "one off" functor.

You don't have to write a functor to use for_each, nor most of the other algorithms. Using your own example (and by the way, your for_each example actually does nothing but waste CPU cycles as written), the following works perfectly fine:

template<class T> T square_plus_one(const T& i)
{
	return (i * i + 1);
}

int main()
{
	vector<int> intVec;
	// fill vector here
	transform(intVec.begin(), intVec.end(), intVec.begin(), square_plus_one<int> ) ;

	return 0;
}

Granted, the Lamda library makes this even easier, and I fully support the use of it. However, for most common loops, this really isn't that complex.

Stephen Hewitt wrote:

What's not true? If I want to see if I get better performance or use less memory using a std::list I simply change one line - this is a fact. Not many people will think it's better to have to make multiple scattered changes and run the risk of introducing an error somewhere along the way. If you've got a specific point make it here as opposed to referring to a book but giving no clue to what to mean.

What I meant is that the containers are not completely interchangible. For some simple things such as iterating through them, they are designed to be the same to make use of the algorithms. However, things like insertion, deletion, allocation of space, etc ... many of them are different (have different function names, don't have certain functions, etc). I directed you to the book because he gives a much better explanation that I could hope to offer on this forum (at least without plaugerizing the book).

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Stephen Hewitt

Zac Howland wrote:

You don't have to write a functor to use for_each, nor most of the other algorithms. Using your own example (and by the way, your for_each example actually does nothing but waste CPU cycles as written), the following works perfectly fine:

Firstly in your example you've written a "one off" function instead of a "one off" functor, the same objection applies in this case. Secondly, your code doesn't seem to work. Try compiling this: #include <iostream> #include <vector> #include <algorithm> #include <iterator> template <typename T> T square_plus_one(const T& i) { return (i * i + 1); } int main() { // For notational convenience. using namespace std; vector<int> intVec; // Fill vector. for (int i=1; i<=10; ++i) { intVec.push_back(i); } // Transform the data. transform(intVec.begin(), intVec.end(), intVec.begin(), square_plus_one<int> ); // Output the results. copy(intVec.begin(), intVec.end(), ostream_iterator<int>(cout, " ")); cout << endl; return 0; } I get the following error: "CommandLine.obj : error LNK2001: unresolved external symbol "int __cdecl square_plus_one(int const &)" (?square_plus_one@@YAHABH@Z)" I'm not sure if this is a compiler bug or what (MSVC6) but regardless it's a problem. As to the “wasted cycles” I concede that I made a mistake in that the results of my calculations are never used (oops). Functors are no less efficient in general however, consider the following. I've altered the code as follows: // Changed function so we compile and made inline. inline int square_plus_one(int i) { return (i * i + 1); } // Added a functor version for comparison: struct functor_square_plus_one : std::unary_function<int, int> { int operator()(int i) const { return (i * i + 1); } }; // Altered transform: transform(intVec.begin(), intVec.end(), intVec.begin(), square_plus_one) ; // Added call to functor ver

Stephen Hewitt

Another interesting library is Boost.Foreach. See details here[^] This enables you to write code like this: foreach (int i, vecInts) { cout << i; } This assumes the following:#include <boost/foreach.hpp> #define foreach BOOST_FOREACH

Steve

Lost User

he he. Java-stylee! One of my favourite helper templates comes straight from Bjorn Karlsson, author of "Beyond the C++ Standard Library: An Introduction to Boost":

template <typename T, typename O> void for_all(T& t, O o)
{
std::for_each(t.begin(), t.end(), o);
}

e.g.:

vector<int> vec;
...
for_all(vec, func);

I use this everywhere. I am also investigating boost::lambda, but it seems to get more complicated when using containers of smart pointers. Early days, but I am head over heels in love with Boost! :-O

Stuart Dootson

Robert Edward Caldecott wrote:

I am also investigating boost::lambda, but it seems to get more complicated when using containers of smart pointers

It does. If you're just using bind, then use boost::bind - it can cope with smart pointers (the boost ones at least!). Otherwise, I've defined macros to do bind the smart pointers get method, as below

#define VALUE(PTR) bind(&Symbols::ValuePtr::get, PTR)

   std::sort(allValues.begin(), allValues.end(), 
             bind(&Value::Address, VALUE(_1)) < bind(&Value::Address, VALUE(_2)));

I suspect Boost.Lambda won't change to cope with smart pointers (I don't know how active its main developer Jaako Jarvi is?). However, Joel de Guzman's developed somethng very similar for Boost.Spirit (it's called Phoenix) and I'm sure I've heard talk of that being merged with lambda...or something. Best place to ask is on the Boost developers list, I guess...

Zac Howland

Stephen Hewitt wrote:

Firstly in your example you've written a "one off" function instead of a "one off" functor, the same objection applies in this case.

Most people's main objection to writing "one off" functors is that they are several extra lines of code (e.g. declare the structure/class, declare the operator, etc.) A function doesn't really add that much to the lines of code, and generally makes the loop easier to read. In this example, it wouldn't matter much, since the loop is fairly easy to follow to begin with; however, I have seen some fairly complex loops in some code I worked on at my last job that simplified greatly using that technique.

Stephen Hewitt wrote:

Secondly, your code doesn't seem to work. Try compiling this: ... I'm not sure if this is a compiler bug or what (MSVC6) but regardless it's a problem.

This is one of the areas where VC6 was not fully compliant with the standard. Passing function templates to the algorithms doesn't quite work with that compiler. I compiled the example (almost identical to what you wrote, by the way) using VS2003.

Stephen Hewitt wrote:

As to the “wasted cycles” I concede that I made a mistake in that the results of my calculations are never used (oops). Functors are no less efficient in general however, consider the following. I've altered the code as follows:

What I was getting at was that the results were never used. I didn't mean to imply that functors are less efficient, because that isn't the case. Most people's main objection to them is the fact that they are creating a separate object that will never be reused. Writing a function for this makes things a bit less "overkill" (at least in my opinion).

Stephen Hewitt wrote:

inline int square_plus_one(int i) { return (i * i + 1); }

Just an FYI, when you pass the function to an algorithm, the compiler immediately ignores the inline request.

Stephen Hewitt wrote:

From what I hear from experts there are cases in which the functor version is actually more efficient as many compilers find it easier to inline a functor then code via a function pointer.

I haven't heard that one, but I do know that when you pass a function via function pointer, the compiler cannot inline it (you can't pass the

Zac Howland

Stuart Dootson wrote:

I suspect Boost.Lambda won't change to cope with smart pointers (I don't know how active its main developer Jaako Jarvi is?). However, Joel de Guzman's developed somethng very similar for Boost.Spirit (it's called Phoenix) and I'm sure I've heard talk of that being merged with lambda...or something.

Several of the Boost libraries are being considered as additions to the next standard. Many of them are already in tr1 (an std extension until the next standard is finalized). I know the smart pointers are already in there (I make use of them fairly heavily), and I think lambda is, but I'm not sure ... something I'll have to double check.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Stephen Hewitt

Zac Howland wrote:

Just an FYI, when you pass the function to an algorithm, the compiler immediately ignores the inline request.

An inspection of the machine code I posted for both examples, the function and the functor, shows that in both cases the code was inlined. And this was with MSVC6, newer compilers may do even better.

Steve

Zac Howland

If it does, great ... just know that the compiler documentation says otherwise: MSDN[^]

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Stephen Hewitt

Well it seems to be a mistake or an oversimplification. From the code I posted here[^] it can be seen that: 1. Both the function and functor versions produce exactly the same code. 2. Both versions have no call instructions. 3. The add and imul instructions which do the actual math can be seen in place. I often find it enlightening to look at the code generated by the compiler. One surprise I had recently was when I was evaluating the Boost BOOST_FOREACH macro. Although when you look at the source there is a fair bit of code behind it, when I actually looked at the code generated in a release build it was actually smaller and more efficient then a hand written loop.

Steve