Re: standard vs. hand crafted loops
In article <1147114512.131883.144670@v46g2000cwv.googlegroups.com>,
pavel.turbin@gmail.com wrote:
I've seen many times advise to use standard loops e.g. for_each,
instead of iterator and for loop. I was trying to follow this hint.
But, it looks it involves extra complexity and the code getting larger
without any benefits for me.
for example: count length of each string wrapped into another object
(MyData):
=======================================================
class MyData // some object
{
std::string m_s;
public:
MyData( const std::string &s ) : m_s(s) {}
size_t len() const { return m_s.length(); }
};
class legngth // helper for for_each
{
size_t cnt; // accumulate result
public:
legngth(){ cnt = 0;} // initialize
void operator ()( const MyData &d) { cnt += d.len(); }
operator size_t() { return cnt; } // return the result
};
int _tmain(int argc, _TCHAR* argv[])
{
std::vector<MyData> vec;
vec.push_back(MyData("a"));
vec.push_back(MyData("b"));
vec.push_back(MyData("c"));
//
// count length for all objects of "vec"
//
size_t l = 0;
// standard algorithm
l = std::for_each( vec.begin(), vec.end(), legngth() );
l = 0;
// hand crafted loop
for( std::vector<MyData>::iterator i=vec.begin(); i < vec.end();
++i)
l += i->len();
return 0;
}
=======================================================
Using for_each it takes 10 lines and auxiliary Function object. Compare
"for" loop it is only 3 lines. Such example looks as very common case,
it is typical to perform two or more operations on container. Create
function object looks as extra useless work.
Is there any better way to write such loops in general?
First, accumulate would be a better choice than for_each:
class MyData {
public:
size_t len() const;
};
size_t add_length( size_t lhs, MyData rhs ) {
return lhs + rhs.len();
}
int main()
{
vector<MyData> vec;
// load up vec
size_t l = accumulate( vec.begin(), vec.end(), 0, &add_length );
}
Using the proper algorithm reduces the size (as in number of lines) to
the same as the for loop, but I suspect you still won't like it.
I suspect that you have no problem using strlen so I will assume that it
isn't the function call you have a problem with.
No, most people seem to have a problem with the functor. They don't mind
using library functions but only if they don't need to customize them at
all. So for example, 'find' and 'count' are used often but 'find_if' and
'count_if' are shunned... Use 'copy'? Sure, but 'transform'? No way.
What are some arguments for using the algorithm over the loop?
(a) A temporary variable can be removed from your mainline function.
With the loop, you need a temp to hold the accumulation, but using
accumulate or even for_each, that temp is naturally buried in a smaller
scope which reduces the chance of error.
(b) Cyclomatic Complexity can be reduced in your program. You end up
with functions that have fewer decisions made which reduces the chance
of bugs as well.
(c) With even moderate reuse, the total line count may be reduced. If
every loop body in your program is different, then the total line count
probably won't go down by using algorithms, but if even a few of them
are the same, you might see significant line count reduction.
(d) Algorithms often do a better job of communicating intent. When you
are finding, counting, accumulating, or transforming, why not make that
explicit in the code by using a standard algorithm. Readers of your code
will like not having to bury their heads into the body of a loop looking
for possible breaks or continues and wondering what the loop is for.
Of course, if you see *none* of these benefits at a particular place in
a particular program, then by all means use a loop instead.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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