Convert a Java iterator loop to C++ STL?

[samuel.y.l.cheung]

Hi,

I am trying to convert a Java iterator loop to C++ STL? the loop looks
like this:

public static boolean func (List aList) {
int minX = 0
for (Iterator iter = aList.listIterator(1); iter.hasNext() {
A a = (A) iter.next();

if (! closeEnough(minX, a.x) ) {
return true;
} else {
minX = a.x;
}
}
return false;
}

I am reading the "Effecitive STL" book, it said i should use STL
algorithm instead of writing my loop if possible. I was thinking of
using for_each(). But in this case, i need to update a local variable
'minX' and stop the iteration if certain condition is met. Should I
use for_each()? or I write my own iterator loop?

Thank you for any idea.

 

[Alan Johnson]

Hi,

I am trying to convert a Java iterator loop to C++ STL? the loop looks
like this:

public static boolean func (List aList) {
int minX = 0
for (Iterator iter = aList.listIterator(1); iter.hasNext() {
A a = (A) iter.next();

if (! closeEnough(minX, a.x) ) {
return true;
} else {
minX = a.x;
}
}
return false;
}

I am reading the "Effecitive STL" book, it said i should use STL
algorithm instead of writing my loop if possible. I was thinking of
using for_each(). But in this case, i need to update a local variable
'minX' and stop the iteration if certain condition is met. Should I
use for_each()? or I write my own iterator loop?

Thank you for any idea.

I think the algorithm you want is "find_if". You can then write your
own predicate. For example, you might have a class named "closeEnough"
to act as your predicate:

#include <functional>

class closeEnough : public std::unary_function<bool, int>
{
private :
int minX ;
public :
closeEnough() : minX(0) {}

bool operator()(int x)
{
// Your logic for the "closeEnough" function goes here.
// Note your ability to change minX as you like.
return true ;
}
} ;


Then your function above would reduce to something like the following
(forgive me if I got some of the logic backwards, but this is the
general idea):

#include <algorithm>
#include <list>

bool func(std::list<int> aList)
{
std::list<int>::iterator i ;
closeEnough cefunc ;

i = std::find_if(aList.begin(), aList.end(), cefunc) ;
return (i == aList.end()) ;
}

 

[samuel.y.l.cheung]

Thanks. This is helpful.

 

[Christopher Dearlove]

// Note your ability to change minX as you like.

return true ;

Not necessarily. Changing state in a predicate is a dangerous business, see

http://www.informit.com/articles/article.asp?p=25142&seqNum=4&rl=1

(Here the order of find_if is guaranteed, so you are more likely to get
away with it. But it's a dodgy practice at best, and likely to teach bad
habits.)

 

[samuel.y.l.cheung]

Do you have a better solution to my problem?
Thanks.

 

[Mark P]

Do you have a better solution to my problem?
Thanks.

Just write a plain old loop.

 

[Jeff Flinn]

Do you have a better solution to my problem?
Thanks.

Depending on just what CloseEnough does, you may just be able to use
min_element as show below. I'm not sure by your example as to whether the
min value is really needed.

#include <algorithm>
#include <list>

bool func( const std::list<int>& aList )
{
typedef std::list<int>::const_iterator tCItr;

int lMinValue = 0;

tCItr i = std::min_element( aList.begin(), aList.end(), cefunc );

if( i != aList.end() )
{
lMinValue = *i;

return false;
}
else
{
return true;
}
}

Jeff