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Meryl Silverburgh

27 Jul 2008 27 Jul '08

5 a.m.

Hi, Can you please tell me if there is an easy way to replace the following for loop using boost:lambda? I have a vector of A, i want to call 'aMethod of each A, and stop it when one of them return true. for (vector::const_iterator iter = aList.begin() ; iter != aList.end(); ++iter) { A* anA = (*iter); // break out the for loop if aMethod return true; if (anA->aMethod()){ break; }

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Niko Vuokko

27 Jul 27 Jul

6:23 a.m.

Meryl Silverburgh wrote:

Hi,

Can you please tell me if there is an easy way to replace the following for loop using boost:lambda?

I have a vector of A, i want to call 'aMethod of each A, and stop it when one of them return true.

for (vector::const_iterator iter = aList.begin() ; iter != aList.end(); ++iter) {

A* anA = (*iter);

// break out the for loop if aMethod return true; if (anA->aMethod()){ break; }

How about trying #include ... iter = find_if( aList.begin(), aList.end(), bind(&A::aMethod, *_1) );

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Marshall Clow

6:31 a.m.

At 12:00 AM -0500 7/27/08, Meryl Silverburgh wrote:

Hi,

Can you please tell me if there is an easy way to replace the following for loop using boost:lambda?

I have a vector of A, i want to call 'aMethod of each A, and stop it when one of them return true.

for (vector::const_iterator iter = aList.begin() ; iter != aList.end(); ++iter) {

A* anA = (*iter);

// break out the for loop if aMethod return true; if (anA->aMethod()){ break; }

Isn't this what std::find_if does? std::find_if ( aList.begin (), aList.end (); bind ( A::Method, *_1 )); [ Code not compiled, just composed in email client, but you get the idea ] http://www.boost.org/doc/libs/1_35_0/doc/html/lambda/le_in_details.html#memb... -- -- Marshall Marshall Clow Idio Software mailto:marshall@idio.com It is by caffeine alone I set my mind in motion. It is by the beans of Java that thoughts acquire speed, the hands acquire shaking, the shaking becomes a warning. It is by caffeine alone I set my mind in motion.

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Steven Watanabe

28 Jul 28 Jul

5:01 p.m.

AMDG Marshall Clow wrote:

At 12:00 AM -0500 7/27/08, Meryl Silverburgh wrote:

...
for (vector::const_iterator iter = aList.begin() ; iter != aList.end(); ++iter) {

A* anA = (*iter);

// break out the for loop if aMethod return true; if (anA->aMethod()){ break; }

Isn't this what std::find_if does?

std::find_if is not quite equivalent to the for loop, because std::find_if is not allowed to modify the input sequence. In Christ, Steven Watanabe

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David Rodríguez Ibeas

29 Jul 29 Jul

7:42 p.m.

On Mon, Jul 28, 2008 at 7:01 PM, Steven Watanabe wrote:

AMDG

Marshall Clow wrote:

...
At 12:00 AM -0500 7/27/08, Meryl Silverburgh wrote:

...
for (vector::const_iterator iter = aList.begin() ; iter != aList.end(); ++iter) {

A* anA = (*iter);

// break out the for loop if aMethod return true; if (anA->aMethod()){ break; }

Isn't this what std::find_if does?

std::find_if is not quite equivalent to the for loop, because std::find_if is not allowed to modify the input sequence.

Ok, here's myquestion: What does exactly mean that std::find_if is not allowed to change the sequence? How does the code. It is not changing the vector in any way (just using a constant iterator) it is not even modifying the contents of the vector. It is just calling a (possibly not constant) method through a (constant) pointer. To me that is not modifying the input sequence. Or is it more a general concern that they are not equivalent even if in this case std::find_if could be used? David

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Steven Watanabe

8:22 p.m.

AMDG David Rodríguez Ibeas wrote:

Ok, here's myquestion: What does exactly mean that std::find_if is not allowed to change the sequence? How does the code. It is not changing the vector in any way (just using a constant iterator) it is not even modifying the contents of the vector. It is just calling a (possibly not constant) method through a (constant) pointer. To me that is not modifying the input sequence.

Or is it more a general concern that they are not equivalent even if in this case std::find_if could be used?

To be precise the standard states: "The function object pred shall not apply any non-constant function through the dereferenced iterator." I presume that non-constant means anything that could make pred(x) != pred(x) be true. In Christ, Steven Watanabe

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participants (5)

David Rodríguez Ibeas
Marshall Clow
Meryl Silverburgh
Niko Vuokko
Steven Watanabe