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Modifying the value of each element stored in a std::vector.

Look at my code:

void modify_element( BigType &x)
{
/* not shown */
}

BigType modify_element_ copy(BigType x)
{
modify_element( x);
return x;
}

void modify_containe r(std::vector<B igType> &v)
{
std::transform( v.begin(), v.end(), v.begin(), modify_element_ copy);
}

Can I replace the call to std::tranform with a call to std::for_each? My
documentation says that for_each must not modify the elements. Here is
modify_containe r using std::for_each rather than std::transform:

void modify_containe r(std::vector<B igType> &v)
{
std::for_each(v .begin(), v.end(), modify_element) ;
}

Any help is appreciated.
Jul 23 '05
11 3939

"Richard Herring" <ju**@[127.0.0.1]> wrote in message
news:Cp******** ******@baesyste ms.com...
In message <42************ **********@news .optusnet.com.a u>, Jason Heyes
<ja********@opt usnet.com.au> writes
"Richard Herring" <ju**@[127.0.0.1]> wrote in message
news:vQ******* *******@baesyst ems.com...
In message <11************ **********@o13g 2000cwo.googleg roups.com>,
Matthew Schaefer <ma************ **@yahoo.com> writes
The for_each algorithm does not modify the elements of the sequence. It
calls Function F for each element in the range [First, Last) and
returns the input parameter F. This function does not modify any
elements in the sequence.

The *algorithm* does not, of itself, modify any elements of the sequence, and it discards the result of function F. That doesn't prevent the
function F from modifying its argument if you pass iterators of an
appropriate type.


The result of F eventually comes back as a return value, I believe.

No. A copy of F itself (the functor or function pointer) is returned:

template <class InputIterator, class Function>
Function for_each (InputIterator first, InputIterator last, Function f)
{
while (first != last) f(*first++);
return f;
}


Jason: Also note that if f() does return a value, it is
ignored. (So there's no point in f() having a return type
other than 'void', unless one plans to use it in other
contexts where the return value would be available).

-Mike

Jul 23 '05 #11
"Richard Herring" <ju**@[127.0.0.1]> wrote in message
news:Cp******** ******@baesyste ms.com...
In message <42************ **********@news .optusnet.com.a u>, Jason Heyes
<ja********@opt usnet.com.au> writes
The result of F eventually comes back as a return value, I believe.

No. A copy of F itself (the functor or function pointer) is returned:

template <class InputIterator, class Function>
Function for_each (InputIterator first, InputIterator last, Function f)
{
while (first != last) f(*first++);
return f;
}


Oh yea. Quite right.
Jul 23 '05 #12

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