昨天羅列了C++11中新增的幾個算法，包括 find_if_not、all_of、any_of、none_of四個算法，這四個算法的共同點就是Non-modifying sequence operations。
所以，今天就來八一八C++11中新增的算法，而這些算法的特點是：Modifying sequence operations。
copy算法我們很熟悉，這裡介紹一下C++11新增的copy_n。
copy_n
原型：
template
OutputIterator copy_n (InputIterator first, Size n, OutputIterator result);
作用：
Copies the first n elements from the range beginning at first into the range beginning at result.
返回值很關鍵：
The function returns an iterator to the end of the destination range (which points to one past the last element copied).
如果n為負數的時候：
If n is negative, the function does nothing.
如果兩個區間有重復的時候：
If the ranges overlap, some of the elements in the range pointed by result may have undefined but valid values.
應用：
#include // std::cout
#include // std::copy
#include // std::vector
int main() {
int myints[] = { 10,20,30,40,50,60,70 };
std::vector myvector;
myvector.resize(8); // allocate space for 7 elements
std::copy_n(myints, 7, myvector.begin()+1);
std::cout << "myvector contains:";
for (std::vector::iterator it = myvector.begin(); it != myvector.end(); ++it)
std::cout << ' ' << *it;
std::cout << '\n';
std::vector myvector2;
myvector2.resize(8);
std::copy_n(myints, -1, myvector2.begin() + 1);
std::cout << "myvector2 contains:";
for (std::vector::iterator it = myvector2.begin(); it != myvector2.end(); ++it)
std::cout << ' ' << *it;
std::cout << '\n';
return 0;
}
//輸出：
myvector contains : 0 10 20 30 40 50 60 70
myvector2 contains : 0 0 0 0 0 0 0 0
copy_if
原型：
template
OutputIterator copy_if (InputIterator first, InputIterator last,
OutputIterator result, UnaryPredicate pred);
作用：
Copies the elements in the range [first,last) for which pred returns true to the range beginning at result.
應用：
#include // std::cout
#include // std::copy_if, std::distance
#include // std::vector
int main () {
std::vector foo = {25,15,5,-5,-15};
std::vector bar (foo.size());
// copy only positive numbers:
auto it = std::copy_if (foo.begin(), foo.end(), bar.begin(), [](int i){return !(i<0);} );
bar.resize(std::distance(bar.begin(),it)); // shrink container to new size
std::cout << "bar contains:";
for (int& x: bar) std::cout << ' ' << x;
std::cout << '\n';
return 0;
}
move和move_backward
move算得上是老生常談了，但需要注意的是：
The value of the elements in the [first,last) is transferred to the elements pointed by result. After the call, the elements in the range [first,last) are left in an unspecified but valid state.
直接給例子代碼：
#include // std::cout
#include // std::move (ranges)
#include // std::move (objects)
#include // std::vector
#include // std::string
int main () {
std::vector foo = {"air","water","fire","earth"};
std::vector bar (4);
// moving ranges:
std::cout << "Moving ranges...\n";
std::move ( foo.begin(), foo.begin()+4, bar.begin() );
std::cout << "foo contains " << foo.size() << " elements:";
std::cout << " (each in an unspecified but valid state)";
std::cout << '\n';
std::cout << "bar contains " << bar.size() << " elements:";
for (std::string& x: bar) std::cout << " [" << x << "]";
std::cout << '\n';
// moving container:
std::cout << "Moving container...\n";
foo = std::move (bar);
std::cout << "foo contains " << foo.size() << " elements:";
for (std::string& x: foo) std::cout << " [" << x << "]";
std::cout << '\n';
std::cout << "bar is in an unspecified but valid state";
std::cout << '\n';
return 0;
}
//輸出：
Moving ranges...
foo contains 4 elements: (each in an unspecified but valid state)
bar contains 4 elements: [air] [water] [fire] [earth]
Moving container...
foo contains 4 elements: [air] [water] [fire] [earth]
bar is in an unspecified but valid state
下面說說move_backward:
原型：
template
BidirectionalIterator2 move_backward (BidirectionalIterator1 first,
BidirectionalIterator1 last,
BidirectionalIterator2 result);
作用：
Moves the elements in the range [first,last) starting from the end into the range terminating at result.
The function returns an iterator to the first element in the destination range.
The function begins by moving (last-1) into (result-1), and then follows backward by the elements preceding these, until first is reached (and including it).
還沒搞懂應用，C++ Reference上的例子在VS2015下不能運行，等下有時候再搞搞。
shuffle
shuffle有 改組 的意思。
原型：
template
void shuffle (RandomAccessIterator first, RandomAccessIterator last, URNG&& g)
作用：
Rearranges the elements in the range [first,last) randomly, using g as uniform random number generator.
應用：
#include // std::cout
#include // std::shuffle
#include // std::array
#include // std::default_random_engine
#include // std::chrono::system_clock
int main() {
std::array foo{ 1,2,3,4,5 };
// obtain a time-based seed:
unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();
shuffle(foo.begin(), foo.end(), std::default_random_engine(seed));
std::cout << "shuffled elements:";
for (int& x : foo) std::cout << ' ' << x;
std::cout << '\n';
return 0;
}
//輸出
//shuffled elements: 4 2 1 3 5
上面的代碼中用到了頭文件chrono，在之前的博客中有講訴過，也算是C++11提供的一些便利工具。
