C++/STL Algorithms Non modifying sequence operations/for each
Содержание
- 1 A function object that computes an integer sum.
- 2 Call custom function in for_each
- 3 Extends a unary_function
- 4 Extends unary_function to do sum
- 5 Use custom function and for_each to calculate mean value
- 6 Use for_each function to add first element to each element in the list
- 7 Use for_each function to add value for each element
- 8 Use for_each function to print all elements in a vector
- 9 Use for_each with predicate
A function object that computes an integer sum.
#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
using namespace std;
class sum : unary_function<int, void> {
public:
argument_type sum;
sum() { sum = 0; }
result_type operator()(argument_type i) {
sum += i;
}
};
int main()
{
vector<int> v;
for(int i=1; i < 11; i++) v.push_back(i);
for(unsigned i=0; i < v.size(); ++i){
cout << v[i] << endl;
}
sum s;
s = for_each(v.begin(), v.end(), sum());
cout << "sum of v: " << s.sum << endl;
return 0;
}
Call custom function in for_each
/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>
#include <functional>
#include <numeric>
/* PRINT_ELEMENTS()
* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/
template <class T>
inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="")
{
typename T::const_iterator pos;
std::cout << optcstr;
for (pos=coll.begin(); pos!=coll.end(); ++pos) {
std::cout << *pos << " ";
}
std::cout << std::endl;
}
/* INSERT_ELEMENTS (collection, first, last)
* - fill values from first to last into the collection
* - NOTE: NO half-open range
*/
template <class T>
inline void INSERT_ELEMENTS (T& coll, int first, int last)
{
for (int i=first; i<=last; ++i) {
coll.insert(coll.end(),i);
}
}
using namespace std;
// function object that adds the value with which it is initialized
template <class T>
class AddValue {
private:
T theValue; // value to add
public:
// constructor initializes the value to add
AddValue (const T& v) : theValue(v) {
}
// the function call for the element adds the value
void operator() (T& elem) const {
elem += theValue;
}
};
int main()
{
vector<int> coll;
INSERT_ELEMENTS(coll,1,9);
// add ten to each element
for_each (coll.begin(), coll.end(), // range
AddValue<int>(10)); // operation
PRINT_ELEMENTS(coll);
// add value of first element to each element
for_each (coll.begin(), coll.end(), // range
AddValue<int>(*coll.begin())); // operation
PRINT_ELEMENTS(coll);
}
/*
11 12 13 14 15 16 17 18 19
22 23 24 25 26 27 28 29 30
*/
Extends a unary_function
#include <algorithm>
#include <functional>
#include <vector>
#include <iostream>
using namespace std;
class MinMax : public unary_function<int, void>{
public:
MinMax();
void operator()(int elem);
int min, max;
protected:
bool first;
};
MinMax::MinMax() : min(-1), max(-1), first(true){}
void MinMax::operator()(int elem){
if (first) {
min = max = elem;
} else if (elem < min) {
min = elem;
} else if (elem > max) {
max = elem;
}
first = false;
}
int main(int argc, char** argv){
vector<int> myVector;
myVector.push_back(1);
myVector.push_back(2);
myVector.push_back(3);
MinMax func;
func = for_each(myVector.begin(), myVector.end(), func);
cout << "The max is " << func.max << endl;
cout << "The min is " << func.min << endl;
return (0);
}
Extends unary_function to do sum
#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
using namespace std;
class sum : unary_function<int, void> {
public:
argument_type sum;
sum() { sum = 0; }
result_type operator()(argument_type i) {
sum += i;
}
};
int main()
{
vector<int> v;
for(int i=1; i < 11; i++) v.push_back(i);
for(unsigned i=0; i < v.size(); ++i)
cout << v[i] << endl;
sum s;
s = for_each(v.begin(), v.end(), sum());
cout << "sum of v: " << s.sum << endl;
v[4]= 99;
s = for_each(v.begin(), v.end(), sum());
cout << "sum of v is now: " << s.sum;
return 0;
}
Use custom function and for_each to calculate mean value
/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>
#include <functional>
#include <numeric>
/* PRINT_ELEMENTS()
* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/
template <class T>
inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="")
{
typename T::const_iterator pos;
std::cout << optcstr;
for (pos=coll.begin(); pos!=coll.end(); ++pos) {
std::cout << *pos << " ";
}
std::cout << std::endl;
}
/* INSERT_ELEMENTS (collection, first, last)
* - fill values from first to last into the collection
* - NOTE: NO half-open range
*/
template <class T>
inline void INSERT_ELEMENTS (T& coll, int first, int last)
{
for (int i=first; i<=last; ++i) {
coll.insert(coll.end(),i);
}
}
using namespace std;
// function object to process the mean value
class MeanValue {
private:
long num; // number of elements
long sum; // sum of all element values
public:
// constructor
MeanValue () : num(0), sum(0) {
}
// function call
// - process one more element of the sequence
void operator() (int elem) {
num++; // increment count
sum += elem; // add value
}
// return mean value (implicit type conversion)
operator double() {
return static_cast<double>(sum) / static_cast<double>(num);
}
};
int main()
{
vector<int> coll;
INSERT_ELEMENTS(coll,1,8);
// process and print mean value
double mv = for_each (coll.begin(), coll.end(), // range
MeanValue()); // operation
cout << "mean value: " << mv << endl;
}
/*
mean value: 4.5
*/
Use for_each function to add first element to each element in the list
/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <list>
#include <algorithm>
using namespace std;
/* PRINT_ELEMENTS()
* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/
template <class T>
inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="")
{
typename T::const_iterator pos;
std::cout << optcstr;
for (pos=coll.begin(); pos!=coll.end(); ++pos) {
std::cout << *pos << " ";
}
std::cout << std::endl;
}
// function object that adds the value with which it is initialized
class AddValue {
private:
int theValue; // the value to add
public:
// constructor initializes the value to add
AddValue(int v) : theValue(v) {
}
// the ""function call"" for the element adds the value
void operator() (int& elem) const {
elem += theValue;
}
};
int main()
{
list<int> coll;
// insert elements from 1 to 9
for (int i=1; i<=9; ++i) {
coll.push_back(i);
}
PRINT_ELEMENTS(coll,"initialized: ");
// add value of first element to each element
for_each (coll.begin(), coll.end(), // range
AddValue(*coll.begin())); // operation
PRINT_ELEMENTS(coll,"after adding first element: ");
}
/*
initialized: 1 2 3 4 5 6 7 8 9
after adding first element: 2 3 4 5 6 7 8 9 10
*/
Use for_each function to add value for each element
/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <list>
#include <algorithm>
using namespace std;
/* PRINT_ELEMENTS()
* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/
template <class T>
inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="")
{
typename T::const_iterator pos;
std::cout << optcstr;
for (pos=coll.begin(); pos!=coll.end(); ++pos) {
std::cout << *pos << " ";
}
std::cout << std::endl;
}
// function object that adds the value with which it is initialized
class AddValue {
private:
int theValue; // the value to add
public:
// constructor initializes the value to add
AddValue(int v) : theValue(v) {
}
// the ""function call"" for the element adds the value
void operator() (int& elem) const {
elem += theValue;
}
};
int main()
{
list<int> coll;
// insert elements from 1 to 9
for (int i=1; i<=9; ++i) {
coll.push_back(i);
}
PRINT_ELEMENTS(coll,"initialized: ");
// add value 10 to each element
for_each (coll.begin(), coll.end(), // range
AddValue(10)); // operation
PRINT_ELEMENTS(coll,"after adding 10: ");
}
/*
initialized: 1 2 3 4 5 6 7 8 9
after adding 10: 11 12 13 14 15 16 17 18 19
*/
Use for_each function to print all elements in a vector
/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
// function that prints the passed argument
void print (int elem)
{
cout << elem << " ";
}
int main()
{
vector<int> coll;
// insert elements from 1 to 9
for (int i=1; i<=9; ++i) {
coll.push_back(i);
}
// print all elements
for_each (coll.begin(), coll.end(), // range
print); // operation
cout << endl;
}
/*
1 2 3 4 5 6 7 8 9
*/
Use for_each with predicate
#include <iostream>
using std::cout;
using std::endl;
#include <algorithm>
#include <numeric>
#include <vector>
#include <iterator>
void outputSquare( int );
int main()
{
std::ostream_iterator< int > output( cout, " " );
int a2[ 10 ] = { 100, 2, 8, 1, 50, 3, 8, 8, 9, 10 };
std::vector< int > v2( a2, a2 + 10 ); // copy of a2
cout << "Vector v2 contains: ";
std::copy( v2.begin(), v2.end(), output );
// output square of every element in v
cout << "\n\nThe square of every integer in Vector v is:\n";
std::for_each( v2.begin(), v2.end(), outputSquare );
cout << endl;
return 0;
}
void outputSquare( int value )
{
cout << value * value << " ";
}
/*
Vector v2 contains: 100 2 8 1 50 3 8 8 9 10
The square of every integer in Vector v is:
10000 4 64 1 2500 9 64 64 81 100
*/