C++ Tutorial/STL Algorithms Non modifying sequence operations/find if

Demonstrate the find() and find_if() algorithms.

<source lang="cpp">#include <iostream>

1. include <vector>
2. include <algorithm>
3. include <string>

using namespace std; bool is_short_str(string str); int main() {

 vector<string> v;
 vector<string>::iterator itr;
 v.push_back("one");
 v.push_back("two");
 v.push_back("three");
 v.push_back("four");
 v.push_back("five");
 v.push_back("six");
 for(unsigned i=0; i < v.size(); ++i)
   cout << v[i] << " ";
 itr = find(v.begin(), v.end(), "two");
 if(itr != v.end()) {
   cout << "Replacing with \"TWO\"" << endl;
   *itr = "TWO";
 }
 return 0;

} // Return true if the string is 3 characters or less. bool is_short_str(string str) {

 if(str.size() <= 3) return true;
 return false;

}</source>

Extract sentences from a vector of chars with help from find_if().

<source lang="cpp">#include <iostream>

1. include <vector>
2. include <algorithm>
3. include <cstring>
4. include <cctype>

using namespace std; bool is_sentence_start(char ch); template<class InIter> void show_range(const char *msg, InIter start, InIter end); int main() {

 vector<char> v;
 vector<char>::iterator itr;
 const char *str = "this is a test";
 for(unsigned i=0; i < strlen(str); i++)
   v.push_back(str[i]);
 show_range("Contents of v: ", v.begin(), v.end());
 cout << endl;
 vector<char>::iterator itr_start, itr_end;
 itr_start = v.begin();
 do {
   itr_start = find_if(itr_start, v.end(), is_sentence_start);
   itr_end = find_if(itr_start, v.end(), is_sentence_start);
   show_range("", itr_start, itr_end);
 } while(itr_end != v.end());
 return 0;

} // Return true if ch is the first letter in a sentence. bool is_sentence_start(char ch) {

 static bool endofsentence = true;
 if(isalpha(ch) && endofsentence ) {
   endofsentence = false;
   return true;
 }
 if(ch=="." || ch=="?" || ch=="!") endofsentence = true;
 return false;

} template<class InIter> void show_range(const char *msg, InIter start, InIter end) {

 InIter itr;
 cout << msg << endl;
 for(itr = start; itr != end; ++itr)
   cout << *itr;
 cout << endl;

}</source>

Find value in a vector with find_if

<source lang="cpp">#include <algorithm>

1. include <vector>
2. include <iostream>

using namespace std; bool perfectScore(int num) {

 return (num >= 100);

} int main(int argc, char** argv){

 int num;
 vector<int> myVector;
 myVector.push_back(1);
 myVector.push_back(2);
 myVector.push_back(3);
 myVector.push_back(4);
 vector<int>::iterator it = find_if(myVector.begin(), myVector.end(),perfectScore);
 if (it == myVector.end()) {
   cout << "No perfect scores\n";
 } else {
   cout << "Found a \"perfect\" score of " << *it << endl;
 }
 return (0);

}</source>

map: find_if

<source lang="cpp">/* 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.
*/

1. include <iostream>
2. include <algorithm>
3. include <map>

using namespace std; /* function object to check the value of a map element

*/

template <class K, class V> class value_equals {

 private:
   V value;
 public:
   // constructor (initialize value to compare with)
   value_equals (const V& v)
    : value(v) {
   }
   // comparison
   bool operator() (pair<const K, V> elem) {
       return elem.second == value;
   }

}; int main() {

   typedef map<float,float> FloatFloatMap;
   FloatFloatMap coll;
   FloatFloatMap::iterator pos;
   // fill container
   coll[1]=7;
   coll[2]=4;
   coll[3]=2;
   coll[4]=3;
   coll[5]=6;
   coll[6]=1;
   coll[7]=3;
   // search an element with value 3.0
   pos = find_if(coll.begin(),coll.end(),    // linear complexity
                 value_equals<float,float>(3.0));
   if (pos != coll.end()) {
       cout << pos->first << ": "
            << pos->second << endl;
   }

}</source>

4: 3

std::find_if with predicate

<source lang="cpp">#include <iostream> using std::cout; using std::endl;

1. include <algorithm>
2. include <vector>
3. include <iterator>

bool greater10( int value ); int main() {

  int a[ 10 ] = { 10, 2, 17, 5, 16, 8, 13, 11, 20, 7 };
  std::vector< int > v( a, a + 10 ); // copy of a
  std::ostream_iterator< int > output( cout, " " );
  cout << "Vector v contains: ";
  std::copy( v.begin(), v.end(), output ); // display output vector
  // locate first occurrence of 16 in v
  std::vector< int >::iterator location;
  // locate first occurrence of value greater than 10 in v
  location = std::find_if( v.begin(), v.end(), greater10 );
  if ( location != v.end() ) // found value greater than 10
     cout << "\n\nThe first value greater than 10 is " << *location
        << "\nfound at location " << ( location - v.begin() );
  else // value greater than 10 not found
     cout << "\n\nNo values greater than 10 were found";
  cout << endl;
  return 0;

} bool greater10( int value ) {

  return value > 10;

}</source>

Vector v contains: 10 2 17 5 16 8 13 11 20 7
The first value greater than 10 is 17
found at location 2

Use find_if to find first element divisible by 3

<source lang="cpp">/* 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.
*/

1. include <iostream>
2. include <vector>
3. include <deque>
4. include <list>
5. include <set>
6. include <map>
7. include <string>
8. include <algorithm>
9. include <iterator>
10. include <functional>
11. 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; int main() {

   vector<int> coll;
   vector<int>::iterator pos;
   INSERT_ELEMENTS(coll,1,9);
   PRINT_ELEMENTS(coll,"coll: ");
   // find first element divisible by 3
   pos = find_if (coll.begin(), coll.end(),
                  not1(bind2nd(modulus<int>(),3)));
   // print its position
   cout << "the "
        << distance(coll.begin(),pos) + 1
        << ". element is the first divisible by 3" << endl;

}</source>

coll: 1 2 3 4 5 6 7 8 9
the 3. element is the first divisible by 3

Use find_if to find first element greater than 3

<source lang="cpp">/* 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.
*/

1. include <iostream>
2. include <vector>
3. include <deque>
4. include <list>
5. include <set>
6. include <map>
7. include <string>
8. include <algorithm>
9. include <iterator>
10. include <functional>
11. 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; int main() {

   vector<int> coll;
   vector<int>::iterator pos;
   INSERT_ELEMENTS(coll,1,9);
   PRINT_ELEMENTS(coll,"coll: ");
   // find first element greater than 3
   pos = find_if (coll.begin(), coll.end(),    // range
                  bind2nd(greater<int>(),3));  // criterion
   // print its position
   cout << "the "
        << distance(coll.begin(),pos) + 1
        << ". element is the first greater than 3" << endl;

}</source>

coll: 1 2 3 4 5 6 7 8 9
the 4. element is the first greater than 3