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Tip of the Day
Language: C++
Expertise: Advanced
Dec 18, 2000

The remove_if() algorithm


The remove_if() algorithms (defined in the standard header <algorithm>) has the following prototype:
 
template <class ForwardIterator, class T>
ForwardIterator  remove (ForwardIterator first,
                         ForwardIterator last,
                         Predicate pred);

The first two arguments mark the sequence's beginning and end. The third argument is a predicate. Unlike remove(), remove_if() uses the predicate provided to select the elements it removes. This enables you to remove elements with different values that fall under the same category, e.g., odd numbers, vowels, uppercase letters etc. Using remove_if() is similar to using remove(). The tricky part in is the definition of the predicate. A concrete example will clarify this issue.

Suppose we want to remove all the vowels from a string object. Our predicate has to distinguish between a vowel and a non-vowel. We define the is_vowel predicate as follows:

 
template <class T> 
class is_vowel: public unary_function
{
public:
 bool operator ()(T t) const
 {
  if ((t=='a')||(t=='e')||(t=='i')||(t=='o')||(t=='u'))
   return true; //t is a vowel
  return false; // t is not a vowel
 }
};

Our goal is to read a string from the standard input, remove all the vowels therein by using remove_if(), and display the resulting vowel-less string. The first part is straightforward:

 
#include <string>
#include <iostream> 
#include <algorithm> // for remove_if
using namespace std;

int main()
{
 string original;
 cin >> original;
 //
Danny Kalev
 
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