Replace and Remove Elements in a Container in Java

Printing QR in Java Replace and Remove Elements in a Container

Headers <algorithm> Classes Functions template <class ForIter, class T> ForIter remove(ForIter start, ForIter end, const T &val) template <class ForIter, class T> void replace(ForIter start, ForIter end, const T &old, const T &new)

The STL provides functions that let you replace or remove elements At the core of this functionality is replace( ) and remove( ) Although both of these operations can be accomplished through the use of container-defined functions, in many cases, these algorithms streamline the task This recipe demonstrates them

To remove or replace one or more elements in a sequence involves these steps: 1 To remove all elements that match a specified value, call remove( ), specifying the range to be modified and the value to remove 2 To replace all occurrences of elements that match a specified value, call replace( ), specifying the range to be modified, the value to replace, and the value to substitute

The remove( ) algorithm removes all occurrences of a specified element from a specified range It is shown here: template <class ForIter, class T> ForIter remove(ForIter start, ForIter end, const T &val) This algorithm removes all elements in the range start through end 1 that are equal to val It returns an iterator to the end of the remaining elements The order of the remaining elements is unchanged Within a specified range, the replace( ) algorithm replaces all occurrences of a specified element with another It is shown here: template <class ForIter, class T> void replace(ForIter start, ForIter end, const T &old, const T &new) Within the specified range start through end 1, replace( ) replaces elements that match the value old with elements that have the value new

lists Therefore, when removing elements in a list, you should use that function rather than the remove( ) algorithm

The following example shows remove( ) and replace( ) in action:

// Demonstrate remove() and replace() #include <iostream> #include <vector> #include <algorithm> using namespace std; template<class InIter> void show_range(const char *msg, InIter start, InIter end);

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int main() { vector<char> v; vector<char>::iterator itr, itr_end; // Create a vector that contains A B C D E A B C D E for(int i=0; i<5; i++) { vpush_back('A'+i); } for(int i=0; i<5; i++) { vpush_back('A'+i); } show_range("Original contents of v:\n", vbegin(), vend()); cout << endl; // Remove all A's itr_end = remove(vbegin(), vend(), 'A'); show_range("v after removing all A's:\n", vbegin(), itr_end); cout << endl; // Replace B's with X's replace(vbegin(), vend(), 'B', 'X'); show_range("v after replacing B with X:\n", vbegin(), itr_end); cout << endl; return 0; } // Show a range of elements from a vector<char> template<class InIter> void show_range(const char *msg, InIter start, InIter end) { InIter itr; cout << msg; for(itr = start; itr != end; ++itr) cout << *itr << " "; cout << endl; }

The output is shown here:

Original contents of v: A B C D E A B C D E v after removing all A's: B C D E B C D E v after replacing B with X: X C D E X C D E

The STL provides several alternatives for removing and replacing elements Two that you will find especially useful are remove_copy( ) and replace_copy( ) Both generate a new sequence that contains the result of the operation Thus, the original sequence is unaltered The prototype for remove_copy( ) is shown here: template <class InIter, class OutIter, class T> OutIter remove_copy(InIter start, InIter end, OutIter result, const T &val) It copies elements from the specified range, removing those that are equal to val It puts the result into the sequence pointed to by result and returns an iterator to one past the end of the result The destination range must be large enough to hold the result The prototype for replace_copy( ) is shown next: template <class InIter, class OutIter, class T> OutIter replace_copy(InIter start, InIter end, OutIter result, const T &old, const T &new) It copies elements from the specified range, replacing elements equal to old with new It puts the result into the sequence pointed to by result and returns an iterator to one past the end of the result The destination range must be large enough to hold the result There are variations on remove( ), replace( ), remove_copy( ), and replace_copy( ) that allow you to specify a unary predicate that determines when an element should be removed or replaced These are called remove_if( ), replace_if( ), remove_copy_if( ), and replace_copy_if( ) Another algorithm that removes elements from a sequence is unique( ) It removes consecutive duplicate elements from a range It has the two forms shown here: template <class ForIter> ForIter unique(ForIter start, ForIter end) template <class ForIter, class BinPred> ForIter unique(ForIter start, ForIter end, BinPred pfn) Consecutive duplicate elements in the specified range are removed The second form allows you to specify a binary predicate that determines when one element is equal to another unique( ) returns an iterator to the end of the resulting range For example, assuming the preceding program, if v contains the sequence AABCCBDE, then after this statement executes

itr_end = unique(vbegin(), vend());

the range vbegin( ) to itr_end will contain ABCBDE The STL also provides unique_copy( ), which works just like unique( ), except that the result is put into another sequence

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