INFO: STL Sample for map: max_size, clear, erase, size Functions

ID: Q157593

The information in this article applies to:

The Standard C++ Library, used with:
- Microsoft Visual C++, 32-bit Editions, versions 4.2, 5.0, 6.0

SUMMARY

The sample code below illustrates how to use the map::max_size, map::clear, map::empty, map::erase, map::size, map::operator[], map::end, map::begin, map::find, map::iterator, map::value_type STL symbols in Visual C++.

MORE INFORMATION

Required Header


   <map>

Prototypes


   size_type max_size() const;
   void clear() const;
   bool empty() const;
   iterator erase(iterator first, iterator last);
   size_type size() const;
   A::reference operator[](const Key& key);    // A is the allocator
   iterator map::begin();
   iterator map::end();
   iterator map::find(const Key& key);

NOTE: The class/parameter names in the prototypes may not match the version in the header file. Some have been modified to improve readability.

Description

The following sample creates a map of strings to ints and fills it first with a map of month names to month numbers, then empties and refills it with a map of week-day names to corresponding ints.

Sample Code


////////////////////////////////////////////////////////////////////// 
// 
// Compile options needed: None
// 
// <filename> :  main.cpp
// 
// Functions:    iterator map::max_size();
//               void clear() const;
//               bool empty() const;
//               iterator erase(iterator first, iterator last);
//               size_type size() const;
//               A::reference operator[](const Key& key);
//               iterator map::begin();
//               iterator map::end();
//               iterator map::find(const Key& key);
// 
// Written by Rick Troemel
// of Microsoft Product Support Services,
// Copyright (c) 1996 Microsoft Corporation. All rights reserved.
////////////////////////////////////////////////////////////////////// 

#pragma warning(disable:4786)

#include <iostream>
#include <string>
#include <map>

using namespace std;

   typedef map<string, int, less<string>, allocator<int> > STRING2INT;

   void main()

#if _MSC_VER > 1020   // if VC++ version is > 4.2
   using namespace std;  // std c++ libs implemented in std
   #endif

typedef map<string, int, less<string>, allocator<int> > STRING2INT;

void main()
{
    STRING2INT MyMap;
    STRING2INT::iterator MyIterator;
    string MyBuffer;

    // print the maximum number of <key,data> pairs that MyMap can hold
    cout << "MyMap is capable of holding " << MyMap.max_size()
         << " <string,int> pairs" << endl;

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;
        cout << "Entering new entries in MyMap" << endl;
    // Fill in MyMap with the months of the year, mapped to their number
    // January - 1, February - 2, etc. using operator[].
    MyMap["January"] = 1;
    MyMap["February"] = 2;
    MyMap["March"] = 3;
    MyMap["April"] = 4;
    MyMap["May"] = 5;
    MyMap["June"] = 6;
    MyMap["July"] = 7;
    MyMap["August"] = 8;
    MyMap["September"] = 9;
    MyMap["October"] = 10;
    MyMap["November"] = 11;
    MyMap["December"] = 12;

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;

    // Ask the user for a month of the year and print the number
    // that corresponds to the month entered
    //MyIterator = MyMap.end();
    while(MyIterator == MyMap.end()){
        cout << "Enter a Month :";
        cin >> MyBuffer;
        if((MyIterator = MyMap.find(MyBuffer)) != MyMap.end())
            cout << (*MyIterator).first << " is Month Number "
                 << (*MyIterator).second << endl;
        else
            cout << "Enter a Valid Month (example: March)" << endl;
    }

    // empty MyMap - note that clear simply calls erase(begin(),end());
    MyMap.clear();

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;
        cout << "Entering new entries in MyMap" << endl;
    // Fill MyMap with the days of the week, each mapped to an int
    MyMap["Monday"] = 1;
    MyMap["Tuesday"] = 2;
    MyMap["Wednesday"] = 3;
    MyMap["Thursday"] = 4;
    MyMap["Friday"] = 5;
    MyMap["Saturday"] = 6;
    MyMap["Sunday"] = 7;

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;

    // Ask the user for a day of the week and print the number
    // that corresponds to the day entered
    MyIterator = MyMap.end();
    while(MyIterator == MyMap.end()){
        cout << "Enter a Day of the Week:";
        cin >> MyBuffer;
        if((MyIterator = MyMap.find(MyBuffer)) != MyMap.end())
            cout << (*MyIterator).first << " is Day Number "
                 << (*MyIterator).second << endl;
        else
            cout <<"Enter a Valid Day of the Week(example: Monday)"<< endl;
    }
// Now clear MyMap again - this time using erase instead of clear
    MyMap.erase(MyMap.begin(), MyMap.end());

    if(!MyMap.empty())
        cout << "MyMap has " << MyMap.size() << " entries" << endl;
    else
        cout << "MyMap is empty" << endl;

}

Program Output is (User input is shown inside []):


MyMap is capable of holding 1073741823 <string,int> pairs
MyMap is empty
Entering new entries in MyMap
MyMap has 12 entries
Enter a Month :[April]
April is Month Number 4
MyMap is empty
Entering new entries in MyMap
MyMap has 7 entries
Enter a Day of the Week:[Friday]
Enter a Valid Day of the Week(example: Monday)
Enter a Day of the Week:[Friday]
Friday is Day Number 5
MyMap is empty

REFERENCES

Visual C++ Books On Line: Visual C++ Books:C/C++:Standard C++ Library Reference.

Additional query words: STL STLSample max_size clear empty erase size operator[] begin end find

Keywords : kbcode kbVC420 kbVC500 kbVC600 kbDSupport STLIss
Version : winnt:4.2,5.0,6.0
Platform : winnt
Issue type : kbinfo