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<h3>Exercise Solutions</h3>

<ol>
<li>Top
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<pre>int IStack::Top () const
{
    assert (_top &gt; 0);
    return _arr [_top - 1];
}

int IStack::Count () const
{    return _top;
}
</pre>
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<li>This example calls Pop on an empty stack, in violation of the contract.
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<pre>int main ()
{
    IStack stack;
    stack.Pop (); 
}</pre>
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<li> This is the interface of the stack of characters:
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<pre>class CharStack
{
public:
    CharStack () :_top (0) {}
    void Push (char c);
    char Pop ();
    char Top () const;
    int Count () const;
private:
    char _arr [maxStack];
    int  _top;
};</pre>
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This is how you reverse a string using a stack of characters:
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<pre>int main ()
{
    CharStack stack;
    char str [] = "esreveR";
    for (int i = 0; str [i] != '\0'; ++i)
        stack.Push (str [i]);
    while (stack.Count () &gt; 0)
        std::cout &lt;&lt; stack.Pop ();
    std::cout &lt;&lt; std::endl;
}</pre>
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<li>Queue
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<pre>#include <iostream>
#include <cassert>

const int maxPuts = 8;

class Queue
{
public:
    Queue ();
    double Get ();
    void Put (double x);
private:
    double    _arr [maxPuts];
    int        _putIdx;
    int        _getIdx;
};

Queue::Queue ()
: _putIdx (0),
_getIdx (0)
{}

double Queue::Get ()
{
    assert (_getIdx &lt; _putIdx);
    ++_getIdx;
    return _arr [_getIdx - 1];
}

void Queue::Put (double x)
{
    assert (_putIdx &lt; maxPuts);
    _arr [_putIdx] = x;
|    ++_putIdx;
}
|
int main ()
{
    Queue queue;
    queue.Put (0.1);
    queue.Put (0.2);
    std::cout &lt;&lt; "Getting: " &lt;&lt; queue.Get () &lt;&lt; ", " 
        &lt;&lt; queue.Get () &lt;&lt; std::endl;
    queue.Put (0.3);
    std::cout &lt;&lt; "Getting more: " &lt;&lt; queue.Get () &lt;&lt; std::endl; 
}
</pre>
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<li>Array of double
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<pre>#include <iostream>
#include <cassert>

const int maxCells = 16;

class DblArray
{
public:
    DblArray ();
    void Set (int i, double val);
    double Get (int i) const;
    bool IsSet (int i) const;
private:
    double    _arr [maxCells];
    bool    _isSet [maxCells];
};

DblArray::DblArray ()
{
    for (int i = 0; i &lt; maxCells; ++i)
        _isSet [i] = false;
}

void DblArray::Set (int i, double val)
{
    assert (i &lt; maxCells);
    assert (!IsSet (i));
    _arr [i] = val;
    _isSet [i] = true;
}

double DblArray::Get (int i) const
{
    assert (i &lt; maxCells);
    assert (_isSet [i]);
    return _arr [i];
}

bool DblArray::IsSet (int i) const
{
    assert (i &lt; maxCells);
    return _isSet [i];
}

int main ()
{
    DblArray arr;
    arr.Set (2, 0.2);
    arr.Set (4, 0.4);
    std::cout &lt;&lt; "IsSet (0) returns " &lt;&lt; arr.IsSet (0) &lt;&lt; std::endl;
    std::cout &lt;&lt; "IsSet (2) returns " &lt;&lt; arr.IsSet (2) &lt;&lt; std::endl;
    std::cout &lt;&lt; "Get (2) returns " &lt;&lt; arr.Get (2) &lt;&lt; std::endl;
    std::cout &lt;&lt; "Get (4) returns " &lt;&lt; arr.Get (4) &lt;&lt; std::endl;
}</pre>
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