Exercise 6-1

Reimplement the frame and hcat operations from S5.8.1/93 and $5.8.3/94 to use iterators.

Solution

The ask for this exercise is to reimplement the frame.cpp and hcat.cpp (as seen in Chapter 5 / my Solution to Exercise 5-0 (Part 3/3)), from index-base access iterator-base access.

I personally think this is a very good exercise to enable us to practice a bit on accessing vector elements by either index or iterator. This is summarised by the diagram below:

Bearing this in mind, it is in fact not that difficult, to convert frame and hcat operations from using index to iterators.

The Project

All the source and header files are essentially the same as my Solution to Exercise 5-0 (Part 3/3). In this exercise we, however, amend the frame.cpp and hcat.cpp to use iterator (instead of using index). I have also updated the main.cpp to enable us to test out the change easily. (i.e. use the Project structure as per Exercise 5-0 (Part 3/3) but use the new main.cpp, frame.cpp, and hcat.cpp. Then run the program to test.

Source File List (Newly re-written)

• main.cpp
• frame.cpp
• hcat.cpp

Source Files (Newly re-written)

main.cpp

#include <iostream>  // cin, cout, endl, getline
#include <vector>    // vector
#include <string>    // string
#include "frame.h"   // frame
#include "hcat.h"    // hcat
#include "vcout.h"   // vcout

using std::cin;
using std::cout;
using std::endl;
using std::getline;
using std::string;
using std::vector;

int main()
{
    string s;            // line
    vector<string> p;    // paragraph

    // read multiple lines to make a paragraph
    while (getline(cin, s))
        p.push_back(s);

    // have a play, manipulate and display paragraph (p) in multiple ways

    cout << "-----------------------------------------------------\n"
            "Display: hcat(p, frame(p))                           \n"
            "-----------------------------------------------------\n";
    vcout(hcat(p,frame(p)));

    cout << "-----------------------------------------------------\n"
            "Display: hcat(frame(p), p)                           \n"
            "-----------------------------------------------------\n";
    vcout(hcat(frame(p),p));


    return 0;
}

frame.cpp

#include <string>      // string
#include <vector>      // vector
#include <algorithm>   // max

using std::string;
using std::vector;
using std::max;

string::size_type width(const vector<string>& v)
{
    string::size_type maxlen = 0;
    for(vector<string>::const_iterator i = v.begin(); i != v.end(); ++i)
        maxlen = max(maxlen, i->size());
    return maxlen;
}

vector<string> frame(const vector<string>& v)
{
    vector<string> ret;
    string::size_type maxlen = width(v);
    string border(maxlen + 4, '*');

    // write the top border
    ret.push_back(border);

    // write each interior row, bordered by an asterisk and a space
    for (vector<string>::const_iterator i = v.begin(); i != v.end(); ++i)
        ret.push_back("* " + (*i) + string(maxlen - i->size(), ' ') + " *");

    // write the bottom border
    ret.push_back(border);

    return ret;
}

hcat.cpp

#include <string>      // string
#include <vector>      // vector
#include "frame.h"     // width

using std::string;
using std::vector;

vector<string> hcat(const vector<string>& left, const vector<string>& right)
{
    vector<string> ret;

    // add 1 to leave a space between pictures
    string::size_type width1 = width(left) + 1;

    // indices to look at elements from left and right respectively
    vector<string>::const_iterator i = left.begin();
    vector<string>::const_iterator j = right.begin();

    // continue until we've seen all rows from both pictures
    while (i != left.end() || j != right.end())
    {
        // construct new string to hold characters from both pictures
        string s;

        // copy a row from the left-hand side, if there is one
        if (i != left.end())
            s = *(i++);

        // pad to full width
        s += string(width1 - s.size(), ' ');

        // copy a row from teh right-hand side, if there is one
        if (j != right.end())
            s += *(j++);

        // add s to the picture we are creating
        ret.push_back(s);
    }

    return ret;
}

Test Program

Running the program gives us the output as expected. This effectively demonstrates that, if the code is written correctly, there is virtually no difference between accessing vector elements by index or by iterators.

Hello world
how are
we today?
^Z
-----------------------------------------------------
Display: hcat(p, frame(p))
-----------------------------------------------------
Hello world ***************
how are     * Hello world *
we today?   * how are     *
            * we today?   *
            ***************
-----------------------------------------------------
Display: hcat(frame(p), p)
-----------------------------------------------------
*************** Hello world
* Hello world * how are
* how are     * we today?
* we today?   *
***************

Reference

Koenig, Andrew & Moo, Barbara E., Accelerated C++, Addison-Wesley, 2000