Chapter 2

[KTC]

Exercise 2-1

Change the framing program so that it writes its greeting with no separation from the frame.

The whole point of the exercise is to show that the program as rewritten in this chapter made it much more flexible than before such that only one line or two need to be change for it to obtain our desired result.

In this case, we only need to change the initialization value of pad to 0.

Code

GeSHi (cpp):
#include <iostream>
#include <string>
 
// say what standard-library names we use
using std::cin;         using std::endl;
using std::cout;        using std::string;
 
int main()
{
    // ask for the person's name
    cout << "Please enter your first name: ";
 
    // read the name
    string name;
    cin >> name;
 
    // build the message that we intend to write
    const string greeting = "Hello, " + name + "!";
 
    // the number of blanks surrounding the greeting
    const int pad = 0;
 
    // the number of rows and columns to write
    const int rows = pad * 2 + 3;
    const string::size_type cols = greeting.size() + pad * 2 + 2;
 
    // write a blank line to separate the output from the input
    cout << endl;
 
    // write rows rows of output
    // invariant: we have written r rows so far
    for (int r = 0; r != rows; ++r) {
 
        string::size_type c = 0;
 
        // invariant: we have written c characters so far in the current row
        while (c != cols) {
 
            // is it time to write the greeting?
            if (r == pad + 1 && c == pad + 1) {
                cout << greeting;
                c += greeting.size();
            } else {
 
                // are we on the border?
                if (r == 0 || r == rows - 1 ||
                    c == 0 || c == cols - 1)
                    cout << "*";
                else
                    cout << " ";
                ++c;
            }
        }
 
        cout << endl;
    }
 
    return 0;
}
 
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-2

Change the framing program so that it uses a different amount of space to separate the sides from the greeting than it uses to separate the top and bottom borders from the greeting.

In this case, we need to update our desired amount of padding as before, and also the statement where we check whether it's time to write the greeting yet, and the calculation for the number of rows/columns to write.

For example, let there be twice as many padding top & bottom as there are left & right.

Code

GeSHi (cpp):
#include <iostream>
#include <string>
 
// say what standard-library names we use
using std::cin;         using std::endl;
using std::cout;        using std::string;
 
int main()
{
    // ask for the person's name
    cout << "Please enter your first name: ";
 
    // read the name
    string name;
    cin >> name;
 
    // build the message that we intend to write
    const string greeting = "Hello, " + name + "!";
 
    // the number of blanks surrounding the greeting
    const int pad = 2;
 
    // the number of rows and columns to write
    const int rows = pad * 4 + 3;
    const string::size_type cols = greeting.size() + pad * 2 + 2;
 
    // write a blank line to separate the output from the input
    cout << endl;
 
    // write rows rows of output
    // invariant: we have written r rows so far
    for (int r = 0; r != rows; ++r) {
 
        string::size_type c = 0;
 
        // invariant: we have written c characters so far in the current row
        while (c != cols) {
 
            // is it time to write the greeting?
            if (r == pad *2 + 1 && c == pad + 1) {
                cout << greeting;
                c += greeting.size();
            } else {
 
                // are we on the border?
                if (r == 0 || r == rows - 1 ||
                    c == 0 || c == cols - 1)
                    cout << "*";
                else
                    cout << " ";
                ++c;
            }
        }
 
        cout << endl;
    }
 
    return 0;
}
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-3

Rewrite the framing program to ask the user to supply the amount of spacing to leave between the frame and the greeting.

Code

GeSHi (cpp):
#include <iostream>
#include <string>
 
// say what standard-library names we use
using std::cin;         using std::endl;
using std::cout;        using std::string;
 
int main()
{
    // ask for the person's name
    cout << "Please enter your first name: ";
 
    // read the name
    string name;
    cin >> name;
 
    // build the message that we intend to write
    const string greeting = "Hello, " + name + "!";
 
    // the number of blanks surrounding the greeting
    int pad;
    cout << "Please enter the amount of spacing: ";
    cin >> pad;
 
    // the number of rows and columns to write
    const int rows = pad * 2 + 3;
    const string::size_type cols = greeting.size() + pad * 2 + 2;
 
    // write a blank line to separate the output from the input
    cout << endl;
 
    // write rows rows of output
    // invariant: we have written r rows so far
    for (int r = 0; r != rows; ++r) {
 
        string::size_type c = 0;
 
        // invariant: we have written c characters so far in the current row
        while (c != cols) {
 
            // is it time to write the greeting?
            if (r == pad + 1 && c == pad + 1) {
                cout << greeting;
                c += greeting.size();
            } else {
 
                // are we on the border?
                if (r == 0 || r == rows - 1 ||
                    c == 0 || c == cols - 1)
                    cout << "*";
                else
                    cout << " ";
                ++c;
            }
        }
 
        cout << endl;
    }
 
    return 0;
}
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-4

The framing program writes the mostly blank lines that separate the borders from the greeting one character at a time. Change the program so that it writes all the spaces needed in a single output expression.

Code

GeSHi (cpp):
#include <iostream>
#include <string>
 
// say what standard-library names we use
using std::cin;         using std::endl;
using std::cout;        using std::string;
 
int main()
{
    // ask for the person's name
    cout << "Please enter your first name: ";
 
    // read the name
    string name;
    cin >> name;
 
    // build the message that we intend to write
    const string greeting = "Hello, " + name + "!";
 
    // the number of blanks surrounding the greeting
    const int pad = 1;
 
    // the number of rows and columns to write
    const int rows = pad * 2 + 3;
    const string::size_type cols = greeting.size() + pad * 2 + 2;
    const string spaces(cols - 2, ' ');
 
    // write a blank line to separate the output from the input
    cout << endl;
 
    // write rows rows of output
    // invariant: we have written r rows so far
    for (int r = 0; r != rows; ++r) {
 
        string::size_type c = 0;
 
        // invariant: we have written c characters so far in the current row
        while (c != cols) {
 
            // is it time to write the greeting?
            if (r == pad + 1 && c == pad + 1) {
                cout << greeting;
                c += greeting.size();
            } else if (r != 0 && r != rows - 1 && r != pad + 1 &&
                       c != 0 && c != cols - 1) {
                cout << spaces;
                c += spaces.size();
            } else {
                // are we on the border?
                if (r == 0 || r == rows - 1 ||
                    c == 0 || c == cols - 1)
                    cout << "*";
                else
                    cout << " ";
                ++c;
            }
        }
 
        cout << endl;
    }
 
    return 0;
}
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-5

Write a set of "*" characters so that they form a square, a rectangle, and a triangle.

Code

GeSHi (cpp):
#include <iostream>
 
// say what standard-library names we use
using std::cin;         using std::endl;
using std::cout;        using std::string;
 
int main()
{
    // the number of rowsto write
    const int rows = 5;
 
    // ** Square **
 
    // write rows rows of output
    // invariant: we have written r rows so far
    for (int r = 0; r != rows; ++r) {
 
        // Square: rows == columns
        const int cols = rows;
 
        // invariant: we have written c columns so far
        for(int c = 0; c != cols; ++c) {
            cout << "*";
        }
 
        cout << endl;
    }
 
    // Separate the shapes
    cout << endl;
 
    // ** Rectangle **
 
    // write rows rows of output
    // invariant: we have written r rows so far
    for (int r = 0; r != rows; ++r) {
 
        // Let's have a rectangle twice as wide as it is high
        const int cols = rows * 2;
 
        // invariant: we have written c columns so far
        for(int c = 0; c != cols; ++c) {
            cout << "*";
        }
 
        cout << endl;
    }
 
    // Separate the shapes
    cout << endl;
 
    // ** Triangle **
 
    // write rows rows of output
    // invariant: we have written r rows so far
    for (int r = 0; r != rows; ++r) {
        // write rows columns of output to form triangle
        // we are writing the r + 1 rows
        // invariant: we have written c columns so far
        for(int c = 0; c != r + 1; ++c) {
            cout << "*";
        }
 
        cout << endl;
    }
 
    return 0;
}
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-6

What does the following code do?

Code:

int i = 0;
while (i < 10) {
    i += 1;
    std::cout << i << std::endl;
}

Noting the increment to i is before the output, we can conclude the program code output the number 1 to 10, each on its own line.

[KTC]

Exercise 2-7

Write a program to count down from 10 to -5 .

Code

GeSHi (cpp):
#include <iostream>
 
using std::cout;    using std::endl;
 
int main()
{
    // Starting from 10, we count down until we reach the first number we don't want
    // namely, -6
    for(int i = 10; i != -6; --i) {
        std::cout << i << std::endl;
    }
 
    return 0;
}
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-8

Write a program to generate the product of the numbers in the range [1, 10).

This is a program I'm sure the book will return to. The product of the numbers in the range [1, N) is the mathematical operation N-1 factorial, or (N-1)!. Factorial is a classical textbook introduction to the concept of recursion when writing our own functions have been introduced.

Code

GeSHi (cpp):
#include <iostream>
 
using std::cout;    using std::endl;
 
int main()
{
    const int N = 10;
 
    // note we start the product off at 1 as we recall anything multiple by 0 is 0
    int product = 1;
 
    // invariant: we have multipled by the number in the range [1, i)
    for(int i=1; i != N; ++i) {
        product *= i;
    }
 
    cout << product << endl;
 
    return 0;
}
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-9

Write a program that asks the user to enter two numbers and tells the user which number is larger than the other.

Code

GeSHi (cpp):
#include <iostream>
 
using std::cin;     using std::cout;
using std::endl;
 
int main()
{
    cout << "The first number: ";
    int num1;
    cin >> num1;
 
    cout << "The second number: ";
    int num2;
    cin >> num2;
 
    cout << "The larger number is " << ((num1 > num2) ? num1 : num2) << endl;
 
    return 0;
}
Created by GeSHI 1.0.7.18

[KTC]

Exercise 2-10

Explain each of the uses of std:: in the following program:

Code:

int main() {
int k =  0;
    while (k != n) {             // invariant: we have written k asterisks so far
        using std::cout;
        cout << "*";
        ++k;
    }
    std::cout << std::endl;      // std:: is required here
    return 0;
}

Code:

using std::cout;

This is a using declaration. We declare cout to be from the std namespace from this point on, until the end of the current scope, in this case, the end of the while loop.

Code:

std::cout << std::endl;      // std:: is required here

In both case, the :: scope resolution operator is used and std is specified to say we want cout and endl from the std namespace.