CISC 3142
Programming Paradigms in C++
Sample Programs

Program #1: Hello World

The Purpose

• Despite being relatively short, the program illustrates the basic structure of a program in the language.
• Input/output (which historically could be somewhat idiosyncratic to the language — in contrast to arithmetic expressions ) is illustrated.

Language Features

• basic program structure
• console-based output
• libraries and include (header) files
• namespaces
• program exit

The Code

1 #include <iostream>
2 
3 using namespace std;
4 
5 int main() {
6      cout << "Hello world" << endl;
7      return 0;
8 }    

Program #2: Primes

Language Features

• conditionals and iteration
• functions and function headers
• console input

The Code

 1 #include <iostream>
 2 #include <cmath>
 3
 4 using namespace std;
 5
 6 bool isPrime(int);
 7 
 8 int main() {
 9 
10	int howFar;
11	cout << "Generate all primes until? ";
12	cin >> howFar;
13
14	bool isFirst = true;
15	for (int i = 2; i <= howFar; i++)
16		if (isPrime(i)) {
17			cout << (isFirst ? "" : ", ") << i;
18			if (isFirst) isFirst = false;
19		}
20	cout << endl;
21 }
22
23 bool isPrime(int n) {
24 	for (int i = 2; i <= sqrt(n); i++) 
25		if (n % i == 0) return false;
26	return true;
27 }

Program #3: File Merge

Language Features

• File I/O
  • Opening, checking for successful open, closing, checking for successful read
• (string-based) exception handling
• program exit and exit codes

The Merge Algorithm

• set current position indexes to the beginning of both files
• compare the values at the current positions; the smaller value is written to the output file, and the corresponding current position index is incremented to the next element
• repeat the above step until the end of one of the files is reached
• output the rest of the other file to the output file

• One could make a modification that would write both equal values out; at a cost of complicating the algorithm

The Code

  1 #include <iostream>
  2 #include <fstream>
  3 #include <cstdlib>
  4 #include <string>
  5 
  6 using namespace std;
  7 
  8 int main() {
  9      try {
 10           ifstream infile1("in1.data");
 11           if (!infile1.good()) throw string("Cannot open in1.data");
 12 
 13           ifstream infile2("in2.data");
 14           if (infile2.fail()) throw string("Cannot open in2.data");
 15 
 16           ofstream outfile("out.data");
 17           if (!outfile)  throw string("Cannot open out.data");
 18 
 19           int i1, i2;
 20 
 21           infile1 >> i1;
 22           infile2 >> i2;
 23 
 24           while (!infile1.eof() && !infile2.eof())
 25                if (i1 < i2) {
 26                     outfile << i1 << " ";
 27                     infile1 >> i1;
 28                }
 29                else {
 30                     outfile << i2 << " ";
 31                     infile2 >> i2;
 32                }
 33 
 34           while (!infile1.eof()) {
 35                outfile << i1 << " ";
 36                infile1 >> i1;
 37           }
 38 
 39           while (!infile2.eof()) {
 40                outfile << i2 << " ";
 41                infile2 >> i2;
 42           }
 43 
 44           infile1.close();
 45           infile2.close();
 46           outfile.close();
 47 
 48           return 0;
 49      } catch(string message) {
 50           cout << message << endl;
 51           exit(1);
 52      }
 53 }

Program #4: Reversing an Array

Language Features

• fixed-size arrays
• const
• c_str()
• some typical array initialization techniques

The Code

  1 #include <iostream>
  2 #include <fstream>
  3 #include <cstdlib>
  4 #include <string>
  5 
  6 
  7 using namespace std;
  8 
  9 void reverse(int a[], int n);
 10 void print(int a[], int n);
 11 int load(string filename, int a[], int capacity);
 12 
 13 int main() {
 14      try {
 15           const int A1_N = 10;
 16           int a1[A1_N];
 17           for (int i = 0; i < A1_N; i++)
 18                a1[i] = i * 100;
 19           print(a1, A1_N);
 20           cout << endl;
 21           reverse(a1, A1_N);
 22           print(a1, A1_N);
 23           cout << endl;
 24 
 25           cout << endl;
 26 
 27           int a2[] = {10, 12, 31, 47, 6, 8};
 28           const int A2_N = sizeof(a2) / sizeof(a2[0]);
 29           print(a2, A2_N);
 30           cout << endl;
 31           reverse(a2, A2_N);
 32           print(a2, A2_N);
 33           cout << endl;
 34 
 35           cout << endl;
 36 
 37           const int A3_CAPACITY = 100;
 38           int a3[A3_CAPACITY];
 39           int a3_size = load("numbers.data", a3, A3_CAPACITY);
 40           print(a3, a3_size);
 41           cout << endl;
 42           reverse(a3, a3_size);
 43           print(a3, a3_size);
 44           cout << endl;
 45 
 46           return 0;
 47      } catch(string message) {
 48           cout << "*** Exception *** " << message << endl;
 49           exit(1);
 50      }
 51 }
 52 
 53 void reverse(int a[], int n) {
 54      for (int i = 0; i < n/2; i++) {
 55           int temp = a[i];
 56           a[i] = a[n-i-1];
 57           a[n-i-1] = temp;
 58      }
 59 }
 60  
 61 void print(int a[], int n) { 
 62      for (int i = 0; i < n; i++) 
 63           cout << a[i] << (i < n-1 ? ", " : "");
 64 }
 65 
 66 int load(string fname, int a[], int capacity) {
 67      ifstream ifs(fname.c_str());
 68      if (!ifs) throw fname + " not found";
 69      int size = 0;
 70 
 71      int num;
 72      ifs >> num;
 73      while (ifs) {
 74           if (size == capacity) throw "array capacity exceeded";
 75           a[size] = num;
 76           size++;
 77           ifs >> num;
 78      }
 79      return size;
 80 }

Program #5: A Counter Class

Language Features

• basic class
• constructor
• member initializer
• c_str()
• some typical array initialization techniques

The Code

 1 #include 
 2 
 3 using namespace std;
 4 
 5 class Counter {
 6 public :
 7      Counter() : val(0) {}
 8 
 9      void up() {val++;}
10      void down() {val--;}
11 
12      int get() {return val;}
13 
14      void print() {cout << val << endl;}
15 
16 private:
17      int val;
18 };
19 
20 
21 int main() {
22      Counter c;
23      c.print();
24 
25      for (int i = 1; i <= 5; i++)
26           c.up();
27      c.print();
28 
29      for (int i = 1; i <= 6; i++)
30           c.down();
31      c.print();
32 
33      while (c.get() != 0)
34           c.up();
35      c.print();
36 
37      return 0;
38 }

Compiling From the Command Line

• To compile a C++ source file:

  weiss> g++ <filename>
  		

  for example,

  weiss> g++ primes.cpp
  		

  • The default name of the executable is a.out; use the -o command-line option to rename the executable to something else:

    weiss> g++ -o primes primes.cpp
    				

• To execute the program, launch the executable file name:

  weiss> a.out
  		

Code Used in this Lecture