C++ For Java Programmers

Arrays (1D)
Arrays (2D)
Classes
Comments
File IO
Functions
Libraries
Loops
Operators
Parameters (const and reference)
Pointers
Program Structure
Selection (decisions)
Simple input/output
Struct
Variable declarations

• In the example code fragments, variable declarations are often omitted, to save time and space. Complete program samples are also available.
• This document is intended to be used by students who have had a 1-semester programming course in Java.  It is not a comprehensive summary of all differences in the Java/C++ language.
  

const int MAX_ITEMS = 10; // common to use const for array size
double array[MAX_ITEMS];  // allocates space for 10 doubles

int count = 0;  // you must maintain the count (may also be true in Java, if array not filled)
do {
    cout << "Enter a double: ";
    cin >> array[count];
    count++;   // count updated
    char reply;
    if (count < MAX_ITEMS)
    {
        cout << "More? ";
        cin >> reply;
    }
} while (count < MAX_ITEMS && reply == 'y');

// As in Java, index bounds are from 0 to size-1.
// A for-loop is often used to process all elements.
for (int i=0; i < count; i++)
    cout << array[i] << endl;

// The following yields unpredictable results
cout << array[10] << endl;

final int MAX_ITEMS = 10;
// declare array, then allocate space (can be done on one line)
double[] array;
array = new double[MAX_ITEMS];
// The following displays 10
System.out.println("Array size is: " + array.length);

// The following causes an exception
System.out.println(array[10]);

int matrix[3][4];
// often processed with nested for loops
for (int row=0; row<3; row++)
    for (int col=0; col<4; col++)
        matrix[row][col] = 0;

ifstream infile;  // ifstream used to read from file
ofstream outfile;  // ofstream used to write to file

infile.open("numbers.dat");  // open makes connection to file on disk
if (!infile)  // error checking
    {
    cout << "Error opening input file numbers.dat\n";
    exit(1);  // aborts program, requires cstdlib
    }

outfile.open("numbers.out");  
if (outfile.fail())  // another way to do error checking
    {
    cout << "Error opening output file numbers.out\n";
    exit(1);  // aborts program, requires cstdlib
    }

double number;
infile >> number;  // reads 1 floating point number from file, stops at white space
outfile << number; // writes 1 value to file

infile.close();
outfile.close();

double sqrt(double);
void printArray(int numbers[], int size);

void printArray(int numbers[], int size)
{
    for (int i=0; i < size; i++)
        cout << numbers[i] << endl;
}

#include <iostream>

// Example prototypes at top of file:

// void functions do not return a value.  
void Welcome(int);
// empty parameter list
void Greetings();
// function that must return a value, parameter names are optional
double doCalc(double a, double b);

//  Example calls in main:

int main()
{
  Welcome(5);    
  Greetings();
  double a = 4.3;  // just to show passing an argument
  double value = doCalc(a, 16.5);
}

// Example function definitions
// Notice that the function header (first line) is the same, except prototypes have a
// semicolon at the end.  

// start with function header, then function body
void Welcome(int numStars)
{
    for (int i=0; i<numStars; i++)
        cout << "*";
    cout << endl << "Welcome\n";  // "\n" forces line feed
}

void Greetings()
{
    cout << "Greetings, earthling\n";
}

double doCalc(double a, double b)
{
    return a*a + b*a;  // must have return of type double
}

char reply;
do
{    
    cout << "Enter y or n: ";
    cin >> reply;
} while (reply != 'y' && reply != 'n');

sum = 0;
count = 1;
while (count < 10)
{
    sum += count;
    count++;
}

sum = 0;
for (int count=1; count<10; count++)
    sum += count;

Mathematical: + - / * % += -= /= *= %=
Relational: < > <= >= !=
Logical: || && !

void doSomething(int x)  // makes a local copy, initialized with parameter value
{
    x = 2 * x; // modifies local copy, in this example x is assigned 20
}

int main()
{
    int y = 10;
    doSomething(y);
    cout << y << endl;  // y will still have the value of 10
}

// parameter MUST be a variable... can't change the value of a literal
void doItAgain(int& x)  // x looks at memory location of parameter
{
    x = 2 * x;
}

int main()
{
    int y = 10;
    doSomething(y);
    cout << y << endl;  // y will now have the value of 20
}

// pointer declaration uses *, must include the data type
int* iptr;  
// new is used to allocate space, just one int in this example
iptr = new int;
// pointer must be dereferenced using * to access contents
*iptr = 3;
cout << "iptr value: " << *iptr << endl;
// without dereferencing, you would be printing the memory address
cout << "address: " << iptr << endl;

// pointer declaration same above, initialize to NULL for safety
int* arrayPtr = NULL;
// by specifying size in brackets, arrayPtr is now an array
arrayPtr = new int[10];
// normal array syntax can be used to access
for (int i=0; i<10; i++)
    arrayPtr[i] = 0;
// assume arrayPtr is used in some way here
// now return the memory to the heap
delete[] arrayPtr;

// Libraries are listed at the top
#include <iostream> // library for IO

using namespace std;
// every C/C++ program must have a main method, which will should be int.  
//main is the function invoked when you execute the program.
int main()            
{   // as in Java, function bodies are surrounded by braces

    cout << "Hello World\n";     // see Simple input/output

}

// program is included within HelloWorld class, main is a static function (can
// be called with no object).  
public class HelloWorld {
        public static void main(String[] args) {
                System.out.println("Hello World");
        }
}

char reply;
cout << "Enter y or n: ";
cin >> reply;
if (reply != 'y' && reply != 'n')
    cout << "Invalid response!\n";

// score is an int containing test grade
if (score >= 90)
    cout << "Grade: A\n";
else if (score >= 80)
    cout << "Grade: B\n";
else if (score >= 70)
    cout << "Grade: C\n";

// can use compound conditions
if (score >= 80 && score < 90)
    cout << "Grade: B\n";

// the following assumes a menu was displayed
int menu;
cin >> menu;
switch (menu) {
    case 1:        // equivalent to if (menu == 1)
        doOne();
        break;  // remember that without this, control continues to next case
    case 2:
        doTwo();
        break;
    default:
        cout <<  "Invalid menu option!\n";
    }

int number;  
cout << "Enter a number: ";  // displays a prompt on the screen
cin >> number;  // reads 1 integer, stops at white space
cout << "You entered " << number << endl;  // endl moves to next line

int number;
System.out.print("Enter a number: ");
number = Integer.parseInt(stdin);  // assumes stdin appropriately declared
System.out.println("You entered " + number); // println forces move to next line

// define the Date data type.  The identifier is called the structure tag.
struct Date
{ 
// list the members
int month, day, year;
};   // end with a semicolon.  

int main()
{ 
// Use the Data datatype, which allocates 3 ints for each variable.
Date birthday, expiration;

// members can be accessed using the dot operator
birthday.month = 1;
birthday.day = 10;
birthday.year = 1980;

// structs can be treated as a whole
expiration = birthday;
expiration.year = 2002;

cout << "Birthday\n";
// members must be printed individually, << is not defined for the struct
cout << birthday.month << "/" << birthday.day << "/" << birthday.year;
cout << "Expiration\n";
cout << expiration.month << "/" << expiration.day << "/" << expiration.year;
}