Learn C++ - C++ Statement

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A C++ program is a collection of functions.

Each function is a collection of statements.

A declaration statement creates a variable. An assignment statement provides a value for that variable.

Example

The following program shows a new capability for cout.

#include <iostream> 
/*from  w w  w.  j  av  a 2 s  .co m*/
int main() { 
     using namespace std; 

     int examples;            // declare an integer variable 

     examples = 25;            // assign a value to the variable 
     cout << "I have "; 
     cout << examples;        // display the value of the variable 
     cout << " examples."; 
     cout << endl; 
     examples = examples - 1;  // modify the variable 
     cout << "I have " << examples << " examples." << endl; 
     return 0; 
} 

The code above generates the following result.

Declaration Statements and Variables

To store an item of information in a computer, you must identify both the storage location and how much memory storage space the information requires.

The program has this declaration statement (note the semicolon):

Assignment Statements

An assignment statement assigns a value to a storage location.

The following statement assigns the integer 25 to the location represented by the variable examples:

The = symbol is called the assignment operator.

One feature of C++ is that you can use the assignment operator serially.

For example, the following is valid code:

int a; 
int b; 
int c; 

a= b = c = 88; 

The assignment works from right to left.

The second assignment statement demonstrates that you can change the value of a variable:

examples = examples - 1;  // modify the variable 

Using cin

The following code uses cin (pronounced "see-in"), the input counterpart to cout.

Also the program shows yet another way to use that master of versatility, the cout object.

#include <iostream> 
//  w w  w  . j a  v a 2 s . c  o  m
int main() 
{ 
     using namespace std; 

     int examples; 

     cout << "How many examples do you have?" << endl; 
     cin >> examples;                // C++ input 
     cout << "Here are two more. "; 
     examples = examples + 2; 
     
     // the next line concatenates output 
     cout << "Now you have " << examples << " examples." << endl; 
     return 0; 
} 

The code above generates the following result.

Note

The following statement reads value into variable.

The iostream file defines the << operator so that you can combine output as follows:

cout << "Now you have " << examples << " examples." << endl;

This allows you to combine string output and integer output in a single statement.

The resulting output is the same as what the following code produces:

cout << "Now you have "; 
cout << examples; 
cout << " examples"; 
cout << endl; 

You can also rewrite the concatenated version this way, spreading the single statement over four lines:

cout << "Now you have " 
     << examples 
     << " examples." 
     << endl; 

The following code outputs the values of expressions.

#include <iostream> 
using namespace std; 
int main() { //w  w  w .  j a  v a  2  s .com
     int x; 

     cout << "The expression x = 100 has the value "; 
     cout << (x = 100) << endl; 
     cout << "Now x = " << x << endl; 
     cout << "The expression x < 3 has the value "; 
     cout << (x < 3) << endl; 
     cout << "The expression x > 3 has the value "; 
     cout << (x > 3) << endl; 
     cout.setf(ios_base::boolalpha);   //a newer C++ feature 
     cout << "The expression x < 3 has the value "; 
     cout << (x < 3) << endl; 
     cout << "The expression x > 3 has the value "; 
     cout << (x > 3) << endl; 
     return 0; 
} 

The code above generates the following result.

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