CS102: Introduction to Computer Science II

So far you have seen only the most rudimentary aspect of the iostreams class. The output formatting available with iostreams also includes features such as number formatting in decimal, octal, and hexadecimal. Here's another example of the use of iostreams:

//: C02:Stream2.cpp
// More streams features
#include <iostream>
using namespace std;

int main() {
  // Specifying formats with manipulators:
  cout << "a number in decimal: "
      
    << dec << 15 << endl;
  cout << "in octal: " << oct << 15 << endl;
  cout << "in hex: " << hex << 15 << endl;
  cout << "a floating-point number: "
      
    << 3.14159 << endl;
  cout << "non-printing char (escape): "
       << char(27) << endl;
} ///:~

This example shows the iostreams class printing numbers in decimal, octal, and hexadecimal using iostream manipulators (which don't print anything, but change the state of the output stream). The formatting of floating-point numbers is determined automatically by the compiler. In addition, any character can be sent to a stream object using a cast to a char (a char is a data type that holds single characters). This cast looks like a function call: char( ), along with the character's ASCII value. In the program above, the char(27) sends an "escape" to cout.

Character array concatenation

An important feature of the C preprocessor is character array concatenation. This feature is used in some of the examples in this book. If two quoted character arrays are adjacent, and no punctuation is between them, the compiler will paste the character arrays together into a single character array. This is particularly useful when code listings have width restrictions:

//: C02:Concat.cpp
// Character array Concatenation
#include <iostream>
using namespace std;

int main()
    {
  cout << "This is far too long to put on a "
    "single line but it can be broken up with "
    "no ill effects\nas long as there is no "
    "punctuation separating adjacent
    character "
    "arrays.\n";
} ///:~

At first, the code above can look like an error because there's no familiar semicolon at the end of each line. Remember that C and C++ are free-form languages, and although you'll usually see a semicolon at the end of each line, the actual requirement is for a semicolon at the end of each statement, and it's possible for a statement to continue over several lines.

Reading input

The iostreams classes provide the ability to read input. The object used for standard input is cin (for "console input"). cin normally expects input from the console, but this input can be redirected from other sources. An example of redirection is shown later in this chapter.

The iostreams operator used with cin is >>. This operator waits for the same kind of input as its argument. For example, if you give it an integer argument, it waits for an integer from the console. Here's an example:

//: C02:Numconv.cpp
// Converts decimal to octal and hex
#include <iostream>
using namespace std;

int main() {
  int number;
  cout << "Enter
    a decimal number: ";
  cin >> number;
  cout << "value in octal = 0" 
       << oct << number << endl;
  cout << "value in hex = 0x" 
      
    << hex << number << endl;
} ///:~

This program converts a number typed in by the user into octal and hexadecimal representations.

Calling other programs

While the typical way to use a program that reads from standard input and writes to standard output is within a Unix shell script or DOS batch file, any program can be called from inside a C or C++ program using the Standard C system( ) function, which is declared in the header file <cstdlib>:

//: C02:CallHello.cpp
// Call another program
#include <cstdlib> // Declare "system()"
using namespace std;

int main() {
  system("Hello");
} ///:~
    

To use the system( ) function, you give it a character array that you would normally type at the operating system command prompt. This can also include command-line arguments, and the character array can be one that you fabricate at run time (instead of just using a static character array as shown above). The command executes and control returns to the program.

This program shows you how easy it is to use plain C library functions in C++; just include the header file and call the function. This upward compatibility from C to C++ is a big advantage if you are learning the language starting from a background in C.