Home > Articles

Calculations

  • Print
  • + Share This
This chapter is from the book

Using an Input Stream

If you are using an Integrated Development Environment (IDE), such as Borland's C++Builder or Microsoft's Visual C++, you may find that running the example creates a window where the result (if any) is displayed, and the window almost instantly disappears when the program stops running. If that happens, you'll need to make the program pause before it ends to see whether the output is what you expect.

Even if you are running your programs directly from the command line of a DOS or shell window, you will find the following code interesting, as it lets the program get information from the user.

  1: #include <iostream>
  2: 
  3: using namespace std;
  4:
  5: int main(int argc, char* argv[])
  6: {
  7:  cout << ((6/2)+3) << endl;
 *8:
 *9:  // Note: You must type something before the Enter key
*10:  char StopCharacter;
*11:  cout << endl << "Press a key and \"Enter\": ";
*12:  cin >> StopCharacter;
*13:
 14:  return 0;
 15:}

Lines 8–13 are new. Lines 8 and 13 are whitespace and line 9 is a comment. Lines 10 –12 are the ones that count.

Line 10 declares a variable. This variable is a place to keep the single character that the user must type to end the pause. You can tell what input is expected because of the word char, which identifies what type of variable this is. Since C++ requires every variable to have a name, this one is called StopCharacter.

Line 11 contains a string. This string will be displayed as follows:

Press a key and "Enter":

Note that the backslashes (\) in line 11 allow you to use quotation marks inside a string literal. Without a \, the compiler sees a quote and assumes that it has reached the end of the string literal. Try taking it out and see what happens. You will probably get an error message when you try to compile the program.

Line 12 waits for a single character from the user (this is the pause) and puts it in StopCharacter. The cin standard input stream from <iostream> is used for this purpose. Here, you can see the use of the >> operator, which is called the extractor. The extractor points the opposite direction from the inserter. Through its direction it shows that information is coming from cin and is being put in the variable.

Now the program runs, and prints 6. Then it prints the string Press a key and "Enter": (this kind request for input is often called a prompt). It waits for the user to press a letter, number, or punctuation key on the keyboard followed by the Enter key. When that happens, the program returns 0 and stops.

NOTE

As indicated by the wording of the prompt and the comment, just pressing Enter will have no effect on the program. You must press some other key first.

Variables

Earlier, you saw how almost everything in a program has a name. Of course, literals are an exception to this rule. They are what they are and they have no name.

Variables let you give a name to a value. Actually, you are naming a place to keep that data in the computer's memory.

When you define a variable in C++, you must tell the compiler not only what its name is, but also what kind of information it will hold: a number, a character (such as char StopCharacter), or something else. This is called a variable's type. You may recall that C++ is said to be a strongly typed language, and identifying the type of a variable is part of what makes strong typing work.

The type of the variable tells the compiler, among other things, how much room to set aside in memory to hold the variable's value. It also lets the compiler make sure that the variable is used appropriately (for instance, it will produce an error message if you try to divide a number by a character).

The smallest unit of memory used for a variable is called a byte.

Size of Storage

In most cases, a character is one byte in size, but for international applications, a character may require more than one byte.

NOTE

The ASCII Character Set Variables of type char typically contain values from the ASCII character set. This is a set of 256 characters standardized for use on computers. ASCII is an acronym for American Standard Code for Information Interchange. Nearly every computer operating system supports ASCII. However, ASCII cannot represent some large character sets such as Japanese, which require characters more than one byte in size (multibyte characters) due to the large size of their alphabet. A type called wchar_t is often used for such characters.

A short int is 2 bytes on most computers, a long int is usually 4 bytes, and an int (without the keyword short or long) can be 2 or 4 bytes. If you are running Windows 95, Windows 98, or Windows NT, your int is likely to be 4 bytes in size, but you should rarely (and then only carefully) depend on this.

  • + Share This
  • 🔖 Save To Your Account