16.3 Binary and ASCII Files

I l @ ve RuBoard

So far we have limited ourselves to ASCII files. "ASCII" stands for American Standard Code for Information Interchange. It is a set of 95 printable characters and 33 control codes. (A complete list of ASCII codes can be found in Appendix A . ) ASCII files are human-readable . When you write a program, the prog . cc file is ASCII.

Terminals, keyboards, and printers deal with character data. When you want to write a number like 1234 to the screen, it must be converted to four characters ("1", "2", "3", and "4") and written.

Similarly, when you read a number from the keyboard, the data must be converted from characters to integers. This is done by the >> operator.

The ASCII character "0" has the value 48, "1" the value 49, and so on. When you want to convert a single digit from ASCII to integer, you must subtract 48:

 int integer;  char ch;  ch = '5';  integer = ch - 48;  std::cout << "Integer " << integer << '\n'; 

Rather than remember that the character "0" is 48, you can just subtract '0':

 integer = ch - '0'; 

Computers work on binary data. When reading numbers from an ASCII file, the program must process the character data through a conversion routine like the integer conversion routine just defined. This is expensive. Binary files require no conversion. They also generally take up less space than ASCII files. The drawback is that they cannot be directly printed on a terminal or printer. (If you've ever seen a long printout coming out of the printer displaying pages with a few characters at the top that look like "!E#(@$%@^Aa^AA^^JHC%^X", you know what happens when you try to print a binary file.)

ASCII files are portable (for the most part). They can be moved from machine to machine with very little trouble. Binary files are almost certainly nonportable. Unless you are an expert programmer, it is almost impossible to make a portable binary file.

Which file type should you use? In most cases, ASCII is best. If you have small to medium amounts of data, the conversion time does not seriously affect the performance of your program. (Who cares if it takes 0.5 seconds to start up instead of 0.3?) ASCII files also make it easy to verify the data.

Only when you are using large amounts of data will the space and performance problems force you to use the binary format.

I l @ ve RuBoard