There are two general methods by which a program can be executed. It can be compiled, or it can be interpreted. Although programs written in any computer language can be compiled or interpreted, some languages are designed more for one form of execution than the other. For example, Java was designed to be interpreted, and C was designed to be compiled. However, in the case of C, it is important to understand that it was specifically optimized as a compiled language. Although C interpreters have been written and are available in some environments, C was developed with compilation in mind. Therefore, you will almost certainly be using a C compiler and not a C interpreter when developing your C programs.
Since the difference between a compiler and interpreter may not be clear to all readers, the following brief description will clarify matters. In its simplest form, an interpreter reads the source code of your program one line at a time, performing the specific instructions contained in that line. This is the way earlier versions of BASIC worked. In languages such as Java, a program’s source code is first converted into an intermediary form that is then interpreted. In either case, a run-time interpreter is still required to be present to execute the program.
A compiler reads the entire program and converts it into object code, which is a translation of the program’s source code into a form that the computer can execute directly. Object code is also referred to as binary code or machine code. Once the program is compiled, a line of source code is no longer meaningful in the execution of your program.
In general, an interpreted program runs slower than a compiled program. Remember, a compiler converts a program’s source code into object code that a computer can execute directly. Therefore, compilation is a one-time cost, while interpretation incurs an overhead each time a program is run.