Compiler, in computer science, computer program that translates source code, instructions in a program written by a software engineer, into object code, those same instructions written in a language the computer’s central processing unit (CPU) can read and interpret. Software engineers write source code using high level programming languages that people can understand. Computers cannot directly execute source code, but need a compiler to translate these instructions into a low level language called machine code.

Compilers collect and reorganize (compile) all the instructions in a given set of source code to produce object code. Object code is often the same as or similar to a computer’s machine code. If the object code is the same as the machine language, the computer can run the program immediately after the compiler produces its translation. If the object code is not in machine language, other programs—such as assemblers, binders, linkers, and loaders—finish the translation.

Most programming languages—such as C, C++, and Fortran—use compilers, but some—such as BASIC and LISP—use interpreters. An interpreter analyzes and executes each line of source code one-by-one. Interpreters produce initial results faster than compilers, but the source code must be re-interpreted with every use and interpreted languages are usually not as sophisticated as compiled languages.

Most computer languages use different versions of compilers for different types of computers or operating systems, so one language may have different compilers for personal computers (PC) and Apple Macintosh computers. Many different manufacturers often produce versions of the same programming language, so compilers for a language may vary between manufacturers.

Consumer software programs are compiled and translated into machine language before they are sold. Some manufacturers provide source code, but usually only programmers find the source code useful. Thus programs bought off the shelf can be executed, but usually their source code cannot be read or modified.