In computer science, a preprocessor is a program that processes its input data to produce output that is used as input to another program. The output is said to be a preprocessed form of the input data, which is often used by some subsequent programs like compilers. The amount and kind of processing done depends on the nature of the preprocessor; some preprocessors are only capable of performing relatively simple textual substitutions and macro expansions, while others have the power of full-fledged programming languages.

A common example from computer programming is the processing performed on source code before the next step of compilation. In some computer languages (e.g., C and PL/I) there is a phase of translation known as preprocessing.

Lexical preprocessors

Lexical preprocessors are the lowest-level of preprocessors as they only require lexical analysis, that is, they operate on the source text, prior to any parsing, by performing simple substitution of tokenized character sequences for other tokenized character sequences, according to user-defined rules. They typically perform macro substitution, textual inclusion of other files, and conditional compilation or inclusion.

In computer aided engineering (CAE) a preprocessor is a program which provides a graphical user interface (GUI) to define physical properties. This data is used by the subsequent computer simulation.

Steps that are followed in Pre-Processing

1> The geometry (physical bounds) of the problem is defined

2> The volume occupied by the fluid is divided into discrete cells (meshing)

3> The physical modeling is defined - Eg. equations of motion + enthalpy + radiation + species conservation

4> Boundary conditions are defined. This involves specifying the fluid behavior and properties at the boundaries of the problem. For transient problems, the initial conditions are also defined.

5> The simulation is started and the equations are solved iteratively as a steady state or transient

6> Finally a post-processor is used for the analysis and visualization of the resting solution