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A user-defined function (UDF) is a common fixture in programming languages, and the main tool of programmers for creating applications with reusable code. Since programs are mostly composed of code that comes from the programmer, or in this case the user, most of it is composed of user-defined functions occasionally punctuated by built-in functions.
User-defined functions allow programmers to create their own routines and procedures that the computer can follow; it is the basic building block of any program and also very important for modularity and code reuse since a programmer could create a user-defined function which does a specific process and simply call it every time it is needed. Their syntax depend entirely on the programming language or application where they are created.
Though part of any programming language, user-defined functions more commonly refer to the special functions that a user creates as scripts or programs in large systems such as databases or spreadsheets like Microsoft Excel. This is because most of the functions being used in a database system or spreadsheet are built-in functions that the user simply has to call and provide parameters, and most of what the application can do is already being done by one or more built-in functions. In this case, user-defined functions are special custom functions that are meant to do something not normally done by the built-in functions. In programming languages such as C, C++ and Java almost every part of the program is user-defined, hence, these functions are no longer referred to as "user-defined" to separate them from the built-in functions, they are simply called functions.