Variable Length Array

In computer programming, a variable-length array (VLA), also called variable-sized, runtime-sized, is an array data structure whose length is determined at run time (instead of at compile time). In C, the VLA is said to have a variably modified type that depends on a value (cf. dependent type).

The main purpose of VLAs is to simplify programming of numerical algorithms.

Programming languages that support VLAs include Ada, Algol 68 (for non-flexible rows), APL, C99 (although subsequently relegated in C11 to a conditional feature which implementations are not required to support; on some platforms, could be implemented previously with alloca() or similar functions) and C# (as unsafe-mode stack-allocated arrays), COBOL, Fortran 90, and J. And also Object Pascal (the language used in Borland Delphi).

The following C99 function allocates a variable-length array of a specified size, fills it with floating-point values, and then passes it to another function for processing. Because the array is declared as an automatic variable, its lifetime ends when read_and_process() returns.

In C99, the length parameter must come before the variable-length array parameter in function calls.

Following is the same example in Ada. Ada arrays carry their bounds with them, so there is no need to pass the length to the Process function.

The equivalent Fortran 90 function is:

when utilizing the Fortran 90 feature of checking procedure interfaces at compile-time; on the other hand, if the functions use pre-Fortran 90 call interface the (external) functions must first be declared, and the array length must be explicitly passed as an argument (as in C):

...
Wikipedia