1. Memory
    Allocation 

2. Implementation
    C99  Ada  Fortran 90  Cobol   C#  

3. References

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).^[1]

In C, the VLA is said to have a variably modified type that depends on a value (see 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;^[2][3] 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).

Memory

Allocation

• The GNU C Compiler allocates memory for VLAs with automatic storage duration on the stack.^[4] VLAs, like all objects in C, are limited to SIZE_MAX bytes.^[5]
• VLAs can also be allocated on the heap and accessed using a pointer to VLA.

Implementation

C99

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.

float read_and_process(int n)

    for (int i = 0; i < n; ++i)        vals[i] = read_val();

}

In C99, the length parameter must come before the variable-length array parameter in function calls.^[1]

Ada

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.

type Vals_Type is array (Positive range <>) of Float;

function Read_And_Process (N : Integer) return Float is

begin

   for I in 1 .. N loop      Vals (I) := Read_Val;   end loop;   return Process (Vals);

end Read_And_Process;

Fortran 90

The equivalent Fortran 90 function is

function read_and_process(n) result(o)

    integer,intent(in)::n    real::o

    real,dimension(n)::vals    integer::i

    do i = 1,n       vals(i) = read_val()    end do    o = process(vals)

end function read_and_process

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):

function read_and_process(n) result(o)

    integer,intent(in)::n    real::o

    real,dimension(n)::vals    real::read_val, process    integer::i

    do i = 1,n       vals(i) = read_val()    end do    o = process(vals,n)

end function read_and_process

Cobol

The following COBOL fragment declares a variable-length array of records DEPT-PERSON having a length (number of members) specified by the value of PEOPLE-CNT:

    05  PEOPLE-CNT          PIC S9(4) BINARY.    05  DEPT-PERSON         OCCURS 0 TO 20 TIMES DEPENDING ON PEOPLE-CNT.        10  PERSON-NAME     PIC X(20).        10  PERSON-WAGE     PIC S9(7)V99 PACKED-DECIMAL.

The COBOL VLA, unlike that of other languages mentioned here, is safe because COBOL requires one to specify the maximal array size – in this example, DEPT-PERSON cannot have more than 20 items, regardless of the value of PEOPLE-CNT.

C#

The following C# fragment declares a variable-length array of integers. The "unsafe" keyword would require an assembly containing this code to be marked as unsafe.

unsafe void declareStackBasedArray(int size)

    int *pArray = stackalloc int[size];    pArray[0] = 123;

}

References

• Juan amarillo River
• Juan Amarillo River
• Juan amarillo river
• Juan Amarillo wetland
• Juan Amarillo Wetland
• Juan Amarillo (wetland)
• Juana María Condesa Lluch
• Juana María Rodríguez
• Juana Matias
• Juana Melara
• Juana Miguelina Gonzalez Sanchez
• Juana Miguelina González
• Juana Miguelina González Sanchez
• Juana Mordó
• Juan Amores
• Juana Mosquera
• Juan Anderson
• Juan Andrada
• Juan Andrade (disambiguation)
• Juan Andres (Burrul)
• Juan Andres Dutra
• Juan Andres Mejia
• Juan Andrés Arango
• Juan Andrés Dutra
• Juan Andrés Mejía