In programming, a variable-length array (or VLA) is an array data structure
Data structure
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
of automatic storage duration whose length is determined at run time (instead of at compile time).
Programming languages that support VLAs include Ada
Ada (programming language)
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
ALGOL 68 isan imperative computerprogramming language that was conceived as a successor to theALGOL 60 programming language, designed with the goal of a...
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
C99 is a modern dialect of the C programming language. It extends the previous version with new linguistic and library features, and helps implementations make better use of available computer hardware and compiler technology.-History:...
; on some platforms, could be implemented previously with alloca or similar functions) and C# (as unsafe-mode stack-allocated arrays), COBOL
COBOL
COBOL is one of the oldest programming languages. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments....
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
.
Memory Allocation
One problem that may be hidden by a language's support for VLAs is that of the underlying memory allocation: in environments where there is a clear distinction between a heap and a stack
Stack-based memory allocation
Stacks in computing architectures are regions of memory where data is added or removed in a last-in-first-out manner.In most modern computer systems, each thread has a reserved region of memory referred to as its stack. When a function executes, it may add some of its state data to the top of the...
, it may not be clear which, if any, of those will store the VLA.
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
function allocates a variable-length array of a specified size, fills it with floating-point values, then passes it to another function for processing. Because the array is declared as an automatic variable, its lifetime ends when the read_and_process function returns.
float read_and_process(int n)
{
float vals[n];
for (int i = 0; i < n; i++)
vals[i] = read_val;
return process(vals, n);
}
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
. Note that Ada arrays
carry their bounds with them, 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
Vals : Vals_Type (1 .. N);
begin
for I in 1 .. N loop
Vals (I) := Read_Val;
end loop;
return Process (Vals);
end Read_And_Process;
COBOL is one of the oldest programming languages. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments....
fragment declares a variable-length array of records, DEPT-PERSON, having a length (number of members) specified by the value of PEOPLE-CNT.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 DEPT-PEOPLE.
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 following C# fragment declares a variable-length array of integers. Please note the "unsafe" keyword which would require assembly containing this code to be marked as unsafe.
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
technically do not provide variable-length arrays, because all array objects in those languages are dynamically allocated on the heap, and therefore do not have automatic storage duration for arrays.
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