Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
. Such a variable is accessible only from the function
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s with only two levels of visibility, local variables are contrasted with global variables. On the other hand, many ALGOL
ALGOL
ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...
-derived languages allow any number of levels of nested function
Nested function
In computer programming, a nested function is a function which is lexically encapsulated within another function. It can only be called by the enclosing function or by functions directly or indirectly nested within the same enclosing function. In other words, the scope of the nested function is...
s with private variables, functions, constants and types hidden within them.
In most languages, local variables are automatic variables stored on the call stack
Call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...
Recursion in computer science is a method where the solution to a problem depends on solutions to smaller instances of the same problem. The approach can be applied to many types of problems, and is one of the central ideas of computer science....
calls itself, local variables in each instance of the function are given separate memory address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
. Hence variables of this scope can be declared, written to, and read, without any risk of side-effects to processes outside of the block in which they are declared.
Programming languages that employ call by value semantics provide a called subroutine with its own local copy of the arguments passed to it. In most languages, these local parameters are treated the same as other local variables within the subroutine. In contrast, call by reference and call by name semantics allow the parameters to act as aliases of the values passed as arguments, allowing the subroutine to modify variables outside its own scope.
Variables of local scope are used to avoid issues with side-effects that can occur with global variables
Global variable
In computer programming, a global variable is a variable that is accessible in every scope . Interaction mechanisms with global variables are called global environment mechanisms...
.
Static local variables
A special type of local variable, called a static local, is available in many mainstream languages (including C
C (programming language)
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....
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
Visual Basic is the third-generation event-driven programming language and integrated development environment from Microsoft for its COM programming model...
Visual Basic .NET , is an object-oriented computer programming language that can be viewed as an evolution of the classic Visual Basic , which is implemented on the .NET Framework...
) which allows a value to be retained from one call of the function to another. In this case, recursive calls to the function also have access to the (single, statically allocated
Static memory allocation
Static memory allocation refers to the process of allocating memory at compile-time before the associated program is executed, unlike dynamic memory allocation or automatic memory allocation where memory is allocated as required at run-time....
) variable. In all of the above languages, static variables are declared as such with a special storage class keyword (e.g., static).
Static locals in global functions can be thought of as global variables, because their value remains in memory for the life of the program. The only difference is that they are only accessible (i.e., scoped
Scope (programming)
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
In computer programming, a static variable is a variable that has been allocated statically — whose lifetime extends across the entire run of the program...
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
has a keyword, local, for “localizing” variables, but in this case, local isn't what most people think of as “local”. It gives a temporary, dynamically-scoped
Scope (programming)
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
value to a global (package) variable, which lasts until the end of the enclosing block. However, the variable is visible to any function called from within the block.
To create lexical variables, which are more like the automatic variables discussed above, use the my operator instead.
To understand how it works consider the following code:
$a = 1;
sub f {
local $a;
$a = 2;
g;
}
sub g {
print "$a\n";
}
g;
f;
g;
this will output:
1
2
1
this happens since the global variable $a is modified to a new temporary (local) meaning inside f, but the global value is restored upon leaving the scope of f.
Using my in this case instead of local would have printed 1 three times since in that case the $a variable is really local to the scope of the function f and not seen by g.
For this reason many consider that the operator local should have had a different name like save.
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
as a language was inspired also by Perl, but in this case, the notation was made simpler: a global variable name must be preceded by a $ sign, like $variable_name, while a local variable has simply no $ sign in front of its name, like variable_name (while in perl all scalar values have a $ in front).
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