Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
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...
that is declared but is not set to a definite known value before it is used. It will have some value, but not a predictable one. As such it is a programming error and a common source of bugs in software.
A common assumption made by novice programmers is that variables are set to a known value, such as zero, when they are declared. While this is true for many languages, it is not true for all of them, and so the potential for error is there. Languages such as 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....
In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...
space for variables, and the collection of variables allocated for a subroutine is known as a stack frame. While the computer will set aside the appropriate amount of space for the stack frame, it usually does so simply by adjusting the value of the stack
Stack (data structure)
In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
itself to any new state. Therefore whatever contents of that memory at the time will appear as initial values of the variables which occupy those addresses.
Here's a simple example in C:
void count( void )
{
int k, i;
for(i = 0; i < 10; i++)
k = k + 1;
printf("%d", k);
}
What is the final value of k? The fact is, it is impossible to tell. The answer that it must be 10 assumes that it started at zero, which may or may not be true. Note that in the example, the variable i is initialized to zero by the first clause of the for statement.
In C, variables with static storage duration that are not initialized explicitly are initialized to zero (or null, for pointers).
Not only are uninitialized variables a frequent cause of bugs, but this kind of bug is particularly serious because it may not be reproducible; in some cases, programs with uninitialized variables may even pass SQA
Software quality assurance
Software quality assurance consists of a means of monitoring the software engineering processes and methods used to ensure quality. The methods by which this is accomplished are many and varied, and may include ensuring conformance to one or more standards, such as ISO 9000 or a model such as...
.
Use in languages
Uninitialized variables are a particular problem in languages such as assembly language, C, and C++, which were designed for systems programming
System programming
System programming is the activity of programming system software. The primary distinguishing characteristic of systems programming when compared to application programming is that application programming aims to produce software which provides services to the user System programming (or systems...
. The development of these languages involved a design philosophy in which conflicts between performance and safety were generally resolved in favor of performance. The programmer was given the burden of being aware of dangerous issues such as uninitialized variables.
In other languages, variables are often initialized to known values when created. Examples include:
VHDL initializes all standard variables into special 'U' value. It is used in simulation, for debugging, to let the user to know when the don't care initial values, through the multivalued logic, affect the output.
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...
does not have uninitialized variables. Fields of classes and objects that do not have an explicit initializer and elements of arrays are automatically initialized with the default value for their type (false for boolean, 0 for all numerical types, null for all reference types). Local variables in Java must be definitely assigned to before they are accessed, or it is a compile error.
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
initializes local variables to NULL (distinct from None) and raises an UnboundLocalError when such a variable is accessed before being (re)initialized to a valid value.
Even in languages where uninitialized variables are allowed, many compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
s will attempt to identify uninitialized variables and report them as compile-time errors
Compilation error
Compilation error refers to a state when a compiler fails to compile a piece of computer program source code, either due to errors from the compiler itself, or syntax errors in the code...
.
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