Return to Data Taxonomy

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Data is tied to the memory where it is used during the Data-In-Use state. Basically, there are six different taxons (i.e., taxonomic groups) that cover Data-In-Use. Although the memory categories presented in Figure 1 are shown as having definite boundaries with definite rules governing the use of the categories; there are situations where these classification and boundaries deviate from the definition. For example, the use of interpretive code using dynamic code written as strings or the use of reflection.

Figure 1: Traditional Memory Taxonomy

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A computer program is an ordered sequence of computer instructions that when executed in order accomplish a goal or task. Each computer architecture defines its own Instruction Set. When a program is executed, a Program Counter is set to the first instruction in the program. As the program executes each individual instruction, the program can modify data within the Variable area or it can modify the program counter (i.e., GOTO a specific instruction in the instruction set). As a general rule, the Instruction data cannot be changed during the execution of a program. When programs do change the contents of the executing program, it is usually considered a fault and the program stops executing (see Segmentation Fault (SEGFAULT)).

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Constant Data is a value that is expected to remain constant over time (i.e., the number of pennies in a dollar, the number of seconds in a minute, the value of Pi, the gravitation constant, the speed of light, etc.). In software, constants are usually hardcoded into the software and require a recompile and link in order to change them. Therefore, although programming constants can change, it is generally not a trivial task. Sometimes constants might need to change or be updated such as when the precision used to represent the constant value needs to change (i.e., the value of Pi going from single-precision 3.1415927 to double-precision 3.14159265358979323846. Constants in programming should not change during the normal execution of the program. Sometimes the “constancy” can be violated or purposely circumvented by overwriting the memory location where the value is stored or by using self-modifying code or dynamic code. Using constants in software can:

• Allow for compile-time or editing-time checking of operations such as assigning a value to a constant. In the following example, most modern editors and all compilers raise errors on the second line.

    const float PI = 3.1415927;
    PI = 42;

• Increase readability (i.e., myVar*PI where PI is the value of the numerical constant 3.1415927 is more readable than myVar*3.1415927)

    const float PI = 3.1415927;
    float radius = 4;
    float areaVar = 0;
    areaVar = PI * (radius*radius); -- PI*r^2

• Increase performance by aiding in compiler optimization (i.e., allocating memory once to hold the constant rather than once per usage)

    const float PI = 3.1415927;
    float radius1 = 4;
    float radius2 = 6;
    float areaVar1 = 0;
    float areaVar2 = 0;
    areaVar1 = 3.1415927 * (radius1*radius1); 
    areaVar2 = 3.1415927 * (radius2*radius2);

There are two kinds of Constants used in most programming languages:

• Literal
• Named

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Literal Constants are the actual numeric values (i.e., 3.1415927) or are characters (i.e., 'H'), or strings of characters (i.e., “Hello World”) used in the software. In the following example, “Hello World! %d”, 42 and 0 are literal constants.

int main()
{  cout<<"Hello World! %d", 42;
   return 0;
}

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Named Constants are similar to variables in most programming languages, however, by convention many programmers will use-upper case names for the Constants to help the programmer visually “see” they are working with a Named Constant rather than a variable. In the following example, ANSWER and GREETING are named constants. “%s %d” and 0 are literal constants.

int main()
{  const int ANSWER = 42;
   const string GREETING = "Hello World";
   cout<<"%s %d", greeting, answerToUniverse;
   return 0;
}

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Variable Data, in contrast to Constant Data, is a data item whose value can change during the program’s execution. In other words, the Variable Data value varies over time. There are two main categories based on the lifespan of the Variable Data:

• Static Data
• Dynamic Data

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Static Data is data that does not change after it has been set. Static Data is usually defined during the startup or initialization of a system or subsystem. Some common examples of Static Data are setup parameters of a system; or properties or attributes of objects within a system (i.e., constructor settings). Data that is static differs from immutable data because immutable data can have newer values, but the original values are never replaced, lost, or destroyed. Generally, newer immutable data has pointers to the previous values of the data.

It is a fixed data set. Experts contrast static data with dynamic data, where dynamic data may change after it is recorded, and has to be continually updated.

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Dynamic Data is usually data that is created, updated, and maintained by the programs. It is generally divided into two areas, the Heap and the Stack. The two areas are organized and managed differently and the boundary between the two areas is fluid (i.e., the amount of space allocated to either the Heap or the Stack can expand and contract as long as the total amount of memory in the Heap or the Stack does not exceed the amount of Dynamic Data allocated for the program.

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Stack Memory is generally allocated and unallocated from contiguous blocks of memory automatically during the execution of a program. When a program is executed, memory is allocated for all the parameters passed to the program and all the variables declared in the program. Upon the completion of the program, all the memory allocated for the program is released to be used by the next program. Often, the first program executed is often referred to as the Main program.

Any other programs (i.e., subprograms) called during the program are again allocated more memory for its parameters and declared variables from the Stack. Upon termination of the subprogram, the memory allocated to the subprogram is released back to the system to be reused for other subprograms. Note: the original memory allocated for the main (or calling program) program remains allocated until that program ends. This repetitive and successive allocation creates a stack of allocation, with the most recent subprogram being at the top of the stack. The stack gets deeper and shallower depending on the depth of subprogram allocations.

Figure 2 illustrates a simple example of how the stack works:

1. A program is written into a file that contains two routines: greetings and a main
2. During compilation of the file, the compiler determines the amount of memory required to run main routine
3. During compilation of the file, the compiler determines the amount of memory required to run greetings routine
4. The main program is executed from the command line and the amount of memory required to run the main is allocated and placed on the stack
5. When the main program executes the line that invokes the greetings routine, the memory required for it is allocated and placed onto the stack
6. When the greetings routine is finished, the memory used to execute the routine is removed from the stack and released for re-use
7. When the main routine is finished, the memory used to execute the routine is removed from the stack and released for re-use

Figure 2: A Simple Example of Stack Memory

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Heap Memory is essentially a large pool of memory (typically per process) from which a running program requests chunks of memory for use within the program. The memory is dynamically allocated during the execution of a program using allocation operations such as malloc or new operators on arrays or objects. When the program (i.e., the process terminates, the chunks of memory allocated during the execution of the program are returned. Because the memory is allocated as it is needed and returned as it is required, the memory available for allocation often gets fragmented into small blocks which are very difficult to use and can result in memory exhaustion. This is why modern programming languages have implemented automatic memory management and a process called garbage collection.

1. A program is written into a file that contains a main. Note: During compilation of the file, the compiler determines the amount of memory required to run main routine
2. The main program is executed from the command line and the amount of memory required to run the main is allocated and placed on the stack
3. When the main program executes the line that invokes the malloc statement, the memory required for it is allocated from the heap
4. When the main program executes the line that invokes the free statement, the memory required for it is returned to the *Heap for later use (see - When the main program executes the line that invokes the next malloc statement, the memory required for it is allocated from the *Heap. Note: This may or may not be the same memory allocated in the previous malloc.
5. When the main program executes the line that invokes the next free statement, the memory required for it is returned to the Heap for later use
6. When the main routine is finished, the memory used to execute the routine is removed from the stack and released for re-use

Figure 3: A Simple Example of Heap Memory

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Often the two terms: Attribute and Property are used interchangeably in Computer Science because we often use them to describe the elements of an Object in Object-Oriented Programming (OOP). However, it is useful to understand the difference between the two grammatically:

• An Attribute is a quality or object that we attribute to someone or something. For example, intelligence is an attribute of a person. You cannot go to the store and buy intelligence; it is an attribute of the individual. Another example would be a planet belonging to a solar system. Planets do not usually exist outside of a solar system.

• A Property is a quality that exists without any attribution. Geometric shapes are properties in their own right. They can be used to independently describe many things. For example, a planet or a ball can be described as a sphere¹⁾. A definition of the sphere exists without the attribution to a planet or a ball. Therefore, we say that a planet or a ball has spherical properties.

In UML, an Attribute and a Property both represent a structural association between two entities. Attributes are most often represented as Composition²⁾, while Property is best represented by Aggregation ³⁾.

In Unified Modeling Language (UML) according to Lenny Delligatti⁴⁾, “UML Attributes are DataTypes (e.g., Strings, Integers, etc.) whereas Systems Modeling Language (SysML) properties are ValueTypes (e.g., can be assigned computed values).”

Some parameters are Mutable and others are Immutable. Mutable parameters can be changed inside an operation, and an immutable parameter can not. When a parameter is marked as Immutable (i.e., in) in some programming languages, a copy of the original value is made and passed to the operation on the Stack. Once the operation is completed, the copied value is lost as the stack is unwound. In some languages such as the C, all values are passed by value (i.e., are immutable) unless the value is passed by reference using a pointer to the variable. When a parameter is marked as mutable, then a new value is returned. In C, the general way to do this is by returning the value via a return statement. If the mutable parameter is a pointer, then the value that the pointer points to can be changed, but not the pointer itself. In C, it is possible to have a pointer to a pointer allowing the value pointed to be changed, but not the original pointer.

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There is often confusion about Arguments and Parameters while discussing functions, procedures, methods or operations. In .NET and Javascript the following definitions are used:

• Arguments are the Keys or Names of the arguments, usually provided during the specification of the function, procedure, method of operation
• Parameters are the Values associated with the Arguments during the invocation of the function, procedure, method of operation

  • that you pass. This enables you to pass arguments in any order, and not necessarily in the order specified by the function declaration.

<!DOCTYPE html>
<html>
<body>

<?php

# Declare a function that looks for a needle in a haystack.
# $haystack is an ARGUMENT
# $needle is an ARGUMENT
function string_contains (
    string $haystack, 
    string $needle
  ): bool {}

# Using Positional Notation, the PARAMETERS must be in the same order as defined in the function
# Foobar is the first PARAMETER
# Foo is the second PARAMETER
string_contains (
    'FooBar', 
    'Foo'
  );

# Using Named Notation, the PARAMETERS are named haystack and needle and can occur in any order
string_contains ( 
    haystack: 'FooBar', 
    needle: 'Foo'
  );
string_contains ( 
    needle: 'Foo',
    haystack: 'FooBar'
  );
?>

</body>
</html>

[nick][✓ char, 2022-03-22]this line doesn't make sense .. especially the end .. "For example, the use of dynamic code written as strings and processed at runtime, interpretive code, and reflection." Processed at runtime is OK but the next don't make sense. Should it be "used as interpretive code, and support reflection"???
[char][✓ char, 2022-03-16]New Section -- review -- finished have comment above -- had to get rid of a number of duplicate sections.