A computer program is a sequence or set of instructions in a programming language for a computer to execute. It is one component of software, which also includes documentation and other intangible components. A computer program in its human-readable form is called source code. Source code needs another computer program to execute because computers can only execute their native machine instructions. Therefore, source code may be translated to machine instructions using a compiler written for the language. (Assembly language programs are translated using an assembler.) The resulting file is called an executable. Alternatively, source code may execute within an interpreter written for the language. If the executable is requested for execution, then the operating system loads it into memory and starts a process. The central processing unit will soon switch to this process so it can fetch, decode, and then execute each machine instruction. If the source code is requested for execution, then the operating system loads the corresponding interpreter into memory and starts a process. The interpreter then loads the source code into memory to translate and execute each statement. Running the source code is slower than running an executable. Moreover, the interpreter must be installed on the computer.
Example computer program The "Hello, World!" program is used to illustrate a language's basic syntax. The syntax of the language BASIC (1964) was intentionally limited to make the language easy to learn. For example, variables are not declared before being used. Also, variables are automatically initialized to zero. Here is an example computer program, in Basic, to average a list of numbers:
Once the mechanics of basic computer programming are learned, more sophisticated and powerful languages are available to build large computer systems.
History
Improvements in software development are the result of improvements in computer hardware. At each stage in hardware's history, the task of computer programming changed dramatically.
Analytical Engine
In 1837, Jacquard's loom inspired Charles Babbage to attempt to build the Analytical Engine. The names of the components of the calculating device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled. The device had a store which consisted of memory to hold 1,000 numbers of 50 decimal digits each. Numbers from the store were transferred to the mill for processing. The engine was programmed using two sets of perforated cards. One set directed the operation and the other set inputted the variables. However, the thousands of cogged wheels and gears never fully worked together. Ada Lovelace worked for Charles Babbage to create a description of the Analytical Engine (1843). The description contained Note G which completely detailed a method for calculating Bernoulli numbers using the Analytical Engine. This note is recognized by some historians as the world's first computer program.
Universal Turing machine
In 1936, Alan Turing introduced the Universal Turing machine, a theoretical device that can model every computation. It is a finite-state machine that has an infinitely long read/write tape. The machine can move the tape back and forth, changing its contents as it performs an algorithm. The machine starts in the initial state, goes through a sequence of steps, and halts when it encounters the halt state. All present-day computers are Turing complete.
ENIAC
The Electronic Numerical Integrator And Computer (ENIAC) was built between July 1943 and Fall 1945. It was a Turing complete, general-purpose computer that used 17,468 vacuum tubes to create the circuits. At its core, it was a series of Pascalines wired together. Its 40 units weighed 30 tons, occupied 1,800 square feet (167 m2), and consumed $650 per hour (in 1940s currency) in electricity when idle. It had 20 base-10 accumulators. Programming the ENIAC took up to two months. Three function tables were on wheels and needed to be rolled to fixed function panels. Function tables were connected to function panels by plugging heavy black cables into plugboards. Each function table had 728 rotating knobs. Programming the ENIAC also involved setting some of the 3,000 switches. Debugging a program took a week. It ran from 1947 until 1955 at Aberdeen Proving Ground, calculating hydrogen bomb parameters, predicting weather patterns, and producing firing tables to aim artillery guns.
Stored-program computers Instead of plugging in cords and turning switches, a stored-program computer loads its instructions into memory just like it loads its data into memory. As a result, the computer could be programmed quickly and perform calculations at very fast speeds. Presper Eckert and John Mauchly built the ENIAC. The two engineers introduced the stored-program concept in a three-page memo dated February 1944. Later, in September 1944, John von Neumann began working on the ENIAC project. On June 30, 1945, von Neumann published the First Draft of a Report on the EDVAC, which equated the structures of the computer with the structures of the human brain. The design became known as the von Neumann architecture. The architecture was simultaneously deployed in the constructions of the EDVAC and EDSAC computers in 1949. The IBM System/360 (1964) was a family of computers, each having the same instruction set architecture. The Model 20 was the smallest and least expensive. Customers could upgrade and retain the same application software. The Model 195 was the most premium. Each System/360 model featured multiprogramming—having multiple processes in memory at once. When one process was waiting for input/output, another could compute. IBM planned for each model to be programmed using PL/I. A committee was formed that included COBOL, FORTRAN and ALGOL programmers. The purpose was to develop a language that was comprehensive, easy to use, extendible, and would replace COBOL and FORTRAN. The result was a large and complex language that took a long time to compile.
Computers manufactured until the 1970s had front-panel switches for manual programming. The computer program was written on paper for reference. An instruction was represented by a configuration of on/off settings. After setting the configuration, an execute button was pressed. This process was then repeated. Computer programs also were automatically inputted via paper tape, punched cards or magnetic-tape. After the medium was loaded, the starting address was set via switches, and the execute button was pressed.
Very Large Scale Integration
A major milestone in software development was the invention of the Very Large Scale Integration (VLSI) circuit (1964). Robert Noyce, co-founder of Fairchild Semiconductor (1957) and Intel (1968), achieved a technological improvement to refine the production of field-effect transistors (1963). The goal is to alter the electrical resistivity and conductivity of a semiconductor junction. First, naturally occurring silicate minerals are converted into polysilicon rods using the Siemens process. The Czochralski process then converts the rods into a monocrystalline silicon, boule crystal. The crystal is then thinly sliced to form a wafer substrate. The planar process of photolithography then integrates unipolar transistors, capacitors, diodes, and resistors onto the wafer to build a matrix of metal–oxide–semiconductor (MOS) transistors. The MOS transistor is the primary component in integrated circuit chips. Originally, integrated circuit chips had their function set during manufacturing. During the 1960s, controlling the electrical flow migrated to programming a matrix of read-only memory (ROM). The matrix resembled a two-dimensional array of fuses. The process to embed instructions onto the matrix was to burn out the unneeded connections. In 1969, Busicom, a Japanese calculator company, contracted with Intel to manufacture 12 integrated circuit chips, each for a specific calculator function. Ted Hoff, the engineer tasked with the assignment, realized he could build one chip that could execute computer programs to perform the 12 calculator functions. The resulting chip was called the Intel 4004 microprocessor.
The terms microprocessor and central processing unit (CPU) are now used interchangeably. However, CPUs predate microprocessors. For example, the IBM System/360 (1964) had a CPU made from circuit boards containing discrete components on ceramic substrates.
x86 series
In 1978, the modern software development environment began when Intel upgraded the Intel 8080 to the Intel 8086. Intel simplified the Intel 8086 to manufacture the cheaper Intel 8088. IBM embraced the Intel 8088 when they entered the personal computer market (1981). As consumer demand for personal computers increased, so did Intel's microprocessor development. The succession of development is known as the x86 series. The x86 assembly language is a family of backward-compatible machine instructions. Machine instructions created in earlier microprocessors were retained throughout microprocessor upgrades. This enabled consumers to purchase new computers without having to purchase new application software. The major categories of instructions are:
Memory instructions to set and access numbers and strings in random-access memory. Integer arithmetic logic unit (ALU) instructions to perform the primary arithmetic operations on integers. Floating point ALU instructions to perform the primary arithmetic operations on real numbers. Call stack instructions to push and pop words needed to allocate memory and interface with functions. Single instruction, multiple data (SIMD) instructions to increase speed when multiple processors are available to perform the same algorithm on an array of data.
Changing programming environment
VLSI circuits enabled the programming environment to advance from a computer terminal (until the 1990s) to a graphical user interface (GUI) computer. Computer terminals limited programmers to a single shell running in a command-line environment. During the 1970s, full-screen source code editing became possible through a text-based user interface. Regardless of the technology available, the goal is to program in a programming language.
Programming paradigms and languages Programming language features exist to provide building blocks to be combined to express programming ideals. Ideally, a programming language should:
express ideas directly in the code. express independent ideas independently. express relationships among ideas directly in the code. combine ideas freely. combine ideas only where combinations make sense. express simple ideas simply. The programming style of a programming language to provide these building blocks may be categorized into programming paradigms. For example, different paradigms may differentiate:
procedural languages, functional languages, and logical languages. different levels of data abstraction. different levels of class hierarchy. different levels of input datatypes, as in container types and generic programming. Each of these programming styles has contributed to the synthesis of different programming languages. A programming language is a set of keywords, symbols, identifiers, and rules by which programmers can communicate instructions to the computer. They follow a set of rules called a syntax.
Keywords are reserved words to form declarations and statements. Symbols are characters to form operations, assignments, control flow, and delimiters. Identifiers are words created by programmers to form constants, variable names, structure names, and function names. Syntax Rules are defined in the Backus–Naur form. Programming languages get their basis from formal languages. The purpose of defining a solution in terms of its formal language is to generate an algorithm to solve the underlining problem. An algorithm is a sequence of simple instructions that solve a problem.
Generations of programming language
The evolution of programming languages began when the EDSAC (1949) used the first stored computer program in its von Neumann architecture. Programming the EDSAC was in the first generation of programming language.
The first generation of programming language is machine language. Machine language requires the programmer to enter instructions using instruction numbers called machine code. For example, the ADD operation on the PDP-11 has instruction number 24576. The second generation of programming language is assembly language. Assembly language allows the programmer to use mnemonic instructions instead of remembering instruction numbers. An assembler translates each assembly language mnemonic into its machine language number. For example, on the PDP-11, the operation 24576 can be referenced as ADD R0,R0 in the source code. The four basic arithmetic operations have assembly instructions like ADD, SUB, MUL, and DIV. Assemblers also have instructions like DW (Define Word) to reserve memory cells. Then the MOV instruction can copy integers between registers and those memory cells. The basic structure of an assembly language statement is a label, operation, operand, and comment. Labels allow the programmer to work with variable names. The assembler will later translate labels into physical memory addresses. Operations allow the programmer to work with mnemonics. Operands tell the assembler which data the operation will process. The assembler will later translate mnemonics and operands into instruction numbers. Comments allow the programmer to articulate a narrative because the instructions alone may be opaque to humans. The key characteristic of an assembly language program is it forms a one-to-one mapping to its corresponding machine language target. The third generation of programming language uses compilers and interpreters to execute computer programs. The distinguishing feature of a third generation language is its independence from particular hardware. Early languages include FORTAN (1958), COBOL (1959), ALGOL (1960), and BASIC (1964). In 1973, the C programming language emerged as a high-level language that produced efficient machine language instructions. Whereas third-generation languages historically generated many machine instructions for each statement, C has statements that may generate a single machine instruction. Moreover, an optimizing compiler might overrule the programmer and produce fewer machine instructions than statements. Today, an entire paradigm of languages fill the imperative, third generation spectrum. The fourth generation of programming language emphasizes what output results are desired, rather than how programming statements should be constructed. Declarative languages attempt to limit side effects and allow programmers to write code with relatively few errors. One popular fourth generation language is called Structured Query Language (SQL). Database developers no longer need to process each database record one at a time. Also, a simple select statement can generate output records without having to understand how they are retrieved.
Imperative languages
Imperative languages specify a sequential algorithm using declarations, expressions, and statements:
A declaration introduces a variable name to the computer program and assigns it to a datatype – for example: var x: integer; An expression yields a value – for example: 2 + 2 yields 4 A statement might assign an expression to a variable or use the value of a variable to alter the program's control flow – for example: x := 2 + 2; if x = 4 then do_something();
Fortran FORTRAN (1958) was unveiled as "The IBM Mathematical FORmula TRANslating system". It was designed for scientific calculations, without string handling facilities. Along with declarations, expressions, and statements, it supported:
arrays. subroutines. "do" loops. It succeeded because:
programming and debugging costs were below computer running costs. it was supported by IBM. applications at the time were scientific. However, non-IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler. The American National Standards Institute (ANSI) developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports:
records. pointers to arrays.
COBOL COBOL (1959) stands for "COmmon Business Oriented Language". Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, so strings were introduced. The US Department of Defense influenced COBOL's development, with Grace Hopper being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal. COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, like object-oriented programming.
Algol ALGOL (1960) stands for "ALGOrithmic Language". It had a profound influence on programming language design. Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable, structured design. Algol was first to define its syntax using the Backus–Naur form. This led to syntax-directed compilers. It added features like:
block structure, where variables were local to their block. arrays with variable bounds. "for" loops. functions. recursion. Algol's direct descendants include Pascal, Modula-2, Ada, Delphi and Oberon on one branch. On another branch the descendants include C, C++ and Java.
Basic BASIC (1964) stands for "Beginner's All-Purpose Symbolic Instruction Code". It was developed at Dartmouth College for all of their students to learn. If a student did not go on to a more powerful language, the student would still remember Basic. A Basic interpreter was installed in the microcomputers manufactured in the late 1970s. As the microcomputer industry grew, so did the language. Basic pioneered the interactive session. It offered operating system commands within its environment:
The 'new' command created an empty slate. Statements evaluated immediately. Statements could be programmed by preceding them with line numbers. The 'list' command displayed the program. The 'run' command executed the program. However, the Basic syntax was too simple for large programs. Recent dialects added structure and object-oriented extensions. Microsoft's Visual Basic is still widely used and produces a graphical user interface.
C C programming language (1973) got its name because the language BCPL was replaced with B, and AT&T Bell Labs called the next version "C". Its purpose was to write the UNIX operating system. C is a relatively small language, making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s. Its growth also was because it has the facilities of assembly language, but it uses a high-level syntax. It added advanced features like:
inline assembler arithmetic on pointers pointers to functions bit operations freely combining complex operators
C allows the programmer to control which region of memory data is to be stored. Global variables and static variables require the fewest clock cycles to store. The stack is automatically used for the standard variable declarations. Heap memory is returned to a pointer variable from the malloc() function.
The global and static data region is located just above the program region. (The program region is technically called the text region. It is where machine instructions are stored.) The global and static data region is technically two regions. One region is called the initialized data segment, where variables declared with default values are stored. The other region is called the block started by segment, where variables declared without default values are stored. Variables stored in the global and static data region have their addresses set at compile time. They retain their values throughout the life of the process. The global and static region stores the global variables that are declared on top of (outside) the main() function. Global variables are visible to main() and every other function in the source code. On the other hand, variable declarations inside of main(), other functions, or within { } block delimiters are local variables. Local variables also include formal parameter variables. Parameter variables are enclosed within the parenthesis of a function definition. Parameters provide an interface to the function. Local variables declared using the static prefix are also stored in the global and static data region. Unlike global variables, static variables are only visible within the function or block. Static variables always retain their value. An example usage would be the following function:
The stack region is a contiguous block of memory located near the top memory address. Variables placed in the stack are populated from top to bottom. A stack pointer is a special-purpose register that keeps track of the last memory address populated. Variables are placed into the stack via the assembly language PUSH instruction. Therefore, the addresses of these variables are set during runtime. The method for stack variables to lose their scope is via the POP instruction. Local variables declared without the static prefix, including formal parameter variables, are called automatic variables and are stored in the stack. They are visible inside the function or block and lose their scope upon exiting the function or block. The heap region is located below the stack. It is populated from the bottom to the top. The operating system manages the heap using a heap pointer and a list of allocated memory blocks. Like the stack, the addresses of heap variables are set during runtime. An out of memory error occurs when the heap pointer and the stack pointer meet. C provides the malloc() library function to allocate heap memory. Populating the heap with data is an additional copy function. Variables stored in the heap are economically passed to functions using pointers. Without pointers, the entire block of data would have to be passed to the function via the stack.
C++ In the 1970s, software engineers needed language support to break large projects down into modules. One obvious feature was to decompose large projects physically into separate files. A less obvious feature was to decompose large projects logically into abstract data types. At the time, languages supported concrete (scalar) datatypes like integer numbers, floating-point numbers, and strings of characters. Abstract datatypes are structures of concrete datatypes, with a new name assigned. For example, a list of integers could be called IntegerList. In object-oriented jargon, abstract datatypes are called classes. However, a class is only a definition; no memory is allocated. When memory is allocated to a class and bound to an identifier, it is called an object. Object-oriented imperative languages developed by combining the need for classes and the need for safe functional programming. A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a method, member function, or operation. Object-oriented programming is executing operations on objects. Object-oriented languages support a syntax to model subset/superset relationships. In set theory, an element of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other people do not have. Object-oriented languages model subset/superset relationships using inheritance. Object-oriented programming became the dominant language paradigm by the late 1990s. C++ (1985) was originally called "C with Classes". It was designed to expand C's capabilities by adding the object-oriented facilities of the language Simula. A C++ class encapsulates related data items with the functions that manipulate them. This is a C++ class for the Grade encapsulation in an application modelling a school:
A constructor operation is a function with the same name as the class name. It is executed when the calling operation executes the new statement. This is a C++ class for the Person encapsulation in an application modelling a school:
This is a C++ class for the Student encapsulation in an application modelling a school:
This is a driver program for demonstration:
Declarative languages
Imperative languages have one major criticism: assigning an expression to a non-local variable may produce an unintended side effect. Declarative languages generally omit the assignment statement and the control flow. They describe what computation should be performed and not how to compute it. Two broad categories of declarative languages are functional languages and logical languages. The principle behind a functional language is to use lambda calculus as a guide for a well defined semantic. In mathematics, a function is a rule that maps elements from an expression to a range of values. Consider the function: times_10(x) = 10 * x The expression 10 * x is mapped by the function times_10() to a range of values. One value happens to be 20. This occurs when x is 2. So, the application of the function is mathematically written as: times_10(2) = 20 A functional language compiler will not store this value in a variable. Instead, it will push the value onto the computer's stack before setting the program counter back to the calling function. The calling function will then pop the value from the stack. Imperative languages do support functions. Therefore, functional programming can be achieved in an imperative language, if the programmer uses discipline. However, a functional language will force this discipline onto the programmer through its syntax. Functional languages have a syntax tailored to emphasize the what. A functional program is developed with a set of primitive functions followed by a single driver function. Consider the snippet:
The primitives are max() and min(). The driver function is range(). For example, printf("%d", range(10, 4, 7)); will print 6. Functional languages are used in computer science research to explore new language features. Moreover, their lack of side-effects have made them popular in parallel programming and concurrent programming. However, application developers prefer the object-oriented features of imperative languages.
Lisp Lisp (1958) stands for "LISt Processor". It is tailored to process lists. A full structure of the data is formed by building lists of lists. In memory, a tree data structure is built. Internally, the tree structure lends nicely for recursive functions. The syntax to build a tree is to enclose the space-separated elements within parenthesis. The following is a list of three elements. The first two elements are themselves lists of two elements: ((A B) (HELLO WORLD) 94) Lisp has functions to extract and reconstruct elements. The function head() returns a list containing the first element in the list. The function tail() returns a list containing everything but the first element. The function cons() returns a list that is the concatenation of other lists. Therefore, the following expression will return the list x: cons(head(x), tail(x)) One drawback of Lisp is when many functions are nested, the parentheses may look confusing. Modern Lisp environments help ensure parenthesis match. As an aside, Lisp does support the imperative language operations of the assignment statement and goto loops. Also, Lisp is not concerned with the datatype of the elements at compile time. Instead, it assigns (and may reassign) the datatypes at runtime. Assigning the datatype at runtime is called dynamic binding. Whereas dynamic binding increases the language's flexibility, programming errors may linger until late in the software development process. Writing large, reliable, and readable Lisp programs requires forethought. If properly planned, the program may be much shorter than an equivalent imperative language program. Lisp is widely used in artificial intelligence. However, its usage has been accepted only because it has imperative language operations, making unintended side-effects possible.
ML ML (1973) stands for "Meta Language". ML checks to make sure only data of the same type are compared with one another. For example, this function has one input parameter (an integer) and returns an integer:
ML is not parenthesis-eccentric like Lisp. The following is an application of times_10():
times_10 2
It returns "20 : int". (Both the results and the datatype are returned.) Like Lisp, ML is tailored to process lists. Unlike Lisp, each element is the same datatype. Moreover, ML assigns the datatype of an element at compile time. Assigning the datatype at compile time is called static binding. Static binding increases reliability because the compiler checks the context of variables before they are used.
Prolog Prolog (1972) stands for "PROgramming in LOGic". It is a logic programming language, based on formal logic. The language was developed by Alain Colmerauer and Philippe Roussel in Marseille, France. It is an implementation of Selective Linear Definite clause resolution, pioneered by Robert Kowalski and others at the University of Edinburgh. The building blocks of a Prolog program are facts and rules. Here is a simple example:
After all the facts and rules are entered, then a question can be asked:
Will Tom eat Jerry?
The following example shows how Prolog will convert a letter grade to its numeric value:
Here is a comprehensive example: 1) All dragons billow fire, or equivalently, a thing billows fire if the thing is a dragon:
2) A creature billows fire if one of its parents billows fire:
3) A thing X is a parent of a thing Y if X is the mother of Y or X is the father of Y:
4) A thing is a creature if the thing is a dragon:
5) Norberta is a dragon, and Puff is a creature. Norberta is the mother of Puff.
Rule (2) is a recursive (inductive) definition. It can be understood declaratively, without the need to understand how it is executed. Rule (3) shows how functions are represented by using relations. Here, the mother and father functions ensure that every individual has only one mother and only one father. Prolog is an untyped language. Nonetheless, inheritance can be represented by using predicates. Rule (4) asserts that a creature is a superclass of a dragon. Questions are answered using backward reasoning. Given the question:
Prolog generates two answers :
Practical applications for Prolog are knowledge representation and problem solving in artificial intelligence.
Object-oriented programming Object-oriented programming is a programming method to execute operations (functions) on objects. The basic idea is to group the characteristics of a phenomenon into an object container and give the container a name. The operations on the phenomenon are also grouped into the container. Object-oriented programming developed by combining the need for containers and the need for safe functional programming. This programming method need not be confined to an object-oriented language. In an object-oriented language, an object container is called a class. In a non-object-oriented language, a data structure (which is also known as a record) may become an object container. To turn a data structure into an object container, operations need to be written specifically for the structure. The resulting structure is called an abstract datatype. However, inheritance will be missing. Nonetheless, this shortcoming can be overcome. This is a C header file for the Grade datatype in an application modelling a school:
The grade_new() function behaves like the C++ constructor, creating a new Grade instance. This is a C source file for the Grade datatype in an application modelling a school:
This is a C header file for the Person datatype in an application modelling a school:
Here is a C source file for the Person datatype in an application modelling a school:
This is a C header file for the Student datatype in an application modelling a school:
This is a C source file for the Student datatype in an application modelling a school:
This is a driver program for demonstration:
The formal strategy to build object-oriented objects is to:
Identify the objects. Most likely these will be nouns. Identify each object's attributes. What helps to describe the object? Identify each object's actions. Most likely these will be verbs. Identify the relationships from object to object. Most likely these will be verbs. For example:
A person is a human identified by a name. A grade is an achievement identified by a letter. A student is a person who earns a grade.
Syntax and semantics
The syntax of a computer program is a list of production rules which form its grammar. A programming language's grammar correctly places its declarations, expressions, and statements. Complementing the syntax of a language are its semantics. The semantics describe the meanings attached to various syntactic constructs. A syntactic construct may need a semantic description because a production rule may have an invalid interpretation. Also, different languages might have the same syntax; however, their behaviors may be different. The syntax of a language is formally described by listing the production rules. Whereas the syntax of a natural language is extremely complicated, a subset of the English language can have this production rule listing:
a sentence is made up of a noun-phrase followed by a verb-phrase; a noun-phrase is made up of an article followed by an adjective followed by a noun; a verb-phrase is made up of a verb followed by a noun-phrase; an article is 'the'; an adjective is 'big' or an adjective is 'small'; a noun is 'cat' or a noun is 'mouse'; a verb is 'eats'; The words in bold-face are known as non-terminals. The words in 'single quotes' are known as terminals. From this production rule listing, complete sentences may be formed using a series of replacements. The process is to replace non-terminals with either a valid non-terminal or a valid terminal. The replacement process repeats until only terminals remain. One valid sentence is:
sentence noun-phrase verb-phrase article adjective noun verb-phrase the adjective noun verb-phrase the big noun verb-phrase the big cat verb-phrase the big cat verb noun-phrase the big cat eats noun-phrase the big cat eats article adjective noun the big cat eats the adjective noun the big cat eats the small noun the big cat eats the small mouse However, another combination results in an invalid sentence:
the small mouse eats the big cat Therefore, a semantic is necessary to correctly describe the meaning of an eat activity. One production rule listing method is called the Backus–Naur form (BNF). BNF describes the syntax of a language and itself has a syntax. This recursive definition is an example of a metalanguage. The syntax of BNF includes:
::= which translates to is made up of a[n] when a non-terminal is to its right. It translates to is when a terminal is to its right. | which translates to or. < and > which surround non-terminals. Using BNF, a subset of the English language can have this production rule listing:
Using BNF, a signed-integer has the production rule listing:
Notice the recursive production rule:
This allows for an infinite number of possibilities. Therefore, a semantic is necessary to describe a limitation of the number of digits. Notice the leading zero possibility in the production rules:
Therefore, a semantic is necessary to describe that leading zeros need to be ignored. Two formal methods are available to describe semantics. They are denotational semantics and axiomatic semantics.
Software engineering and computer programming
Software engineering is a variety of techniques to produce quality computer programs. Computer programming is the process of writing or editing source code. In a formal environment, a systems analyst will gather information from managers about all the organization's processes to automate. This professional then prepares a detailed plan for the new or modified system. The plan is analogous to an architect's blueprint.
Performance objectives The systems analyst has the objective to deliver the right information to the right person at the right time. The critical factors to achieve this objective are:
The quality of the output. Is the output useful for decision-making? The accuracy of the output. Does it reflect the true situation? The format of the output. Is the output easily understood? The speed of the output. Time-sensitive information is important when communicating with the customer in real time.
Cost objectives Achieving performance objectives should be balanced with all of the costs, including:
Development costs. Uniqueness costs. A reusable system may be expensive. However, it might be preferred over a limited-use system. Hardware costs. Operating costs. Applying a systems development process will mitigate the axiom: the later in the process an error is detected, the more expensive it is to correct.
Waterfall model The waterfall model is an implementation of a systems development process. As the waterfall label implies, the basic phases overlap each other:
The investigation phase is to understand the underlying problem. The analysis phase is to understand the possible solutions. The design phase is to plan the best solution. The implementation phase is to program the best solution. The maintenance phase lasts throughout the life of the system. Changes to the system after it is deployed may be necessary. Faults may exist, including specification faults, design faults, or coding faults. Improvements may be necessary. Adaption may be necessary to react to a changing environment.
Computer programmer A computer programmer is a specialist responsible for writing or modifying the source code to implement the detailed plan. A programming team is likely to be needed because most systems are too large to be completed by a single programmer. However, adding programmers to a project may not shorten the completion time. Instead, it may lower the quality of the system. To be effective, program modules need to be defined and distributed to team members. Also, team members must interact with one another in a meaningful and effective way. Computer programmers may be programming in the small: programming within a single module. Chances are a module will execute modules located in other source code files. Therefore, computer programmers may be programming in the large: programming modules so they will effectively couple with each other. Programming-in-the-large includes contributing to the application programming interface (API).
Program modules Modular programming is a technique to refine imperative language programs. Refined programs may reduce the software size, separate responsibilities, and thereby mitigate software aging. A program module is a sequence of statements that are bounded within a block and together identified by a name. Modules have a function, context, and logic:
The function of a module is what it does. The context of a module are the elements being performed upon. The logic of a module is how it performs the function. The module's name should be derived first by its function, then by its context. Its logic should not be part of the name. For example, function compute_square_root( x ) or function compute_square_root_integer( i :