Programming Languages: Types of Languages

• A computer programs is a set of instructions for the computer to carry out (execute)

• Programs are written using a programming language

– Types of languages:

∗ Machine language

· The only language a computer understands is machine language

· ML is binary - it consists of zeroes and ones only

· Every processor type has its own machine language

· Machine code is usually called object code

∗ Assembly language

· Uses mnemonic instructions

· 1:1 correspondence between assembly language and machine language

· For example: LDA 96

· CPU cannot directly execute program because not binary

· Assembler: Program that converts assembly language into machine lan-guage

∗ High-level languages

· Use English-like instructions

· Uses symbolic references (variables) to memory

· For example: x = y + 6

· Example languages: BASIC, FORTRAN, COBOL, C, C++, Lisp, Java,Python

· These are independent of any CPU

· Programs written in high-level languages must be converted to machinelanguage

· Such programs are called source code

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Programming Languages: Converting High-level Programs to Object Code

• High-level source code must be converted into object code in order to execute aprogram on a computer

1. Interpreter

– Reads one HL instruction at a time

– Translates that instruction to machine code

– Executes the machine code

– Repeats until every HL instruction executed

2. Compiler

– Entire HL program converted into a machine language program

– Machine program executed

• Comparison of interpreters v compilers

– Interpreters:

∗ Programs execute slowly (must wait for each instruction to be translated)

∗ Catch errors exactly where they occur

– Compilers:

∗ Programs execute fast (cause is machine code)

∗ Generated errors may occur far from the actual problem

∗ Programs must be recompiled after every change

• There must be a compiler/interpreter for every family of chips

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Programming Languages: Converting High-level Programs to Object Code (2)

• Comparison of high-level v low-level languages

– Low-level:

∗ Most efficient code

∗ Can take advantage of chip’s special features

– High-level

∗ Faster to write (∼ 10X faster)

∗ Self-documenting

∗ Allows programmer to ignore low-level details

∗ Portable

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Programming Languages: Program Structure/Format

• General differences among languages include

1. Fixed v free format

– Fixed format languages (e.g., FORTRAN, Python) expect instructions to beplaced just so on a line, usually one instruction per line

– Free format languages can place instructions anywhere without restriction

∗ Lines separated by special characters called delimiters

2. Case sensitive v non-case sensitive

– In languages that are case-sensitive, upper v lower case makes a difference

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Programming Languages: Language Components

• In general, all programming languages have same set of components

1. An alphabet

– A set of symbols from which the language is constructed

2. Reserved (key) words

– A set of words with special meaning in the language

– They cannot be redefined by the programmer

3. Syntax

– The rules governing valid sentences in the language

– Rules called a grammar

4. Semantics

– The meaning behind the constructs of the language

• Learning a language is learning these components

• Major differences are in syntax and language model (imperative, object-oriented,functional, declarative)

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Programming Languages: Syntax and Semantics

• Every language characterized by

1. Syntax

– Rules that specify the form of valid sentences in the language

– Rules called a grammar

– BNF (Backus-Naur Form) is one technique for describing syntax

– The following will be used to describe syntax in these notes

∗ Terminal symbols (symbols that represent themselves/ stand for them-selves): Boldface, or normal font

∗ Nonterminal symbols (symbols that represent a gropu of constructs): italicsor delimited by <>

∗ Choices/options: separated by |∗ Optional components: Delimited by [ and ]

∗ Zero or more occurences of a construct: construct∗ or {construct}∗

∗ One or more occurences of a construct: construct+ or {construct}+ or con-struct...

2. Semantics

– The meanings associated with language constructs

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Programming Languages: Program Constructs

• In general, all programming languages have same set of constructs

1. Comments

– Documentation used to

(a) Indicate history (e.g., programmer names, creation date, date last modified)

(b) Explain algorithms used

(c) Explain uses of variables

(d) Etc.

– Comments ignored by compiler

2. Identifiers

– Symbols that represent program components, e.g.,

(a) Variables

(b) Constants

(c) Reserved words

3. Variables

– Represent values stored in memory

– Have specific data type allocated specific amount of storage

4. Declarations

– Specify data type (integer, float, string, ...) of construct

– Needed so

(a) Computer allocates proper amount of storage

(b) Computer knows how to interpret the binary value stored there

5. IO statements

– Accept input from mouse, keyboard, etc.

– Write output to file, display, printer, etc.

6. Assignment statement

– Assigns values to variables

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Programming Languages: Program Constructs (2)

7. Control statements

– Control order in which statements are executed

∗ Sequence (default)

· Statements simply executed in order that they appear in the program

∗ Loop (iteration, repetition)

· Causes set of statements to be repeated

∗ Conditional (if)

· Determines which of a set of instructions are executed based on a condition

∗ Subprogram call

· Causes control to jump to a subprogram (function, method, subroutine)

· Subprogram executes like a miniprogram

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Programming Languages: Program Development

• Every program has 3 aspects:

1. Input - Data to be processed is made accessible to program

2. Process - Data is processed (producing meaningful info)

3. Output - Processing results are made available to user

• Software Life Cycle (Program Development Cycle)

– General process describing steps used to develop programs:

1. Analyze

∗ Basic problem-solving - identifying what the problem is

∗ Generate program requirements/specifications: Id input and output

2. Design

∗ Identify program constructs and processing required to solve problem

∗ Involves designing an algorithm:

· A step-by-step sequence of unambiguous instructions for solving a problemin a finite amount of time

3. Implement

∗ Write code

∗ Compile

∗ Debug

· Syntax errors: Errors in statement structure

4. Test

∗ Generate test cases

∗ Run/execute the program

∗ Debug

· Logic errors: Errors in problem solving strategies

· Semantic errors: Errors in use of language

5. Maintain

∗ Includes documentation and tweaking as needed

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Programming Languages: Programming Environments

• A programming environment includes all the software that is used to create, debug,execute, and maintain programs

• The simplest environment consists of a set of unrelated programs

– An editor to create the program

– The compiler/interpreter to create the object code and/or execute the program

– Possibly debugging software to help id problems

• Environments generally consist of one of two types:

– Command line

∗ In this approach, the user types commands directly to the operating system

∗ This is a strictly text-based approach

– GUI (Graphical User Interface)

∗ In this approach, a windowed environment is provided

∗ Standard mouse click, drag-and-drop, etc. methods are used

• Many languages provide an IDE (Integrated Development Environment)

– An IDE is a GUI-based system that incorporates all of the above aspects into asingle integrated interface for programming

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