Synapseindia dot net development computer programming

Learning Objectives •First course in Computer Science – No previous knowledge is assumed ! •By the end of the course, students will: – Understand fundamental concepts of computer programming/imperative structured programming languages – Design algorithms to solve (simple) problems – Use the C programming language

Fundamentals – Chapter outline: • Classical model for computing machines • Programming • Programming languages • Compiling • Operating system Setting the basic concepts and terminology …

Model of a computing machine • Computing machine (Computer): “a machine that stores and manipulates information under the control of a changeable program that is stored in its memory.” – Pocket calculator: not a computer ! Manipulates information, but is built to do a specific task (no changeable stored program) • This model is named the “von Neumann architecture” (John von Neumann – 1945; EDVAC - Electronic Discrete Variable Automatic Computer – the first stored-program computer) • Stored-program concept: earlier ideas in theoretical articles of: Alan Turing (1936), Konrad Zuse (1936)

The von Neumann architecture CPU Input Device Output Device ALU CU Main memory (RAM) Secondary storage

The von Neumann architecture • Central Processing Unit (CPU): the “brain” of the machine. – CU: Control Unit – ALU: Arithmetic and Logic Unit • Carries out all basic operations of the computer • Examples of basic operation: adding two numbers, testing to see if two numbers are equal. • Main memory (called RAM for Random Access Memory): stores programs and data – Fast but volatile • Secondary memory: provides permanent storage • Human-computer interaction: through input and output devices. – keyboard, mouse, monitor – Information from input devices is processed by the CPU and may be sent to the main or secondary memory. When information needs to be displayed, the CPU sends it to the output device(s).

How it works • How does a computer execute a program ? (example programs: a computer game, a word processor, etc) • the instructions that comprise the program are copied from the permanent secondary memory into the main memory • After the instructions are loaded, the CPU starts executing the program. • For each instruction, the instruction is retrieved from memory, decoded to figure out what it represents, and the appropriate action carried out. (the fetch- execute cycle) • Then the next instruction is fetched, decoded and executed.

Machine level programming • Example: suppose we want the computer to add two numbers, and if the preliminary result is less than 10, then add 10 to the result • The instructions that the CPU carries out might be : [INSTR1] Load into ALU the number from mem location 15 [INSTR2] Load into ALU the number from mem location 7 [INSTR3] Add the two numbers in the ALU [INSTR4] If result is bigger than 10 jump to [INSTR6] [INSTR5] Add 10 to the number in the ALU [INSTR6] Store the result from ALU into mem location 3 • The processors instruction set: all basic operations that can be carried out by a certain type of processor

Machine level programming • the instructions and operands are represented in binary notation (sequences of 0s and 1s). – Why binary ? Because computer hardware relies on electric/electronic circuits that have/can switch between 2 states – bit (binary digit) – Byte: 8 bits • The program carried out by the CPU, on a hypothetical processor type, could be: 1010 1111 1011 0111 0111 … • This way had to be programmed the first computers ! • The job of the first programmers was to code directly in machine language and to enter their programs using switches

Example: old computer frontpanel LEDS display the current memory address and contents of current memory location or registers SWITCHES allow programmer to enter binary data / instructions

Higher level languages • Assembly language – First step from machine language – Uses symbolic names for operations – Example: a hypothetical assembly language program sequence: 1010 1111 LD1 15 1011 0111 LD2 7 0111 ADD 0011 1010 CMP 10 0010 1100 JGE 12 0110 1010 ADD 10 … …

• Assembly language (cont) – Translation of assembly language into machine language: in the beginning done manually, later done by a special computer program – the assembler – Disadvantages: Low-level language: • programmer must learn the instruction set of the particular processor • Program must be rewritten in order to run on a different processor type – program is not portable

Higher level languages • High level languages – Using more abstract instructions – Portable programs result – Example: a hypothetical program sequence: DEFVAR a,b,c; BEGIN READ a READ b READ c c := a+b IF (c <10) THEN c:=c+10 PRINT c END …

• High level languages – Writing portable programs, using more abstract instructions – A high level instruction (statement) is translated into many machine instructions – Translation of high level language into machine instructions: done by special computer programs – compilers or interpreters

Compilers/Interpreters Compiler Source Code Machine Code Executable Program Input data Output data Interpreter Source Code Input data Output data Compiler: analyzes program and translates it into machine language Executable program: can be run independently from compiler as many times => fast execution Interpreter: analyzes and executes program statements at the same time Execution is slower Easier to debug program

Operating Systems • Operating system: a program that controls the entire operation of a computer system: – Handles all input and output (I/O) operations that are performed on a computer – manages the computer system’s resources – handles the execution of programs (including multitasking or multiuser facilities) • Most famous OS families: – Windows – Unix

Higher Level Languages • Programming Paradigms: – Imperative Programming: describes the exact sequences of commands to be executed • Structured programming, procedural programming – FORTRAN, C, PASCAL, … • Object oriented programming – C++, Java, C#, … – Declarative programming: program describes what it should do, not how • Functional programming – Lisp, ML, … • Logic Programming – Prolog

The C Programming Language • Developed by Dennis Ritchie at AT&T Bell Laboratories in the early 1970s • Growth of C tightly coupled with growth of Unix: Unix was written mostly in C • Success of PCs: need of porting C on MS-DOS • Many providers of C compilers for many different platforms => need for standardization of the C language • 1990: ANSI C (American National Standards Institute) • International Standard Organization: ISO/IEC 9899:1990 • 1999: standard updated: C99, or ISO/IEC 9899:1999

The first C program #include <stdio.h> int main (void) { printf ("Programming is fun.n"); return 0; } uses standard library input and output functions (printf) the program begin of program statements end of program main: a special name that indicates where the program must begin execution. It is a special function. first statement: calls a routine named printf, with argument the string of characters “Programming is fun n” last statement: finishes execution of main and returns to the system a status value of 0 (conventional value for OK)

The format in C • Statements are terminated with semicolons • Indentation is nice to be used for increased readability. • Free format: white spaces and indentation is ignored by compiler • C is case sensitive – pay attention to lower and upper case letters when typing ! – All C keywords and standard functions are lower case – Typing INT, Int, etc instead of int is a compiler error • Strings are placed in double quotes • New line is represented by n (Escape sequence)

Compiling and running C programs Editor Compiler Linker Source code file.c Object code file.obj Executable code file.exe Libraries IDE (Integrated Development Environment)

C Compilers and IDE’s • One can: – use a text editor to edit source code, and then use independent command-line compilers and linkers – use an IDE: everything together + facilities to debug, develop and organize large projects • There are several C compilers and IDE’s that support various C compilers • Lab: Dev-C++ IDE for C and C++, Free Software (under the GNU General Public License) – Works with gcc (GNU C Compiler) • supports the C99 standard • available on Windows and Unix – The GNU Project (http://www.gnu.org/): launched in 1984 in order to develop a complete Unix-like operating system which is free software - the GNU system.

Debugging program errors Editor Compiler Linker Source code file.c Object code file.obj Executable code file.exe Syntactic Errors Libraries Semantic Errors

Syntax and Semantics • Syntax errors: violation of programming language rules (grammar) – "Me speak English good." – Use valid C symbols in wrong places – Detected by the compiler • Semantics errors: errors in meaning: – "This sentence is excellent Italian." – Programs are syntactically correct but don’t produce the expected output – User observes output of running program

Second program #include <stdio.h> int main (void) { printf ("Programming is fun.n"); printf ("And programming in C is even more fun.n"); return 0; }

Displaying multiple lines of text #include <stdio.h> int main (void) { printf ("Testing...n..1n...2n....3n"); return 0; } Output: Testing... ..1 ...2 ....3 It is not necessary to make a separate call to printf for each line of output !

Variables • Programs can use symbolic names for storing computation data and results • Variable: a symbolic name for a memory location – programmer doesn’t has to worry about specifying (or even knowing) the value of the location’s address • In C, variables have to be declared before they are used

Using and Displaying Variables #include <stdio.h> int main (void) { int sum; sum = 50 + 25; printf ("The sum of 50 and 25 is %in", sum); return 0; } Variable sum declared of type int Variable sum assigned expression 50+25 Value of variable sum is printed in place of %i The printf routine call has now 2 arguments: first argument a string containing also a format specifier (%i), that holds place for an integer value to be inserted here

Displaying multiple values #include <stdio.h> int main (void) { int value1, value2, sum; value1 = 50; value2 = 25; sum = value1 + value2; printf ("The sum of %i and %i is %in",value1, value2, sum); return 0; } The format string must contain as many placeholders as expressions to be printed

Using comments in a program • Comment statements are used in a program to document it and to enhance its readability. • Useful for human readers of the program – compiler ignores comments • Ways to insert comments in C: – When comments span several lines: start marked with /*, end marked with */ – Comments at the end of a line: start marked with //

Using comments in a program /* This program adds two integer values and displays the results */ #include <stdio.h> int main (void) { // Declare variables int value1, value2, sum; // Assign values and calculate their sum value1 = 50; value2 = 25; sum = value1 + value2; // Display the result printf ("The sum of %i and %i is %in", value1, value2, sum); return 0; }

Synapseindia dot net development computer programming

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