Overview of Computers and Programming

Main Components of a computer

• CPU - Central Processing Unit: The "brain" of the computer
  • ISA - Instruction Set Architecture: the specific set of low-level instructions available to a CPU. Differs for various CPU types (Intel Pentium, Mac G4, etc)
• ALU - Arithmetic & Logic Unit
  • responsible for performing arithmetic calculations, as well as logical operations (comparisons for equality, inequality, for instance).
• Main Memory (RAM - Random Access Memory)
  • storage close to CPU
  • Faster to access than hard disk
  • stores executing programs and data being currently worked on
• Secondary Memory
  • hard disk, floppy disk, CD, DVD, etc.
• Input devices
  • mouse, keyboard, scanner, network card, etc.
• Output devices
  • screen/console, printer, network card, etc.
• Operating System
  • Examples: Mac OS, Windows XP, Linux
  • Controls computer operations
  • Manages allocation of resources for currently running applications

Memory Concepts

• bit: a binary digit
  • Stores the value 0 or 1
  • Smallest unit of storage in a computer
• byte: 8 bits
  • Smallest addressable unit of storage in a computer
  • Storage units (variables) in a program are 1 or more bytes
  • Each byte in memory has an address (a number that identifies the location)

Programming, and Programming Languages

• Program - a set of instructions for a computer to execute
• Evolution of Programming languages
  • Machine Language
    • Based on machine's core instruction set
    • Needed by computer, hard for humans to read (1's and 0's)
    • Example: 1110110101010110001101010
      • Say what?
  • Assembly Language
    • translation of machine instructions to symbols, slightly easier for humans to read
    • Example: ADD $R1, $R2, $R3
      • Well, we know it has something to do with addition!
  • High-level procedural languages
    • Abstraction of concepts into more human-readable terms
    • Closer to "natural language" (i.e. what we speak)
    • Easy to write and design, but must be translated for computer
    • Examples include C, Pascal, Fortran
  • Object-oriented languages
    • Abstraction taken farther than procedural languages
    • Objects model real-world objects, not only storing data (attributes), but having inherent behaviors (operations, functions)
    • Easier to design and write good, portable, maintainable code
    • Examples include Smalltalk, C++, Java
• Bridging the gap between high-level code and machine code
  • Interpreted languages -- source code is directly run on an interpreter, a program that runs the code statements
  • Compiled Languages
    • A compiler program translates source code (what the programmer writes) to machine language (object code)
    • A linker program puts various object code files together into an executable program (or other target type, like a DLL)
    • C and C++ are compiled languages

Building Programs

Software development

• Analysis and problem definition
• Design - includes design of program or system structure, algorithms, user-interfaces, and more
• Implementation (coding)
• Testing - can be done during design, during implementation, and after implementation
• Maintenance - usually the major cost of a software system. Not part of "development", but definitely part of the software life cycle

Basic Creation and Execution of a C program

1. Create source code with a text editor, store to disk
   • Source code is just a plain text file, usually given a filename extension to identify the programming language (like .c for C, or .cpp for C++)
2. Preprocessor -- Part of compiler process, performs any pre-processing tasks on source code
3. Compilation -- syntax checking, creation of object code
   • Object code is the machine code translation of the source code
4. Linking -- Final stage of the creation of an executable program. Linking of object code files together with any necessary libraries (also already compiled)
5. Execution of program
   • Program loaded into memory, usually RAM
   • CPU executes code instructions

Integrated Development Environments

• An Integrated Development Environment (IDE) is a software package that includes all necessary tools for creating a program. Typcially includes:
  • Text editor
  • Compiler
  • Linker
  • Debugger
  • ability to launch and run applications from within IDE environment
• Not every compiler is an IDE, but pretty much any compiler software includes: preprocessor, compiler, linker
• Examples of C++ IDEs
  • Microsoft Visual C++ 6.0
  • Metrowerks CodeWarrior
  • Borland Builder

Programming is about Problem Solving

• Algorithm - a finite sequence of steps to perform a specific task
  • To solve a problem, you have to come up with the necessary step-by-step process before you can code it
  • This is often the trickiest part of programming
• Some useful tools and techniques for formulating an algorithm
  • Top-down Refinement: Decomposing a task into smaller and simpler steps, then breaking down each step into smaller steps, etc
  • Pseudocode: Writing algorithms informally in a mixture of natural language and general types of code statements
  • Flowcharting: If you can visualize it, it's often easier to follow and understand!
• Testing - algorithms must also be tested!
  • Does it do what is required?
  • Does it handle all possible situations?
• Syntax vs. Semantics
  • Syntax -- the grammar of a language.
    A syntax error: "I is a programmer."
  • Semantics -- the meaning of language constructs
    Correct syntax, but a semantic error: "The car ate the lemur."

A sample C program

#include <stdio.h>		/* Pre-processor directive */

int main()			/* main function, start of program */
{
  int year = 2006;		// a integer variable declaration
  char entry;			// a character variable

  printf("Welcome to C Programming, Summer %d\n", year);
  printf("Enter 'x' to quit: ");
  scanf("%c", &entry);		// read input from user

  printf("Goodbye!\n");

  return 0;			// return value to operating system
}

• A C program has exactly one main() function block
  • Format is:

      int main()
      {
        // code statements here
    
        return 0;
      }
      

• The lines that look like these are known as "comments":

    /* This is a block comment */
    // This is a line comment
  

  They allow users to document code by writing explanatory notes inside. The compiler ignores all comments. Comments are for people.
   
• A C program will usually make use of external libraries of code
  • stdio.h library used here ("standard input/output" library)
  • brought in with the #include directive
  • stdio.h has declarations of the functions printf and scanf
• Declarations of variables (variables must be declared before they are used)
  • int year = 2006;
    char entry;
• The statements with printf are doing output of data to the screen
  • printf -- function that prints to standard output (which by default is the monitor/console)
  • scanf -- function that reads data from standard input (default is the keyboard) and loads the data into variables for storage inside the program