1 I.Introduction to Algorithm and Programming Algoritma dan Pemrograman – Teknik Informatika UK Petra 2009

2 Computer Programming A form of problem solving Or, more accurately, a way to solve problems What we will be studying is “How to solve problems” using computers

3 What’s a Problem? A question that someone wants an answer to A problem usually has information that represents a starting point, initial assumptions, facts, or data that will be needed in answering the question For example, “What is my grade point average?”

4 How Do We Solve the Problem? With pencil and paper we can compute our grade point average if we know all of our grades and the units for each of the courses taken What are the advantages of writing a computer program to compute our grade point average?

5 How Do We Solve Problems? Four Steps 1. Understand the problem 2. Devise a plan to solve the problem 3. Implement the plan 4. Evaluate the solution  From George Polya’s book, “How to Solve It” written in 1945.

6 What is an Algorithm? A plan for solving a problem  A series of steps to follow to arrive at a solution  A highly technical and precise “recipe” for solving a problem  A “good” algorithm is “correct”, “complete”, and “unambiguous”  “Good” algorithms are also general rather than specific

7 What Does an Algorithm Look Like? It can be written in (precise) English/Indonesia It can be drawn as a diagram (e.g., a flowchart) with steps and a path to follow from one step to the next To make it precise (and computer executable) algorithms are translated into a computer understandable form (e.g., machine language)

8 A Simple Example Problem: Calculate the average of a list of numbers.  Let the number of numbers in the list be n  Assume n is greater than or equal to 1  Calculate the sum of the n numbers  Call this sum s  Then the average is given by s/n

9 Example Expanded Problem: Calculate the average of a list of numbers.  Let the number of numbers in the list be n.  Assume n is greater than or equal to 1.  Pick off the first number on the list and place it on the blackboard.  As long as there are more numbers on the list, pick the next number off the list and add it to the number on the blackboard and replace the number on the blackboard with this sum. (Repeat this step until the list is empty.)  Call the number on the blackboard s.  Then the average is given by s/n.

10 Final Note about Algorithms To qualify as an algorithm, the instructions must be expressed so completely and precisely that somebody could follow the instructions without having to make any assumptions or ask for more information or help of any kind.

11 Computer Basics Hardware and Memory Programs Programming Languages Compilers

12 Hardware and Memory The processor or CPU (Central Processing Unit)  Executes instructions Memory  Main memory (RAM)  Secondary (auxiliary) memory Disk drives (hard drives) Diskettes (floppies) Compact disks (and DVDs) USB Flash Drives

13 Bits, Bytes, and Memory Organization A bit is a single binary digit (i.e., it can assume the value 0 or 1) A byte is a string of 8 bits (enough to hold the representation of a character) The number of a byte is called its “address” or “memory location” When more than one byte is needed to store information the computer uses a string of adjacent bytes

14 Why Do Computers Use Just Zeros and Ones? It’s easier to make a physical device that has only two physical states (e.g., “on” and “off”). There is nothing special about zero and one. The only important thing is that the physical device have two stable states.

15 Programs A program is simply a set of instructions for a computer to follow. A computer “executes” the instructions in the program to process the “data” or information provided as input in order to produce the results or output desired. It is actually the “operating system” (just another program) running on the computer that executes your program.

16 Programming Languages High level languages (languages designed for people to use)  Pascal, FORTRAN, C, C++, Visual Basic, Java Low level languages (languages that computers can understand)  Machine languages, assembly languages

17 Compilers Compilers are programs that translate high level languages into low level languages Compilers are generally machine dependent because different machines have different machine languages The input to the compiler is called the “source code” and the output is called the “object code”

18 Testing and Debugging A mistake in a program is called a bug. The process of removing bugs from a program is called debugging. There are three main classes of bugs or errors in programs:  Syntax errors  Runtime errors  Logic errors

19 Syntax Errors A syntax error is a grammatical mistake in a program. There are strict grammatical rules for writing programs including required punctuation marks. The compiler will catch syntax errors and give its best guess as to the problem, but is often wrong regarding the actual mistake made.

20 Run-Time Errors Errors caught when the program is run. The computer will give an error message, but such messages are often hard to understand. You can be sure that there is an error in your program but it may take some time to find.

21 Logic Errors If the program runs successfully to completion but produces the wrong result, you have a logic error. What you have written is a valid program, but it doesn’t do what it is supposed to do. These are the most difficult errors to find.