CS111: PROGRAMMING LANGUAGE1. Lecture 2: Algorithmic Problem Solving

Transcription

1 CS111: PROGRAMMING LANGUAGE1 Lecture 2: Algorithmic Problem Solving

2 Agenda 2 Problem Solving Techniques Pseudocode Algorithm Flow charts Examples

3 How People Solve Problems 3 A Problem exists when what we have (Data) is not the same as what we want (information) People create a solution (called an Algorithm) which manipulates Data into Information People do this quickly and often in a complex way

4 How Computers Solve Problems 4 Computers also use Algorithms to solve problems, and change data into information Computers can only perform one simple step at a time Complex Human Algorithms must be broken down into simple step-by-step instructions BEFORE they can be translated into computer code

5 What is Programming? 5 Planning or scheduling the performance of a task. Consciously thinking about each step Example: Accelerating in a car 1. Move right foot to gas pedal 2. Apply pressure to gas pedal with right foot 3. If speed is too high, apply less pressure. 4. If speed is too low, apply more pressure.

6 ALGORITHMS AND FLOWCHARTS 6 A typical programming task can be divided into two phases: Problem solving phase produce an ordered sequence of steps that describe solution of problem this sequence of steps is called an algorithm Implementation phase implement the program in some programming language

7 Algorithms 7 (source : wikipedia) In mathematics, computing, linguistics and related disciplines, an algorithm is a procedure (a finite set of well-defined instructions) for accomplishing some task which, given an initial state, will terminate in a defined endstate. Algorithm: A step-by-step problem-solving process in which a solution is arrived at in a finite amount of time

8 Problem Solving 8 Problem Solving is the ability to understand what you have, what you want, and creating a set of instructions to change what you have into what you want Good Problem Solving Skills are based on knowledge, experience and logic Good Programmers NEVER make assumptions

9 9

10 Problem Solving Approach 10 In the programming environment, the problemsolving process involves the following steps: 1. Analyze the problem and outline the problem and its solution requirements. 2. Design an algorithm to solve the problem. 3. Implement the algorithm in a programming language, such as Java. 4. Verify that the algorithm works. 5. Maintain the program by using and improving it, and modifying it if the problem domain changes.

11 11 Problem analysis coding execution cycle

12 Expressing the Algorithms 12 A Standard way of describing an algorithm must exist if we expect our solution to be understood by others easily There are standards in programming: PSEUDOCODE FLOWCHARTS

13 Pseudo Code 13 Pseudo means pretend or false Pseudo Code is pretend or false computer code; generic English-like terms that are somewhat like computer code Pseudo Code is not as standardized as flowcharts, and does not facilitate the breaking down of problems as well as a flowchart does

14 14 Pseudocode (wikipedia) Pseudocode (derived from pseudo and code) is a compact and informal high-level description of a computer programming algorithm that uses the structural conventions of programming languages, but omits detailed subroutines, variable declarations or language-specific syntax. The programming language is augmented with natural language descriptions of the details, where convenient.

15 15 Pseudo Code Example design an algorithm to find the perimeter and area of a rectangle

16 16 Flowcharts Graphical representations of algorithms Tool to translate algorithms into software A Flowchart uses easy-to-understand symbols to represent actions on data and the flow of data Flowcharts aid in breaking down a problem into simple steps

17 Flowchart Symbols 17 Name Symbol Use in Flowchart Oval Denotes the beginning or end of the program Parallelogram Denotes an input operation Rectangle Denotes a process to be carried out e.g. addition, subtraction, division etc. Diamond Denotes a decision (or branch) to be made. The program should continue along one of two routes. (e.g. IF/THEN/ELSE) Hybrid Denotes an output operation Flow line Denotes the direction of logic flow in the program

18 flowcharting 18

19 19 Example Example 1: Write an algorithm and draw a flowchart to convert the length in feet to centimeter. hint: I feet = 30 cm

20 Pseudocode 20 Input the length in feet (Lft) Calculate the length in cm (Lcm) by multiplying LFT with 30 Print length in cm (LCM)

21 Algorithm 21 Flowchart Step 1: Input Lft Step 2: Lcm Lft x 30 Step 3: Print Lcm START Input Lft Lcm Lft x 30 Print Lcm STOP

22 Example 2 22 Write an algorithm and draw a flowchart that will read the two sides of a rectangle and calculate its area.

23 Example 2 23 Pseudocode Input the width (W) and Length (L) of a rectangle Calculate the area (A) by multiplying L with W Print A

24 Example 2 24 Algorithm Step 1: Step 2: Step 3: Input W,L A L x W Print A START Input W, L A L x W Print A STOP

25 Example 3 Write an algorithm and draw a flowchart that will calculate the roots of a quadratic equation 2 ax bx c 0 Hint: the roots are: x1 = ( b + d)/2a and x2 = ( b d)/2a where, d = sqrt ( 2 ) b 4ac 25

26 Example 3 26 Pseudocode: Input the coefficients (a, b, c) of the quadratic equation Calculate d Calculate x1 Calculate x2 Print x1 and x2

27 Example 3 27 START Algorithm: Step 1: Input a, b, c Step 2: d sqrt ( b b 4 a c ) Step 3: x1 ( b + d) / (2 x a) Step 4: x2 ( b d) / (2 x a) Step 5: Print x1, x2 Input a, b, c d sqrt(b x b 4 x a x c) x 1 ( b + d) / (2 x a) X 2 ( b d) / (2 x a) Print x 1,x 2 STOP

28 28 That s all for Today!! Text book [1] chapter 1 (pages 13-16)