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•A typical programming task can be divided into two phases:

1-Problem solving phase

•produce an ordered sequence of steps that describe solution of problem

•this sequence of steps is called an algorithm

2-Implementation phase

•implement the program in some programming language

Steps for writing an algorithm for a program:

1. Keep in mind that algorithm is a step-by-step process.

2. Depending upon programming language, include syntax where necessary.

3. Begin.

4. Include variables and their usage.

5. If they are any loops, try to give sub number lists.

6. Try to give go back to step number if loop or condition fails.

7. Use jump statement to jump from one statement to another.

8. Define expressions.

9. Use break and stop to terminate the process.

Qualities of a good algorithm

1. Input and output should be defined precisely.

2. Each steps in algorithm should be clear and unambiguous.

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3. Algorithm should be most effective among many different ways to

solve a problem.

4. An algorithm shouldn't have computer code. Instead, the algorithm

should be written in such a way that, it can be used in similar

programming languages.

Example 1: Write an algorithm to determine a student’s final grade and

indicate whether it is passing or failing. The final grade is calculated as the

average of four marks.

Detailed Algorithm

•Step 1: Input M1,M2,M3,M4

Step 3: if (GRADE < 50) then

Print “FAIL”

else

Print “PASS”

Endif

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Flowchart:

A graphical representation of the sequence of operations in an information

system or program. Information system flowcharts show how data flows

from source documents through the computer to final distribution to users.

Program flowcharts show the sequence of instructions in a single program or

subroutine. Different symbols are used to draw each type of flowchart.

A Flowchart

•shows logic of an algorithm

•emphasizes individual steps and their interconnections

•e.g. control flow from one action to the next

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Flowchart Symbols Basic:

Name Symbol Use in Flowchart

Oval Denotes the beginning or end of the

program

Parallelogram Denotes an input operation

Rectangle Denotes an output operation

Diamond Denotes a decision (or branch) to be made.

The program should continue along one of two routes. (e.g. IF/THEN/ELSE)

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Hybrid Denotes a process to be carried out e.g.

addition, subtraction, division etc. Flow line

Flow line Denotes the direction of logic flow in the

program

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EXAMPLE

•Write an algorithm and draw a flowchart to convert the length in feet to

centimeter.

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EXAMPLE

Write an algorithm and draw a flowchart that will read the two sides of a

rectangle and calculate its area.

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EXAMPLE

•Write an algorithm and draw a flowchart that will calculate the roots of a

quadratic equation.

• Hint: d = sqrt ( ), and the roots are: x1 = (–b + d)/2a

and x2 = (–b – d)/2a

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DECISION STRUCTURES

•The expression A>B is a logical expression

•it describes a condition we want to test

•if A>B is true (if A is greater than B) we take the action on left

•print the value of A

•if A>B is false (if A is not greater than B) we take the action on right

•print the value of B

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IF–THEN–ELSE STRUCTURE

•The structure is as follows

If condition then

true alternative

else

false alternative

endif

•The algorithm for the flowchart is as follows:

If A>B then

print A

else

print B

endif

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RELATIONAL OPERATORS

EXAMPLE

•Write an algorithm that reads two values, determines the largest value and

prints the largest value with an identifying message.

ALGORITHM

Step 1: InputVALUE1, VALUE2

Step 2: if (VALUE1 > VALUE2) then

else

endif

Step 3: Print “The largest value is”, MAX

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NESTED IFS

•One of the alternatives within an IF–THEN–ELSE statement

•may involve furtherIF–THEN–ELSE statement

EXAMPLE

•Write an algorithm that reads three numbers and prints the value of the

largest number.

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Exercise:

•Flowchart: Draw the flowchart of the above Algorithm.

Now, if we take definition of algorithm as: “A sequence of activities to be

processed for getting desired output from a given input.” Then we can say

that:

1. Getting specified output is essential after algorithm is executed.

2. One will get output only if algorithm stops after finite time.

3. Activities in an algorithm to be clearly defined in other words for it to be

unambiguous.

Before writing an algorithm for a problem, one should find out what is/are

the inputs to the algorithm and what is/are expected output after running the

algorithm.

General Rules for flowcharting

1. All boxes of the flowchart are connected with Arrows. (Not lines)

2. Flowchart symbols have an entry point on the top of the symbol with no

other entry points. The exit point for all flowchart symbols is on the bottom

except for the Decision symbol.

3. The Decision symbol has two exit points; these can be on the sides or the

bottom and one side.

4. Generally a flowchart will flow from top to bottom. However, an upward

flow can be shown as long as it does not exceed 3 symbols.

5. Connectors are used to connect breaks in the flowchart. Examples are:

• From one page to another page.

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• From the bottom of the page to the top of the same page.

• An upward flow of more then 3 symbols

6. Subroutines and Interrupt programs have their own and independent

flowcharts.

7. All flow charts start with a Terminal or Predefined Process (for interrupt

programs or subroutines) symbol.

8. All flowcharts end with a terminal or a contentious loop.

Advantages and Disadvantages of Flow Chart

Advantages

1. Communication: Flow Charts are better way of communicating the logic

of a system to all concerned.

2. Effective Analysis: With the help of flow chart, problem can be analyzed

in more effective way.

3. Proper Documentation: Program flow charts serve as a good program

documentation, which is needed for various purposes.

4. Efficient Coding: The flow charts act as a guide or blueprint during the

systems analysis and program development phase.

5. Proper Debugging: The flow chart helps in debugging process.

6. Efficient Program Maintenance: The maintenance of operating program

becomes easy with the help of flow chart. It helps the programmer to put

efforts more efficiently on that part.

Disadvantages

Complex logic: Sometimes, the program logic is quite complicated. In that

case, flow chart becomes complex and clumsy.

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1. Alterations and Modifications: If alterations are required the flow chart

may require redrawing completely.

2. Reproduction: As the flow chart symbols cannot be typed, reproduction of

flow chart becomes a problem.

3. The essentials of what is done can easily be lost in the technical details of

how it is done.

H.W. Write an algorithm and draw a flow chart to calculate the factorial of a

number (N).

Verify your result by a trace table by assuming N = 5.

Hint: The factorial of N is the product of numbers from 1 to N.

H.W. Write an algorithm and draw a flowchart to calculate 24

. Verify your

result by a trace table.

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H.W. Write down an algorithm and draw a flowchart to find and print the

largest of three numbers. Read numbers one by one. Verify your result by a

trace table. (Use 5, 7, 3 as the numbers read)

H.W. Write an algorithm and draw a flowchart to print the square of all

numbers from 1 to10.

H.W. Write an algorithm and draw a flowchart to print the SUM of numbers

from LOW to HIGH. Test with LOW=3 and HIGH=9.

H.W. Write an algorithm and draw a flowchart to print all numbers between

LOW and HIGH that are divisible by NUMBER.

H.W. Draw a flowchart for a program that reads 10 numbers from the user

and prints out their sum, and their product.

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Example:

Find the biggest of 100 prices and reduce it by 10%

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Example

Output the numbers x,y,z in ascending order.