Programs_Problem_Solving_Algorithms.ppt

Programming Fundamentals -->
Ch1. Problem solving
1
Algorithms and Problem Solving

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Problem Solving
• Programming is a process of problem
solving (Problem Solution by computer)
• Algorithm ?
 Step-by-step problem-solving process
 Solution achieved in finite amount of time

3
Problem Solving Process
• Phase 1 - Analyze the problem
 Outline the problem and its requirements
 Design (algorithm) to solve the problem ( Flow chart, pseudo code)
 Algorithm tracing
• Phase 2 - Implement the algorithm
 Implement the algorithm in code (in Programming Language 
Program)
 Verify that the algorithm works
• Phase 3 - Maintenance
 Use and modify the program if the requirements changes

4
Analyze the Problem (1)
Outline the problem and its requirements
• Understand the problem
• Understand problem requirements
 Does program require user interaction?
 Does program manipulate data?
 What is the output?
 are all possible circumstances handled?
• If the problem is complex, divide it into
subproblems
 Analyze each subproblem as above

5
Design Algorithm
1. Flowcharts
2. pseudo-code

6
Flow Chart Symbols
Start and End
Input / output
Selection
Calculation
Data
Flow
• A Flowchart is
 An algorithm graphical representation.
 Written as a combination of the following graphical
notations:

7
• Pseudo-code:
<Algorithm name>
// input ? The comment lines “//”
// function?
// Output?
Begin
<data definition>
<actions>
End

8
Algorithm Tracing
 Draw flowchart
 Find all possible paths
 Check each path with appropriate input data
 Observed Outputs not conform to the expected ones 
error in the algorithm.

9
Problem Example.
Convert a student mark from decimal mode to ABC mode.
Understand problem requirements
 Is the problem statement complete and unambiguous?
 Does program require user interaction? Input the mark
 Does program manipulate data?  covert mark Control
Requirements (IF statement)
 What is the output? The mark converted to A or B or C or
error
 Is there subproblem? No

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Efficiency:
- an algorithm may work correctly but be inefficient – by taking
more time and using more resources than required to
solve the problem.
- becomes more important for larger programs.

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Design Algorithm  Algorithm Discovery
• To discover an algorithm is to solve the problem!
1. Working backwards (modification of an old solution)
Example: Min of a and b is known. Deduce Max a and b.
2. Look for a related problem that has been solved before (similar
solutions reuse)
The precedent algorithm of mark conversion is for single student. It
may be generalized to many students
3. Stepwise Refinement (new solution)
 Break the problem into several sub-problems
 Solve each subproblem separately
 Produces a modular structure

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•Example. Making tea. Suppose we have a robot
which carries out household tasks. We wish to
program the robot to make a cup of tea. An initial
attempt at an algorithm might be:
1. Put tea leaves in pot
2. Boil water
3. Add water to pot
4. Wait 5 minutes
5. Pour tea into cup

Design Algorithm  Algorithm Discovery Strategies  Stepwise
Refinement

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Analyze the Problem(3)
•the designer must know when to stop refining.
•In this example the instruction Switch on kettle is
directly executable by the robot, but that Fill kettle
with water is not.
•Experience will tell us when a step is directly
implementable or not.
•The above algorithm consists of a sequence of
steps, executed exactly once and in order

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Analyze the Problem(3)
•Control requirements: while
Another common requirement is the need for
iteration.
— Example. Repeat the mark conversion for a
class room of 30 students. Each student having a
unique student number from 1 to 30.

16
Implement the algorithm (Coding)
•After testing your algorithm, you can code it
in any programming language.
•In our lab, we are going to use C Language.

C Language Elements
• The general form of a C program
// File: filename.c
// Program description
# include compiler directives
void main ( )
{
declarations section
executable statement section
} Programming Fundamentals -->
17

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 Computer Stage: after completing the
program, the computer must do the
following:
- Compilation:
- Execution:

Stages of Compilation
o Performed by a program called the compiler
o Translates the preprocessor-modified source code
into object code (machine code)
o Checks for syntax errors and warnings
o Saves the object code to a disk file
o If any compiler errors are received, no object code file
will be generated.
o An object code file will be generated if only warnings,
not errors, are received.
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Execution
• when the program become don’t has errors, the
computer execute it to produce the ……output.
21

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Implement the algorithm
Verify that the algorithm works  source of
errors
• Many errors made in:
- analyzing the problem,
- developing an algorithm, and/or
- coding the algorithm

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• Errors are of three types:
. syntax errors
. run-time errors
. logic errors
• Syntax errors: detected by the C compiler
. source code does not conform to one or more of C’s
grammar rules
. examples of syntax errors: undeclared variable, …
• Often one mistake leads to multiple error messages –
can be confusing

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• Run-time errors. detected and displayed by computer during
execution.
- Occur when program directs computer to perform illegal
operation. Example: int x=y/0;
- will stop program execution and display message
• Logic errors.
- caused by faulty algorithm
- sign of error: incorrect program output
- cure: thorough testing and comparison with expected
results
A logic error is referred to as a bug, so finding logic errors is
called debugging.

Programs_Problem_Solving_Algorithms.ppt

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