Computer Programming - Lecture 2

Lecture 2
Algorithms and
Problem Solving

Computer Programming I 1

Overview
 Algorithm
 Program design
 Pseudocode
 Structure diagram
 Flowcharts
 Introduction to C++
 Testing and debugging
 Program errors

Algorithm
 Algorithm
 A sequence of precise instructions which
leads to a solution

 Program
 An algorithm expressed in a language the
computer can understand


Program Design
 Programming is a creative process
 No complete set of rules for creating a program

 Program Design Process
 Problem Solving Phase
• Result is an algorithm that solves the problem
 Implementation Phase
• Result is the algorithm translated into a programming
language


Problem Solving Phase
 Be certain the task is completely specified
 What is the input?
 What information is in the output?
 How is the output organized?

 Develop the algorithm before implementation
 Experience shows this saves time in getting your
program to run.
 Test the algorithm for correctness


Implementation Phase
 Translate the algorithm into a programming language
 Easier as you gain experience with the language

 Compile the source code
 Locates errors in using the programming language

 Run the program on sample data
 Verify correctness of results

 Results may require modification of the algorithm and
program


Object Oriented Programming
 Abbreviated OOP

 Used for many modern programs

 Program is viewed as interacting objects
 Each object contains algorithms to describe
its behavior
 Program design phase involves designing
objects and their algorithms


Sample problems
Write a program calculating the sum of two numbers
Input Processing Output

5, 10 15

1) Declare variables 3) Process
2) Assign values
input_1 sum = input_1 + input_2
input_1 = 5
input_2
input_2 = 10
sum

The computer (and so C++)
Names for our cells
provides basic arithmetic 8
operations. If the operation
you want to use is not provided,
you have to compose it.


There are many models supporting the
development of the code. We will see now the
same algorithm expressed as:

1)Pseudocode
2)Structure diagram
3)Flowcharts

and finally in C++.


Pseudocode
 Mixture of C++ and ordinary English
 Allows us to make our algorithm precise
without worrying about the details of C++
syntax


Pseudocode
Version 1: Version 2:
PROGRAM Add Two Numbers PROGRAM Add Two Numbers
READ two numbers READ First
ADD the numbers READ Second
WRITE the sum Sum = First + Second
END PROGRAM WRITE Sum
END PROGRAM


Structure Diagram
 Helpful to break the algorithm into more
manageable pieces

Version 1: PROGRAM
Add Two Numbers

READ ADD WRITE
Two Numbers Two Numbers The Sum


Structure Diagram

Version 2: PROGRAM
Add Two Numbers

READ ADD WRITE
Two Numbers Two Numbers The Sum

READ READ Sum =
Input_1 Input_2 Input_1 + Input_2


Rules for Structure Diagram
 A module which resides above others is
referred to as a Calling module
 A module which resides below another is
referred to as a Called module
 A module can be both a calling and called
module
 A called module can only be called by one
calling module


Flowchart
 Diagram that shows the logical flow of a program
 Stress on structured programming
 Useful for planning each operation a program
performs, and in order in which the operations
are to occur
 By visualizing the process, a flowchart can
quickly help identify bottlenecks or inefficiencies
where the process can be streamlined or
improved
 The final visualization can then be easily
translated into a program

Flowcharting symbols
Input/Output (used for all I/O operations)

Processing (used for all arithmetic and data transfer
operations).

Decision (used to test for a condition).

Terminal (used to indicate the beginning and end of a
program or module).

Connector (used to indicate the point at which a transfer of
control operation occurs).

Predefined (used to indicate the name process of a module
to be executed).

Connecting all the symbols and showing the flow


START

READ First
READ Second

Sum = First + Second

WRITE Sum

END


Flowchart Conventions
1) Each symbol denotes a type of operation.
2) A note is written inside each symbol to indicate the
specific function to be performed.
3) The symbols are connected by flow-lines.
4) Flowcharts are drawn and read from top to bottom
unless a specific condition is met that alters the path.
5) A sequence of operations is performed until a terminal
symbol designates the sequence's end or the end of the
program.
6) Sometimes several steps or statements are combined
in a single processing symbol for ease of reading.


start
A flowchart to accept two numbers as
input and prints out the maximum
Input A

Input B

False True
A>B

print B print A

end


Structured Programming
 Structured Programming is a technique using logical control constructs
that make programs easier to read, debug, and modify if changes are
required.

true false

true

Sequence Selection Repetition


Different selection structures
If a > 10 then do S1 If a > 10 then do nothing else do S2

false true
true false A>10
A>10

S1 S2

If a > 10 then do S1 else do S2 If a <= 10 then do S1

True False
true false
A>10 A<=10

S1 S2 S1


Loop structures
False
S1 A<=10

true
S2
S1

true
A<=10
S2

False
Repeat While A is less than or equal to
S1 10 repeat

S2 S1

As long as A is Less than or S2
equal to 10 otherwise exit the End loop
loop

Computer Programmingthe
What is I difference ?
23

Loop example (do..While)
Draw a flowchart to allow the input of 5 numbers Start
and displays out the sum of these numbers
1 C=1
Assume the numbers given to A
are 3,2,4,5,6 in order
2 Sum=0

C=1 C=1 C=2
C=1
Sum = 0 Sum = 3 Sum = 3
Sum = 0
A=3 A=3 A=3
3 Input A
1,2 3 4 5
4 Sum = Sum + A

C=2 C=2 C=2 C=3
Sum = 3 Sum = 3 Sum = 5 Sum = 5 5 C=C+1
A=3 A=2 A=2 A=2
C <=5 true
true
6 3 4 5 6 c<=5
False
C=3 C=3 C=3 C=4 Output
Sum = 5 Sum = 5 Sum = 9 Sum = 9 7 Sum
A=3 A=4 A=4 A=4
C <=5 true
Computer Programming 4
6 3 I 5 24 End

Loop example (while…)
Draw a flowchart to allow the input of Start
5 numbers and displays out the sum
of these numbers
1 C=1

1 C=1 Assume the numbers given to
2 Sum=0
A are 3,2,4,5,6 in order

2 C=1
False
Sum = 0 3 c<=5
4 5 6
true
3
C=1 C=1 C=1 C=2 4
Sum = 0 Sum = 3 Sum = 3 Sum = 3 Input A
C <=5 true A=3 A=3 A=3
5 Sum = Sum + A

C=2 C=2 C=2 C=3 6 C=C+1
Sum = 3 Sum = 3 Sum = 5 Sum = 5
C <=5 true A=2 A=2 A=3
Output
3 4 5 6 7 Sum

Computer Programming I 25 End

Prime number example flowchart
Start 1
Pseudocode algorithm to solve
this problem: Input M 2
1. Start 3
I=2
2. Input a number M
3. Set an Index (I) to start from 2
4. Divide the number M by the Index (I) R=M%I 4
value and store the remainder in R True
False
5. If R is equal to zero then output “Not R=0? 5
Prime” and goto to Step 10
6. Increment Index (I) by 1 I=I+1 6
7. If the Index (I) value is less than the True
number M go to Step 4 I<M? 7

8. Output “Prime” False
9. End Output Output 8
Prime Not Prime

End 9

Example of structured flowchart


Find the Maximum

28

Find the Maximum - Structured

29

Find the Maximum - Structured

30

Unstructured Flowchart

break…


Introduction to C++
 Where did C++ come from?
 Derived from the C language
 C was derived from the B language
 B was derived from the BCPL(Basic Combined
Programming Language) language

 Why the ‘++’?
++ is an operator in C++


C++ History
 C developed by Dennis Ritchie at
AT&T(American Telephone & Telegraph
Company) Bell Labs in the 1970s.
 Used to maintain UNIX systems
 Many commercial applications written in C
 C++ developed by Bjarne Stroustrup at AT&T
Bell Labs in the 1980s.
 Overcame several shortcomings of C
 Incorporated object oriented programming
 C remains a subset of C++


A Sample C++ Program
 A simple C++ program begins this way

#include <iostream>
using namespace std;
int main()
{

 And ends this way

return 0;
}


Comments
Comments are pieces of source code discarded from the code by the
compiler. They do nothing. Their purpose is only to allow the programmer
to insert notes or descriptions embedded within the source code.
C++ supports two ways to insert comments:
// line comment
/* block comment */
/* my second program in C++
with more comments */
#include <iostream.h>
int main ()
{
cout << "Hello World! "; // says Hello World!
return 0;
}


My first program in C++ Hello World!

a comment line

a pound sign (#) is a directive for
the preprocessor. It is not
// my first program in C++ executable code but indications
for the compiler.
#include <iostream.h>
tells the compiler's preprocessor to
int main () include the iostream standard header
file.
{
cout << "Hello World!"; Corresponds to the beginning of the
main function declaration. The main
return 0; function is the point where all C++
programs begin their execution.
}

to terminate a program cout is the standard output stream in C++

Layout of a Simple C++ Program
#include <iostream>

int main()
{
variable_declarations

statement_1
statement_2
…
statement_last

return 0;
}


Program Layout (1/2)
 Programmers format programs so they
are easy to read
 Place opening brace ‘{‘ and closing brace ‘}’
on a line by themselves
 Indent statements
 Use only one statement per line


Program Layout (2/2)
 Variables are declared before they are used
 Typically variables are declared at the beginning of
the program
 Statements (not always lines) end with a semi-colon

 Include Directives
#include <iostream>
 Tells compiler where to find information about items
used in the program
 iostream is a library containing definitions of cin and
cout


Program Layout
 Tells the compiler to use names in iostream in
a “standard” way

 To begin the main function of the program
int main()
{
 To end the main function
return 0;
}
 Main function ends with a return statement

Running a C++ Program
 C++ source code is written with a text editor

 The compiler on your system converts
source code to object code.

 The linker combines all the object code
into an executable program.


Concepts
Compiler: is a program that translates a high-level
language program, such as a C++ program, into a
machine-language program that the computer can
directly understand and execute.
Linking: The object code for your C++ program
must be combined with the object code for
routines (such as input and output routines) that
your program uses. This process of combining
object code is called linking and is done by a
program called a linker. For simple programs,
linking may be done for you automatically.

Run a Program
 Obtain code
 Compile the code
 Fix any errors the compiler indicates and
re-compile the code
 Run the program
 Now you know how to run a program on
your system


Testing and Debugging
 Bug
 A mistake in a program
 Debugging
 Eliminating mistakes in programs
 Term used when a moth caused a failed relay
on the Harvard Mark 1 computer. Grace Hopper
and other programmers taped the moth in logbook
stating:
“First actual case of a bug being found.”


Program Errors
 Syntax errors
 Violation of the grammar rules of the language
 Discovered by the compiler
• Error messages may not always show correct location of
errors
 Run-time errors
 Error conditions detected by the computer at run-time
 Logic errors (warning)
 Errors in the program’s algorithm
 Most difficult to diagnose
 Computer does not recognize an error


End of Lecture 2


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