Problem Solving

1

• Programming Review

Lecture 2

• Problem Solving
• and
• Program Design

2
Remember Your First Program?
Preprocessor statements
// a simple program include ltiostreamgt using
namespace std int main() cout ltlt "Hello
world!" ltlt endl return 0
Comments
Function named main() indicates start of program
Print statement
Ends execution of main() which ends program
Function
3
C Software Development

• Major activities
• Editing (to write the program)
• Compiling (creates .obj file)
• Linking with compiled files (creates .exe file)
• Object files
• Library modules
• Loading and executing
• Testing the program

4
Problem Solving

• Define the problem.
• Develop a solution.
• Outline solution first.
• Then write down the solution steps in detail.
• Test the solution and revise if necessary.
• Document and maintain the solution.

5
Example 1

• Define Problem
• Find the best way to travel from HKUST to Central
  quickly and cheaply.

6
Example 1

• Develop Solution
• Evaluate all possible routes and modes of
  transportation
• Select the route that meets our goal (fastest and
  cheapest)

7
Example 1

• Testing
• Test the route and make sure conditions have not
  changed (e.g., increased bus fares, road
  construction).
• Documentation
• Give description of solution and explain it.
• Maintenance
• Test and revise the route as needed.

8
Programming as Problem Solving

• Define the problem.
• What is the input output?
• What constraints must be satisfied?
• What information is essential?
• Develop a solution
• Construct an algorithm (steps that must be done)
• Implement a program.
• Compile, test, and debug the program.
• Document and maintain the program.

9
What Makes a Good Program?

• Correctness
• Meets the problem requirements
• Produces correct results
• Easy to read and understand
• Easy to modify
• Easy to debug
• Efficient
• Fast
• Requires less memory

10

• Programming Review

• Branching

11
Three Program Structures

• Sequence - executable statements which the
  computer processes in the given order
• Choice/Branching - sequence(s) selected depending
  on some condition
• if ltcondition existsgt
• ltdo Pgt
• Iteration - repetitively executed sequences
• while ltcondition existsgt
• ltdo Pgt

12
Sequence

• It is natural to write a program as a sequence of
  program structures such as sequences, choices,
  and iterations

• Program
• Structure 1

• Program
• Structure 2

• Program
• Structure 3

13
Choice Constructs

• Provide
• Ability to control whether a statement list is
  executed
• Two constructs
• if statement
• if
• if-else
• if-else-if
• switch statement

14
The Basic if Statement

• Syntax
• if(Expression)
• Action
• Example absolute value
• if(value lt 0)
• value -value

Expression
true
false
Action
15
Absolute Value

• // program to read number print its absolute
  value
• include ltiostreamgt
• using namespace std
• int main()
• int value
• cout ltlt "Enter integer "
• cin gtgt value
• if (value lt 0)
• value -value
• cout ltlt "The absolute value is " ltlt value ltlt
  endl
• return 0

16
Choice (if)

• Put multiple action statements within braces
• if ltit's raininggt
• lttake umbrellagt
• ltwear raincoatgt

17
Sorting Two Numbers

• int value1
• int value2
• int temp
• cout ltlt "Enter two integers "
• cin gtgt value1 gtgt value2
• if (value1 gt value2)
• temp value1
• value1 value2
• value2 temp
• cout ltlt "The input in sorted order "
• ltlt value1 ltlt " " ltlt value2 ltlt endl

18
Relational Operators

• Relational operators are used to compare two
  values
• Math C Plain English
• equals example if(ab)
• (ab) means put the value of b into a
• lt lt less than
• ? lt less than or equal to
• gt gt greater than
• ? gt greater than or equal to
• ? ! not equal to

19
Relational Expressions

• Examples
• numberOfStudents lt 200
• 20 j 10 i

20
Operator Precedence

• Which comes first?
• /
• -
• lt lt gt gt
• !

Answer
21
The if-else Statement

• Syntax of if-else
• if ltit's sunnygt
• ltwalk catgt
• else
• lttake cat with umbrellagt
• Example
• if (v 0)
• cout ltlt "v is 0"
• else
• cout ltlt "v is not 0"

Expression
false
true
Action1
Action2
22
Finding the Bigger One

• int value1
• int value2
• int larger
• cout ltlt "Enter two integers "
• cin gtgt value1 gtgt value2
• if (value1 gt value2)
• larger value1
• else
• larger value2
• cout ltlt "Larger of inputs is " ltlt larger ltlt endl

23
Selection

• Often we want to perform a particular action
  depending on the value of an expression
• Two ways to do this
• if-else-if statement
• if-else statements glued together
• switch statement
• An advanced construct

24
if-else-if Statements

• Top10

• if ltMon, Wed, or Fri AMgt
• ltgoto COMP103gt
• else if ltTues, Thurs AMgt
• ltgoto MATH/PHYSgt
• else if lt1PM or 7PMgt
• lteatgt
• else
• ltsleepgt

25
if-else-if Statement Grade

• Example
• if(score gt 90)
• cout ltlt "Grade A" ltlt endl
• else if(score gt 80)
• cout ltlt "Grade B" ltlt endl
• else if(score gt 70)
• cout ltlt "Grade C" ltlt endl
• else if(score gt 60)
• cout ltlt "Grade D" ltlt endl
• else
• cout ltlt "Grade F" ltlt endl

26
switch Statement Grade

• switch(int(score)/10)
• case 10
• case 9 cout ltlt "Grade A" ltlt endl
• break
• case 8 cout ltlt "Grade B" ltlt endl
• break
• case 7 cout ltlt "Grade C" ltlt endl
• break
• case 6 cout ltlt "Grade D" ltlt endl
• break
• default cout ltlt "Grade F" ltlt endl

27
Switch Calculator

• int left
• int right
• char oper
• cout ltlt "Enter simple expression "
• cin gtgt left gtgt oper gtgt right
• cout ltlt left ltlt " " ltlt oper ltlt " " ltlt right
• ltlt " "
• switch (oper)
• case '' cout ltlt left right ltlt endl break
• case '-' cout ltlt left - right ltlt endl break
• case '' cout ltlt left right ltlt endl break
• case '/' cout ltlt left / right ltlt endl break
• default cout ltlt "Illegal operation" ltlt endl

28
A Boolean Type

• C contains a type named bool which can have one
  of two values
• true (corresponds to non-zero value)
• false (corresponds to zero value)
• Boolean operators can be used to form more
  complex conditional expressions
• The and operator is
• The or operator is
• The not operator is !
• Warning
• and are also operators

29
A Boolean Type

• Example logical expressions
• bool P truebool Q falsebool R
  truebool S P Qbool T !Q Rbool U
  !(R !Q)

30
More Operator Precedence

• Precedence of operators (from highest to lowest)
• Parentheses ( )
• Unary operators !
• Multiplicative operators /
• Additive operators -
• Relational ordering lt lt gt gt
• Relational equality !
• Logical and
• Logical or
• Assignment

31
More Operator Precedence

• Examples
• 5 ! 6 7 lt 3
• (5 !6) (7 lt 3)
• 5 15 4 13 12 lt 19 !false 5 lt 24

32
Nested if Statements

• Nested means that one complete statement is
  inside another
• Example
• if ltit is Mondaygt
• if ltit is time for classgt
• ltgo to COMP104gt
• ltcall your friendsgt

• Top10

33
Dangling Else Problem

• Problem Nested if statements can seem ambiguous
  in their meaning.
• What is the value of c after the following is
  executed?
• int a-1, b1, c1
• if(agt0)
• if(bgt0)
• c 2
• else
• c 3

34
Dangling Else Problem

• C groups a dangling else with the most recent
  if.
• The following indentation shows how C would
  group this example (answer c1).
• int a-1, b1, c1
• if(agt0)
• if(bgt0)
• c 2
• else // dangling else grouped to nearest if
• c 3

35
Dangling Else Problem

• Use extra brackets to clarify the intended
  meaning, even if not necessary.
• int a-1, b1, c1
• if(agt0)
• if(bgt0)
• c 2
• else // parenthesis avoid dangling else
• c 3

36
Exercise Leap Year

• Develop an algorithm that takes the year as
  input, and report the number of days for that
  year.
• Leap year 366 days (with Feb 29)
• A year is NOT leap year if it is not divisible by
  4. If the year is divisible by 100, it's not a
  leap year UNLESS it is also divisible by 400.
• Write a C program according to your algorithm

37

• Sample Program I/O
• Leap Year Calculation
• Enter the year 1900
• 1900 is NOT a leap year.
• The number of days in 1900 is 365.
• Test with the following input
• 1985, 1988, 1990, 2004, 2000
• If you dont get every answer correct, check your
  algorithm and/or program