CSCI6370: Topics in Computer Science Advanced Topics in Algorithms and Applications Fall Semester, 2002

1
Lecture 2 Logical Problems with Choices
2
Problem Solving

• Before writing a program
• Have a thorough understanding of the problem
• Carefully plan an approach for solving it
• While writing a program
• Know what building blocks are available
• Using good programming principles

3
Algorithms

• Computing problems
• All can be solved by executing a series of
  actions in a specific order
• Algorithms An algorithm is a clearly specified
  set of simple instructions to be followed to
  solve a problem
• Actions to be executed
• The order in which these actions are to be
  executed
• Program control
• Specify order in which statements are to be
  executed

4
Control Structures

• Sequential execution
• Normally, statements are executed one after the
  other in the order written
• Transfer of control
• When the next statement executed is not the next
  one in sequence
• Overuse of goto statements led to many problems
• All C programs written in term of 3 control
  structures
• Sequence structures
• Programs executed sequentially by default
• Selection structures
• Three types if, if else, and switch
• Repetition structures
• Three types while, do while and for

5
Pseudocode

• Artificial, informal language that helps develop
  algorithms
• Similar to everyday English
• Not actually executed on computers
• Help think out a program before writing it
• Easy to convert into a corresponding C program
• Consists only executable statement
• Definitions are not executable statements

6
Flowchart

• Graphical representation of an algorithm
• Drawn using certain special-purpose symbols
  connected by arrows called flowlines
• Special-purpose symbols
• Rectangle (action) symbol
• any type of action.
• Oval symbol
• the beginning or end of a program
• Small circle (connector) symbol
• the entry or exit of a portion of an algorithm.
• Diamond symbol
• indicate that a decision is to be made.
• Single-entry/single-exit

Flowcharting Cs sequence stucture
7
if Selection Statement

• Selection structure
• Used to choose among alternative courses of
  action
• Example - Pseudocode
• if students grade is no less than 60
• print Passed
• If condition true
• Print statement executed and program goes on to
  next statement.
• If condition false
• Print statement is ignored and the program goes
  onto the next statement

8
if Selection Statement

• Pseudocode statement in C
• if ( grade gt 60)
• printf( Passed\n )
• C code corresponds closely to the pseudocode
• Flow chart for the if selection statement
• Diamond symbol (decision symbol)
• Indicates decision is to be made
• Contains an expression that can be true or false
• Test the condition, follow appropriate path

9
if else Selection Statement

• if else
• Specifies an action to be performed both when the
  condition is true and when it is false
• if Only performs an action if the condition is
  true.
• Problem
• Pseudocode

Read in 2 numbers and print in non-decreasing
order.
Read in two numbers, num1 and num2. If num1 is no
larger than num2 Print num1 num2 else
Print num2 num1
10
if else Selection Statement

• Flowchart

begin
Read in two numbers num1 and num2
true
false
Print num1 num2
Print num2 num1
end
11
if else Selection Statement

• C code

12
Nested if else Statements

• Test for multiple cases by placing ifelse
  selection statements inside ifelse selection
  statement
• Once condition is met, rest of statements skipped
• Example - Pseudocode

If students grade is greater than or equal to
90 Print A else If students grade is
greater than or equal to 80 Print B
else If students grade is greater than
or equal to 70 Print C
else If students grade is greater
than or equal to 60 Print
D else Print F
13
Nested if else Statements

• Flowchart

Print A
Print B
Print C
false
Print D
Print F
14
Nested if else Statements
/ Convert a student's score to grade
/ include ltstdio.hgt / function main
begins program execution / int main( void
) int score / score of the
student / printf( "Enter the
score\n" ) / prompt / scanf( "d",
score ) / read an integer /
if ( score gt 90 ) printf( "The grade
is 'A'.\n" ) else if ( score gt 80
) printf(
"The grade is 'B'.\n" )
else if ( score gt 70
) printf( "The grade is 'C'.\n"
) else if ( score
gt 60 ) printf( "The grade
is 'D'.\n" ) else
printf( "The grade is 'F'.\n"
) return 0 / indicate that
program ended successfully / / end
function main /

• C code

15
Compound Statement

• Set of statements within a pair of braces
• Example
• if ( grade gt 60 )
• printf( "Passed.\n" )
• else
• printf( "Failed.\n" )
• printf( "You must take this course
  again.\n" )
• Without the braces, the statement
• printf( "You must take this course
  again.\n" )would be executed automatically.
• A compound statement can be placed anywhere in a
  program that a single statement can be placed.

16
In-Class Programming Exercise
Write a program that completes the
following Read in three integers and determine
the largest. You should only use if/else
statements for the logic in your code. Submit
your maxnum.c file to the dropbox called Maximum
Number and complete Program Quiz 2.