Ch3.1

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Lecture 3
Control Structures
Selection: else/if and switch

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2.1 Introduction
 Before writing a program
 Have a thorough understanding of problem
 Carefully plan your approach for solving it
 While writing a program
 Know what “building blocks” are available
 Use good programming principles

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2.2 Algorithms
 Computing problems
 Solved by executing a series of actions in a
specific order
 Algorithm is a procedure determining
 Actions to be executed
 Order to be executed
 Example: recipe
 Program control
 Specifies the order in which statements are
executed

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2.3 Pseudocode
 Pseudocode
 Artificial, informal language used to develop
algorithms
 Similar to everyday English
 Not executed on computers
 Used to think out program before coding
 Easy to convert into C++ program
 Only executable statements
 No need to declare variables

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2.4 Control Structures
 Sequential execution
 Statements executed in order
 Transfer of control
 Next statement executed not next one in sequence
 3 control structures (Bohm and Jacopini)
 Sequence structure
 Programs executed sequentially by default
 Selection structures
 if, if/else, switch
 Repetition structures
 while, do/while, for

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 C++ keywords
 Cannot be used as identifiers or variable names
C++ Keywords
Keywords common to the
C and C++ programming
languages
auto break case char const
continue default do double else
enum extern float for goto
if int long register return
short signed sizeof static struct
switch typedef union unsigned void
volatile while
C++ only keywords
asm bool catch class const_cast
delete dynamic_cast explicit false friend
inline mutable namespace new operator
private protected public reinterpret_cast
static_cast template this throw true
try typeid typename using virtual
wchar_t

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 Flowchart
 Graphical representation of an algorithm
 Special-purpose symbols connected by arrows (flowlines)
 Rectangle symbol (action symbol)
 Any type of action
 Oval symbol
 Beginning or end of a program, or a section of code (circles)
 Single-entry/single-exit control structures
 Connect exit point of one to entry point of the next
 Control structure stacking

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2.5 if Selection Structure
 Selection structure
 Choose among alternative courses of action
 Pseudocode example:
If student’s grade is greater than or equal to 60
Print “Passed”
 If the condition is true
 Print statement executed, program continues to next
statement
 If the condition is false
 Print statement ignored, program continues
 Indenting makes programs easier to read
 C++ ignores whitespace characters (tabs, spaces, etc.)

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 Translation into C++
If student’s grade is greater than or equal to 60
Print “Passed”
if ( grade >= 60 )
cout << "Passed";
 Diamond symbol (decision symbol)
 Indicates decision is to be made
 Contains an expression that can be true or false
 Test condition, follow path
 if structure
 Single-entry/single-exit

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 Flowchart of pseudocode statement
true
false
grade >= 60 print “Passed”
A decision can be made on
any expression.
zero - false
nonzero - true
Example:
3 - 4 is true

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2.6 if/else Selection Structure
 if
 Performs action if condition true
 if/else
 Different actions if conditions true or false
 Pseudocode
if student’s grade is greater than or equal to 60
print “Passed”
else
print “Failed”
 C++ code
if ( grade >= 60 )
cout << "Passed";
else
cout << "Failed";

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 Nested if/else structures
 One inside another, test for multiple cases
 Once condition met, other statements skipped
Print “A”
else
Print “B”
else
Print “C”
else
Print “D”
else
Print “F”

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 Example
if ( grade >= 90 ) // 90 and above
cout << "A";
else if ( grade >= 80 ) // 80-89
cout << "B";
else if ( grade >= 70 ) // 70-79
cout << "C";
else if ( grade >= 60 ) // 60-69
cout << "D";
else // less than 60
cout << "F";

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Importance of Curly Braces
 Print “We have a problem” if examGrade < 60
 Print “We have a real problem” if examGrade < 60 and
quizGrade < 10
 Print “Ok” if examGrade >= 60
int examGrade, quizGrade;
if (examGrade < 60)
cout << “We have a problem” << endl;
if (quizGrade < 10)
cout << “We have a real problem” << endl;
else
cout << “Ok”;

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Exam Grade Flowchartint examGrade, quizGrade;
if (examGrade < 60)
if (quizGrade < 10)
else
cout << “Ok”;
examGrade < 60
“We have a problem”
“We have a real problem”
true
quizGrade < 10
“Ok”
truefalse

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Writing Cases
 Print “We have a problem” if examGrade < 60
 Print “We have a real problem” if examGrade < 60 and
quizGrade < 10
 Print “Ok” if examGrade >= 60
examGrade < 60 quizGrade < 10 Action
Case 1 true false “We have a problem”
Case 2 true true “We have a problem” and
Case 3 false true/false “Ok”

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Putting it all together
examGrade < 60 quizGrade < 10 Action
Case 1 true false “We have a problem”
Case 2 true true “We have a problem” and
Case 3 false true/false “Ok”
if (examGrade < 60)
System.out.println(“We have a problem”);
if (quizGrade < 10)
System.out.printl(“We have a real problem”);
else
System.out.println(“Ok”);
if (examGrade < 60) {
if (quizGrade < 10)
}
else
cout << “Ok”;

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boolean Operators
 Combines multiple boolean expressions
 If person’s age greater than or equal to 13 and less
than 17, he can go to G and PG-13 rated movies,
but not R rated movies
 if (age >= 13 && age < 17)
cout << “You can go to G and PG-13”
<< “ rated movies, but not R” +
<< “ rated movies.”) << endl;
 boolean operators
 and - && (true if all conditions are true)
 or - || (true if one condition is true)
 not - ! (true if conditions is false)

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Expression Combinations
operand1 operand2 operand1 && operand2
true true true
true false false
false true false
false false false
if (age >= 13 && age < 17)
cout << “You can go to G and PG-13”
<< “ rated movies, but not R” +
<< “ rated movies.” << endl;
Let age = 12
Let age = 16
Let age = 17
The && (and) operator

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Expression Combinations
operand1 operand2 operand1 || operand2
true true true
true false true
false true true
false false false
The || (or) operator
operand !operand
true false
false true
The ! (not) operator
Example
if ( !( grade == sentinelValue ) )
cout << "The next grade is "
<< grade << endl;
Alternative:
if ( grade != sentinelValue )
cout << "The next grade is "
<< grade << endl;

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Playing Cards
 Exercise with playing cards
 Numbers represent the rank and suit of cards
// Codes for suits
const int SPADES = 0;
const int HEARTS = 1;
const int DIAMONDS = 2;
const int CLUBS = 3;
// Codes for nonnumeric ranks
const int ACE = 1;
const int JACK = 11;
const int QUEEN = 12;
const int KING = 13;

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Prints a Card Name
 Print “rank of suit”
 Consider just the rank part
if (rank == JACK)
cout << "Jack";
else if (rank == QUEEN)
cout << "Queen";
else if (rank == KING;
cout << "King";
else if (rank == ACE)
cout << "Ace";
else
cout << rank;
Notice:
comparing rank to
a number of
different value

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2.16switch Multiple-Selection
Structure
 switch
 Test variable for multiple values
 Series of case labels and optional default case
switch ( variable ) {
case value1: // taken if variable == value1
statements
break; // necessary to exit switch
case value2:
case value3: // taken if variable == value2 or == value3
statements
break;
default: // taken if variable matches no other cases
statements
break;
}

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2.16switch Multiple-Selection
Structure
true
false
.
.
.
case a case a action(s) break
case b case b action(s) break
false
false
case z case z action(s) break
true
true
default action(s)

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Converting if/else to a switch
if (rank == JACK)
cout << "Jack";
else if (rank == QUEEN)
cout << "Queen";
else if (rank == KING;
cout << "King";
else if (rank == ACE)
cout << "Ace";
else
cout << rank;
switch (rank)
{
case JACK:
cout << "Jack";
break;
case QUEEN:
cout << "Queen";
break;
case KING:
cout << "King";
break;
case ACE:
cout << "Ace";
break;
default:
cout << rank;
}

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