Chapter 3

Applications & Logical Constructs

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Application 1

Temperature Conversion: Write a program that will convert a Celsius temperature to the corresponding Fahrenheit temperature.

SOLUTION: Assume that the temperature converting formula is given by

F = (9/5) C + 32INPUT: A temperature in degrees CelsiusOUTPUT: A temperature in degrees Fahrenheit Programs T_conv_1.f and T_conv_2.f BASIC STEPS FOR ALGORITHM

STEP1 --> Enter CSTEP2 --> Calculate FSTEP3 --> Display F

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Application 2

Area of a Triangle: Write a program to read values for the three sides a, b, c of a triangle and then calculate its perimeter and its area. These should be displayed together with the values of a, b, c using appropriate labels. (for the area, you might use Hero’s formula for the area of a triangle:

where s is one-half the perimeter.)

SOLUTION: See the source code area_tri.f for the solution

))()(( csbsassarea

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Selective Execution

Three basic methods of control are used in designing algorithms and

programs:

1) Sequence: Each step are performed exactly once and steps are performed in a strictly sequential manner.

2) Selection: One of a number of alternative actions is selected and executed

3) Repetition: One or more steps are performed repeatedly.

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Logical Expressions

Logical expressions may be either simple or compound. Simple logicalexpressions are logical constants or variables or relational expressionsof the form.

expression1 relational-operator expression2

where both expression1 and expression2 are numeric or character (or

logical) expressions, and the relational-operator may be any of thefollowing:

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Simple Logical Expressions

Symbol Meaning< or .LT. Is less than

> or .GT. Is greater than

== or .EQ. Is equal to

<= or .LE. Is less than or equal to

>= or .GE. Is greater than or equal to

/= or .NE. Is not equal to

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Simple Logical Expressions

Examples: x < 6.7 number == 1999ASCII uses codes in the range 0 through 255 (i.e. A=65,

B=66, …Z=90).

(See complete table of ASCII characters in Appendix D of Ellis’s book)

”A” < ”G” is true logical expression ”cat” < ”dog” true ”June” < ”July” true

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Compound Logical Expressions

They are formed by combining logical expressions by using the logical

operators .NOT. (negation)

.AND. (conjunction).OR. (disjunction).EQV. (equivalence)

.NEQV. (nonequivalence)

all possible values are displayed in truth tables (next page)

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Compound Logical Expressions

p q p.AND.q p.OR.q p.EQV.q p.NEQV.q

.TRUE. .TRUE. .TRUE. .TRUE. .TRUE. .FALSE.

.TRUE. .FALSE. .FALSE. .TRUE. .FALSE. .TRUE.

.FALSE. .TRUE. .FALSE. .TRUE. .FALSE. .TRUE.

.FALSE. .FALSE. .FALSE. .FALSE. .TRUE. .FALSE.

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IF Constructs

A simple IF construct is as follows

if (logical-expression) then statement-sequence end if

EXAMPLE: if (X >= 0) then Y=X * X Z=sqrt(X) end if

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Logical IF Statement

Fortran also provides a simpler IF construct called Logical IF statement

if (logical-expression) statement

if (1.5 <= X .AND. X<= 2.5) print *, X

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General form of the IF construct

if (logical-expression) then statement-sequence1

else statement-sequence2

end if

If the logical expression is true, statement-sequence1 is executed and

statement-sequence2 is bypassed.

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Applications

(Problem #1) Solve the quadratic equation

using an IF construct.

SOLUTION: Read : The coefficients A, B, and COutput : The roots of the equation

Check the source code Quad_eq_1.f !

02 CBxAx

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Applications

(Problem #2) Rank two numbers Two real data values, X and Y are given. Write a

program that finds the ranking between two entered numbers.

Use If construct for the solution in your program.

Check the source code rank_two.f !

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IF-ELSE IF Constructs It is possible to use the IF construct to design selection

structures that contain more than two alternatives.

if (logical-expression1) then

statement-sequence1

else if (logical-expression2) then

statement-sequence2

else if (logical-expression3) then

statement-sequence3...

else statement-sequencen

end if

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IF-ELSE IF Constructs

When an if-else if construct is executed, the logical expressions are evaluated to determine the first expression that is true; the associated sequence of statements is executed, and execution then continues with the next statement following the next construct. If none of the logical expressions is true, the statement sequence associated with the else statement is executed, and execution then continues with the statement following the construct.

This if construct thus implements an n-way selection structure in which exactly one of statement-sequence1, statement-sequence2, statement-sequence3,…, statement-sequencen is executed.

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Named Constructs

if and if-else if constructs may be named by attaching a label at the

beginning and end of the construct

name: if (logical-expression) thenend if name

For example: update: if (x > largest) then largest=x position=N end if update

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Named Constructs

Naming prevents the possible mixing within the nested if’s. The name used in an if or if-else if construct may also be attached following the keyboard then in any else if part of the construct and following the keyboard else in an else part.

For example:

EmpType: if (EmployeeType == ”S”) then ! Salaried employee print *,”Enter employee’s annual salary” read *, Salary pay = salary / 52 else EmpType ! Hourly employee print *,”Enter hours worked and hourly rate:” read *, HoursWorked, HourlyRate Overtime: if (HoursWorked > 40.0) then

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Named Constructs

pay = 40.0 * HourlyRate & + Multiplier * HourlyRate*(HoursWorked-40.0) else Overtime pay=HoursWorked*HourlyRate end if Overtimeend if EmpType

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The CASE Construct

The case construct is not as general as the if-else if construct. But itis useful for implementing some selection structures. The form of acase construct is as follows:

select case (selector) case (label-list1)

statement-sequence1

case (label-list2)

statement-sequence2...

case (label-listn)

statement-sequencen

end select

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The CASE Construct

Where the selector is an integer, character, or logical expression; each of the (label-list1) is a list of one or more possible values of the selector, enclosed in parenthesis. The values in this list may have any of the forms

valuevalue1 : value2

value1 :

: value2

to denote a single value, the range of values from value1 through value2, the set of all values greater than or equal to value1, or the set of all values less than or equal to value2, respectively. When a case construct is executed, the selector is evaluated; if this value is in label-list1 , statement-sequence1 is executed, and execution continues with the statement following the end select statement.

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Named CASE Construct

If the value is not in any of the lists of values, the sequence of statements associated with default is executed.

Named case constructs

A name may be attached to a case construct:

name: select case (selector)...

end select name

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Named CASE Construct

For example:

Class: select case (ClassCode) case (1) print *, ”Freshman” case (2) print *, ”Sophomore” case (3) print *, ”Junior” case (4) print *, ”Senior” case (5) print *, ”Graduate” default print *, ”Illegal class code”,Class code end select Class

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Exercises & Reading (Ellis’s Book)

Study examples 5.1, 5.3, 5.4 and 5.5 on pages 112, 118, 124 and 126 respectively.

Read summary on page 131.

Study Chapter 5: Pages between 103 and 131.

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The LOGICAL data type

Two logical constants in Fortran are .TRUE. and .FALSE. A logical variable is declared using a logical type statement logical :: listwhere list is a list of variables being typed as logical. An

assignment statement of the form logical-variable = logical-expressioncan be used to assign a value to a logical variableFor example: logical :: RootExists, End_of_Data End_of_Data=.true. RootExists=Discriminant >= 0are valid assignment statements and assigns .TRUE. to

RootExists if Discriminant is nonnegative and assigns .FALSE. otherwise.

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The LOGICAL data type

Display of logical variables are as follows:

print *, A,B,C, .true., .false.

produces T F F F T Fas output.