ICS 101: Computer Programming in FORTRAN

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ICS 101 Computer Programming in FORTRAN

• Lecture 2 Logical Operations

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Lecture Objectives

• Logical Operators
• Relational Operators
• Logical Expressions
• Writing a Complete Program

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Logical Operators

• Basic logical operators .AND. .OR. .NOT.
• LOGICAL P, Q

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Logical Operators

• Precedence of logical operators
• Example .FALSE. .OR. .NOT. .TRUE. .AND. .TRUE.

.FALSE.
.FALSE.
.FALSE.
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Relational Operators
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Relational Operators

• .EQ. and .NE. operators may not work properly
  with real values. This is because real values
  cannot be stored exactly.
• Example

REAL X, Y LOGICAL EQUALS PRINT, ENTER REAL
NUMBERS READ, X Y X (1.0 / X) EQUALS Y
.EQ. 1.0 PRINT, Y EQUALS 1? , EQUALS END
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Relational Operators

• Integers also affect the result of a relational
  operator.
• For integer K and L
• K / L L .EQ. K
• When k is divisible by L, the value of the
  expression is .TRUE. Otherwise, the value will be
  .FALSE.

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Logical Expressions

• A logical expression evaluates to either .TRUE.
  or .FALSE.
• It can have arithmetic, relational and logical
  operators.
• In a logical expression, evaluation is done in
  this order
• Arithmetic operators.
• Relational operators.
• Logical operators.

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Example

• X 3.0 Y 5.0 Z 10.0 FLAG .FALSE.

.NOT. FLAG .AND. X Y .GT. Z .OR. X Y .GT. Z
8.0
15.0
.TRUE.
.FALSE.
.TRUE.
.TRUE.
.TRUE.
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Writing Programs

• Read the problem carefully and identify the given
  information and what the requirements (the final
  results or output of the program) are.
• Develop a solution for each requirement. To do
  that, think about how this requirement can be
  achieved. For example, a formula to find out the
  output or the needed steps or strategy to get the
  final result.
• If there are more than one way to solve the
  problem choose the best one in terms of cost and
  time to do that solution.
• Implement the solution in a program and test it.

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Example

• The Problem
• Write a program that computes the average of
  three numbers entered by the user and then
  display their average.
• The solution
• The requirements are
• Three numbers are entered by the user.
• Compute the average.
• Display the average.
• Requirement solutions
• Use READ statement for entering the numbers.
• The average
• Use PRINT statement for displaying the average.

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Example

• Implementation
• All variables you need in the solution, declare
  it in the beginning of the program.
• Special statements should follow FORTRAN format.
• Any expressions needed should be converted into
  FORTRAN format and gives the expected result.
• Add comments to explain the purpose of the
  program.
• Add END statement at the end of the program.
• Testing
• Run the program and verify that it gives the
  desired result.
• If not, trace the running of the program and
  discover any error.

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Example
Computer
C THIS PROGRAM READS 3 REAL C NUMBERS AND
COMPUTES AND PRINTS C THE AVERAGE REAL NUM1,
NUM2, NUM3, AVER PRINT, ENTER 3 NUMBERS
READ, NUM1, NUM2, NUM3 AVER (NUM1 NUM2
NUM3) / 3 PRINT, THE AVERAGE IS , AVER
END
NUM1
NUM2
8.0
9.0
NUM3
AVER
10.0
9.0
Output
ENTER 3 NUMBERS
9.0 8.0 10.0
THE AVERAGE IS 9.0
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Exercise

• What is the output of this program
• Assume the input is 3 2
• The output is 1.0 3 2 27

INTEGER XLM, NUM1, NUM2 REAL PNM READ, NUM1,
NUM2 PNM NUM1 / NUM2 XLM 3 / PNM 3.0
NUM2 PRINT, PNM, NUM1, NUM2, XLM END
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Exercise

• Convert the following into FORTRAN expression
• Write logical expressions for the following
  conditions
• A and B have the same sign (both are negative or
  both are positive).
• X is less than 3 or Y is less than 3 but not
  both.
• P Q R.

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Exercise

• The length of the line segment joining two points
  (x1, y1) and (x2, y2) is given by
• And the midpoint of the segment has coordinates
• Write a program that reads two points coordinates
  and compute the length of the line segment and
  the midpoint of the segment. Display these
  results.

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Exercise

• Write a program that reads triples of real
  numbers and assigns the appropriate value of true
  or false to the following logical variables
• TRIANG true if the real numbers represent
  lengths of the sides of a triangle (the sum of
  any two sides is greater than the third) and
  false otherwise.
• EQUIL true if equilateral (the three sides are
  equal).
• ISOS true if it is TRIANG and isosceles (at
  least two sides are equal).
• Sample output of the program should look like

ENTER 3 LENGTHS 2 3 3 TRIANG T EQUIL
F ISOS T