BIL106E

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BIL106E

• Selective EXECUTION
• Control of EXECUTION

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Contents

• Prelude
• Logical Expressions Logical Variable
• IF, IF - ELSE IF constructs
• Comparing Numbers
• Comparing Character Strings
• Case construct
• Named constructs

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Prelude

• This chapter introduces the concept of comparison
  between two numbers or two character strings, and
  explains how such comparisons can be used to
  determine which of two, or more, alternatives
  sections of the code obeyed.
• In everyday life we frequently encounter a
  situation which involves several possible
  alternative courses of action, requiring us to
  choose one of them based on some decision-making
  criteria.
• The ability of a program to specify how these
  decisions are to be made is one of the most
  important aspects of programming

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Prelude

• EXAMPLE Q How do I get to Adana from
  Istanbul ?
•   A It depends how you want to
  travel.
• if you are in hurry then
• ? you should fly from Atatürk airport to Adana
  airport
• but if you are a romantic or like trains
  then
• ? you should take the Toros Express from Istanbul
  to Adana
• but if you have plenty of time then
• ? you can travel on one of the boats which ply
  along the shoreline.
• otherwise
• ? you can always go by road

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Prelude

• F provides a very similar construction for this
  decision-making problem as can be seen below

• in F syntax
• if (criterion_1) then
• action_1
• else if (criterion_2) then
• action_2
• else if (criterion_3) then
• action_3
• else
• action_4
• endif

• If criterion 1 then
• action 1
• but if criterion 2 then
• action 2
• but if criterion 3 then
• action 3
• otherwise
• action 4

What does this criterion mean and how to
implement it?
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• Prelude
• Logical Expressions Logical Variable
• IF, IF - ELSE IF constructs
• Comparing Numbers
• Comparing Character Strings
• Case construct
• Named constructs

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Logical Expressions Logical Variable

• Logical expressions may be either simple or
  compound.
• Simple logical expressions are logical constants
  or variables or relational expressions of the
  form.
• expression1 relational-operator expression2
• your-choice "train"
• where both expression1 and expression2 are
  numeric or character (or logical) expressions,
  and the relational-operator may be any of the
  following

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Simple Logical Expressions

• Relational operators and expressions()
• Symbol Meaning
• altb or a.LT.b a is less than b
• agtb or a.GT.b a is greater than b
• ab or a.EQ.b a is equal to b
• altb or a.LE.b a is less than or equal to b
• agtb or a.GE.b a is greater than or equal to b
• a/b or a.NE.b a is not equal to b

• b2 gt 4 a c
• b2 - 4 a c gt 0
• Current-year 2003

() Relational expressions are simple form of
logical expressions
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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

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The logical operators .or. .and.

• L1 L2 L1.or.L2 L1.and.L2
• true true true true
• true false true false
• false true true false
• false false false false

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The logical operators .eqv. .neqv.

• L1 L2 L1.eqv.L2 L1.neqv.L2
• true true true false
• true false false true
• false true false true
• false false true false

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Logical operator priorities

• The operations are performed in the following
  order (priority rules)
•   1.- arithmetic operations and functions
• 2.- relational operations
• 3.- logical operarions in the order mentioned
  below
• Operator Priority
• .not. Highest
• .and.
• .or.
• .eqv. and .neqv. Lowest

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Examples

• Valid Expressions
• (altb) .or. (cgtd)
• (xlty) .and. (yltz)
• .not. (altb) .eqv. (xlty)
• altb .neqv. xlty
• Invalid expressionsI 1.or.2 (A.and.B) /
  0.0       x gt 0.0 .and. gt 1.0         0.0 lt x
  lt 1.0

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Logical Variable

• An object of logical type has the valuetrue or
  false, or (0 or 1), or (yes or no).
• in F syntax.TRUE..FALSE.
• logical variable declaration in F logical
  flag1, flag2
• Exampleflag1 .TRUE.if(flag1) then flag2
  .FALSE. ... if(flag1 .and.
  flag2)then print , "Both flags are TRUE" end
  ifend if

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• Prelude
• Logical Expressions Logical Variable
• IF, IF - ELSE IF constructs
• Comparing Numbers
• Comparing Character Strings
• Case construct
• Named constructs

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Comparing numbers ()

• Accuracy/round-off
• Number of significant digits for real numbers
• Do not test whether two numbers are equal
• Test whether their difference is acceptably small

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• Prelude
• Logical Expressions Logical Variable
• IF, IF - ELSE IF constructs
• Comparing Numbers
• Comparing Character Strings
• Case construct
• Named constructs

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Comparing character strings

• F language lays down 6 rules for collecting
  sequence of letters, digits and other characters
  based on ASCII standard.
• 26 upper-case letters can be collected in the
  following order ABCDEFGHIJKLMNOPRSTUVWXYZ
• 26 lower-case letters can be collected in the
  following order abcdefghijklmnoprstuvwxyz 
• the 10 digits can be collected in the following
  order 0 1 2 3 4 5 6 7 8 9
• a space (or blank) is collected before both
  letters and digits
• digits are all collected before the letter A.
• upper-case letters are collacted before any
  lower-case letters.

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ASCII character codes

• ASCII uses codes in the range 0 through 255 (i.e.
  A65, B66,Z90).
• (See complete table of ASCII characters in
  Appendix D of Elliss book)

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httpwww.asciitable.com
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ASCII charecter codes 32-126
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Comparing character strings

• When 2 character operands are being compared
  there are 3 distinct stages in the process
•  1.- If two operands are not the same length,
  the shorter one is treated as though it were
  extended on the right with blanks (spaces) until
  it is the same length as the longer one.
•  2.- The two operands are compared character by
  character, starting with the left-most character,
  until either a difference is found or the end of
  the operand is reached.
•  3.- If a difference is found, then the
  relationship between these two different
  characters defines the relationship between the
  two operands, with the character which comes
  earlier in collating sequence being deemed to be
  the lesser of the two. If no difference is found,
  then the strings are considered to be equal.

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Examples

• "A" lt "F" is a "true" logical expression
• "m" gt "b" is a "true" logical expression
• Comparisons of 2 strings is done character by
  character, considering the numeric codes. If the
  first characters of the strings are the same, the
  second characters are compared, if these are the
  same, then the third characters are compared,
  etc.
• "cat" lt "dog" ! is a "true" logical
  expression.
•  "cat" gt "car" ! is a "true" logical
  expression.
•  "June" gt "July" ! is a "true"
  logical expression.
• Note Comparision is always proceeded regarding
  to the ASCII code of characters

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Examples

• Two strings with different lengths are compared
  so that blanks are appended to the shorter
  string.
•  "cat" lt "cattle"
• ! is a "true" logical expression, because
  blank characters (b) preceds all letters
  ("catbbb" lt "cattle") 
• "Adam" gt "Eve"
• ! is false, because A comes before E, thus, less
  than E
•  "Adam" lt "Adamant" ! "Adambbb" lt "Adamant"
• ! is true, because "Adam" has been extended
  using 3 blanks a blank always comes before a
  letter
•  "120" lt "1201" ! "120b" lt "1201"
• ! is true because a blank comes before a digit
•  "ADAM" lt "Adam"
• ! is true because the second digit "D" lt "d"
  (i.e. upper-case letters come before lower-case
  letters.

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• Prelude
• Logical Expressions Logical Variable
• IF, IF - ELSE IF constructs
• Comparing Numbers
• Comparing Character Strings
• Case construct
• Named constructs

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IF Constructs

• In the simplest selection structure, a sequence
  of statements (called a block of
  statements) is executed or bypassed depending on
  whether a given logical expression is true or
  false.
• If the logical expression is true, then the
  specified sequence of statements is executed
  otherwise it is bypassed.
• In either case, execution continues with the
  statement in the program following the end if
  statement.
•  

if ( x gt 0.0 ) then y x x z sqrt
(x) end if
if (logical_expression) then statement
sequence end if
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Logical IF Statement

• Fortran also provides a simpler IF construct
  called Logical IF statement
• if ( logical-expression) statementif (1.5 lt X
  .AND. Xlt 2.5) print , X

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General form of the IF construct

• if (logical expression 1) then
• block of F statements-1
• else if (logical expression 2) then
• block of F statements-2
• else
• block of F statements-3
• end if
• if 1.LE is true then 1. SB is executed, if 1. LE
  is false and if 2.LE is true then 2. SB is
  executed if neither of them is true then last SB
  is executed...

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Example Code

• Example 5.5, p126Write a program to read the
  coefficients of a quadratic equation and print
  its roots.

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Code-1 Ellis, ex. 5.5, p 126

• program quadratic_by_block_if
• real a,b,c,d,sqrt_d,x1,x2
• print ,"Please type a,b and c"
• read ,a,b,c
• d b2 - 4.0ac
• if(dgt0.0) then
• sqrt_dsqrt(d)
• x1(-bsqrt_d)/(aa)
• x2(-b-sqrt_d)/(aa)
• print ,"Two roots",x1,x2
• else if (d0.0l) then
• x1-b/(aa)
• print ,"one root",x1
• else
• print ,"no real roots"
• end if
• end program quadratic_by_block_if

Direct comparision of two real number
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Code-2
Test of the diffrence between two real number

• program quadratic_by_block_if
• real, parameter SMALL1e-6
• real a,b,c,d,sqrt_d,x1,x2
• print ,"Please type a,b and c"
• read ,a,b,c
• db2-4.0ac
• if(dgtSMALL) then
• sqrt_dsqrt(d)
• x1(-bsqrt_d)/(aa)
• x2(-b-sqrt_d)/(aa)
• print ,"Two roots",x1,x2
• else if (dgt-SMALL) then
• x1-b/(aa)
• print ,"one root",x1
• else
• print ,"no real roots"
• end if
• end program quadratic_by_block_if

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Code 3

• !This program inputs a character
• !and test it is whether a UPPER or
• !LOWER letter or is a DIGIT
• program isValid
• character (len 1) ch
• print , "Input one character"
• read , ch
• if( "A" lt ch .and. ch lt "Z" )then
• print , ch , " is a UPPER char"
• else if("a" lt ch .and. ch lt "z") then
• print, ch, " is a LOWER char "
• else if("0" lt ch .and. ch lt "9") then
• print, ch, " is a DIGIT"
• else
• print ,ch , " is None of them"
• end if
• end program isValid

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• Prelude
• Logical Expressions Logical Variable
• IF, IF - ELSE IF constructs
• Comparing Numbers
• Comparing Character Strings
• Case construct
• Named constructs

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The CASE Construct

• A case construct uses an integer expression or a
  character expression to determine the required
  course of action.
• The structure of a case construct
• select case (case expression)
• case (case selector)
• block of statements
• case (case selector)
• block of statements
• ...
• case default
• block of statements
• end select
• The case selector can take one of four forms
• case_value low_value high_value low_valuehigh_v
  alue

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• Case expression
• either integer or character expression
• Case selector
• case_value case(10) case(10, 13, 18)
• case_expression case_value
• lower_limit case(0)
• low_limit lt case_expression
• higher_limit case(1)
• case_expression lt high_limit
• low_valuehigh_value
• lower_limit lt case_expression .and.
  case_expression lt higher_limit case(010)

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Example

• !classcode.f
• program classcode_prg
• integer ClassCode
• print , "Input Class Code"
• read , ClassCode
• select case (ClassCode)
• case (1)
• print , "Freshman"
• case (2)
• print , "Sophomore"
• case (3)
• print , "Junior"
• case (4)
• print , "Senior"
• case (5)
• print , "Graduate"
• case default
• print , "Illegal class code", ClassCode
• end select

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Code-1

• program quadratic_by_block_case
• real, parameter small1e-6
• real a,b,c,d,sqrt_d,x1,x2
• integer selector
• print ,"Please type a,b and c"
• read ,a,b,c
• db2-4.0ac
• selectord/small
• select case (selector)
• case (1)
• sqrt_dsqrt(d)
• x1(-bsqrt_d)/(aa)
• x2(-b-sqrt_d)/(aa)
• print ,"Two roots",x1,x2
• case (0)
• x1-b/(aa)
• print ,"one root",x1
• case(-1)
• print ,"no real roots"

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Code-2

• !days.f
• !Example Days of the month
• program D17
• implicit none
• integer Month, Year, NDays
• write (unit , fmt ) " Please enter the
  year (4 digits). "
• read (unit , fmt ) Year
• write (unit , fmt ) " Please enter the
  month number (1 to 12). "
• read (unit , fmt ) Month
• select case (Month)
• case (9, 4, 6, 11) ! Sep, Apr, Jun, Nov
• NDays 30
• case (2) ! February
• if (modulo( Year, 4 ) 0) then
• NDays 29 !Leap year
• else
• NDays 28
• end if
• case default ! Jan, Mar, May, Jul, Aug,
  Oct, Dec

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Code-3

• !Semesters.f
• ! Example Fall and spring semesters, by months.
• program D19
• implicit none
• integer M
• write (unit , fmt ) "Please enter a month
  number between 1 and 12"
• read (unit , fmt ) M
• select case (M)
• case (9 12, 1)
• write (unit , fmt ) " Month ", M, "
  is in the fall semester. "
• case (2 6)
• write (unit , fmt ) " Month ", M, "
  is in the spring semester. "
• case default
• write (unit , fmt ) " School is not
  in session during month ", M
• end select
• end program D19

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• Prelude
• Logical Expressions Logical Variable
• IF, IF - ELSE IF constructs
• Comparing Numbers
• Comparing Character Strings
• Case construct
• Named constructs

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Named Constructs

• if, if-else if, case constructs may be named by
  attaching a label at the beginning and end of the
  construct
• name if ( logical-expression) then...else
  name...end if name
• name select case ( selector)...case( case
  selector)...end select name
• For exampleupdate if (x gt largest)
  thenlargestxpositionNend if update

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Example

• !nested-if.f
• !verison2 Named if constructs
• !condition x
• !if x is greater than equal to 1 and y is equal
  to 2 then assign 3 to z.
• !if x is greater than equal to 1 and y is equal
  to 2 and
• !condition z
• ! z is equal to 3 or n is equal to 5 then
  assign 8 to m.
• !if x is greater than equal to 1 and y is equal
  to 2 and
• ! z is equal to 3 or n is equal to 5
  and
• !condition k
• ! k is in between 1 and 10 (is also equal to 1
  and 10) or m is equal to 5
• ! or m is less than 0 then
• ! assign 22 to p.
• program NestedIfNamed
• integer x,y,z,n,m,k,p

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Cont'

• print , "x y"
• read , x, y
• CndX if( x gt 1 .and. y 2 ) then
• z 3
• print , "z" , z
• print , "n"
• read , n
• CndZ if( z 3 .or. n 5 ) then
• m 8
• print , "m" , m
• print , "k"
• read , k
• CndK if( ( 1 lt k .and. k lt 10 ) .or. m 5
  .or. m lt 0 ) then
• p 22
• print , "p" , p
• end if CndK
• end if CndZ
• end if CndX
• end program NestedIfNamed

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Homework2

• Write a program to calculate the following
  piecewise continuous function by using if
  construct.
• Due Date 27/02/2003