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Published on March 9, 2014

Author: fyjordan9

Source: slideshare.net

Basic Scientific Programming Selective Execution

Selective Execution Selective execution gives the programmer the ability to execute a part of the code if a condition is met. Else that part of code is not executed. The selection is based on a logical expression.

Logical Expressions    Logical expressions may be simple or compound. Simple logical expression expression1 logical operator expression2 Compound logical expression formed by combining simple logical expressions by using the following logical operators: NOT, AND, OR, EQV, NEQV.

Relational Operators Symbol Meaning Example < Is less than x<y > Is greater than x>y == Is equal to x == y <= Is less than or equal to x <= y >= Is greater than or equal to x >= y /= Is not equal to x /= y

 The result of a logical operation is either “true” or “false”. 5>4 9<6 3>3 true. false. false.

Characters & Relational Operators   Numeric codes are used to establish an ordering for the character set. ASCII & EBCDIC “Refer to App. A” “CAT” < “DOG” true C precedes D in the table. “June” < “July” false l precedes n in the table. “June” < “june” true J precedes j in the table. Two strings are compared as though blanks are appended to the shorter string. “cat” < “cattle” true a blank space precedes t in the table.

 Logical expressions formed by comparing real values with == are often evaluated as false, why? because real values are stored with some error. Real:: value value = 10 if ( (1/value) == 0.1) then do something end if In this example (1/value) might not result in 0.1 due to storage limitations, it might be 0.998 or 0.10001

Declaring logical variables There are two logical constants in Fortran .TRUE. .FALSE. Declaration: LOGICAL :: VARIABLE_LIST Reading logical variables is similar to reading other data types. Ex. Logical :: A,B,C Read *, A,B,C Input : .T. , F, .False “any form is valid input”

Logical Operators      .NOT. .AND. .OR. .EQV. .NEQV. A F T .NOT. A T F

AND/ OR AND A F B F F T T T OR A .AND. B F A F B F A .OR.B F F F T T F F T F T T T T T T

EQV/NEQV EQV A F B F F T T T NEQV A .EQV. B T A F B A .NEQV. B F F F F T T F F T F T T T T T F

Priorities  In logical expression containing several of these operators, the operations are performed in the following order .NOT.  .AND.  .OR.  .EQV. (or .NEQV.) Parenthesis may be used to indicate the subexpressions that must be evaluated first. Ex: A .AND. B .OR. .NOT. C A .AND. (B .OR. .NOT. C)

The general case The most general case is to have logical expressions containing arithmetic, relational and logical operators. The operations are performed in the following order: 1. Arithmetic operations 2. Relational operations 3. Logical operations Ex: (((b**2 – 4* a* c) >=5) .and. c+2 <6)

Flowchart Symbols Begin or End Input or Output Assignment Point of Selection Multiway Selection Flow Lines join

IF Construct IF (Logical Expression) then Statement-Sequence End IF Selection is made between either executing the sequence of statements or bypassing it. if Stat. seq

General form of IF Construct IF (Logical Expression) then Statement-Sequence1 ELSE Statement-Sequence2 End IF Selection is made between either executing sequence1 or sequence2. if Stat. seq1 Stat. seq2

Nested IF  The sequence of statements in an IF construct may itself contain other IF constructs. Ex: if(X<=0) then value = -X else if (X<1.0)then value = X**2 else value=1.0 end if end if

IF ELSE IF Construct IF(logical expression1) then statement sequence 1 ELSE IF (logical expression2) then statement sequence 2 ELSE IF (logical expression3) then statement sequence 3 . . . ELSE statement sequence n END IF

IF   ELSE IF Construct This structure is used in cases involving multiple alternatives. Only one of the statement sequences will be executed, the others will be skipped.

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