1. Basic Scientific Programming
Selective Execution

2. 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.

3. 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.

4. 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

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

6. 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.

7. 
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

8. 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”

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

10. 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

11. 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

12. 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)

13. 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)

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

15. 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

16. 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

17. 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

18. 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

19. 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.