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Logical & comparison expressions

Logical & comparison expressions

Topics

• Relational operators.

• Logical expressions.

• Character comparison.

Relational operators

Form

• Relational operators are used to compare two objects and are of the form:

• Where rel_operator may be one of the following operators:
  • < less than,

  • <= less than or equal to,

  • > greater than,

  • >= greater than or equal to,

  • == equal to,

  • /= not equal to.

• The result of a comparison is a default logical value, .TRUE. or .FALSE.

Example

INTEGER :: age, my_age, your_age(5)

CHARACTER(len=5) :: name

...

test = 5 < 6 !true

test = 5 <= 6 !true

test = 5 > 6 !false

test = 5 => 6 !false

test = 5 == 6 !false

test = 5 /= 6 !true

...

ntest = your_age > 34

test = your_age(1) /= my_age

test = name == `Smith'

test = (age*3) /= your_age(3)

The objects compared may be variables, constants, array elements (numeric or character) or expressions.

Logical expressions

Form

• Logical expressions may be combined using logical operators.

• Where logical may be one of the following operators:
  • .AND.

  • .OR.

  • .NOT.

  • .EQV.

  • .NEQV.

• The result of a logical expression is a default logical value, .TRUE. or .FALSE.

Example

...

part1 = 5*3 > 12 !true

part2 = 6*2 < 12 !false

...

test = part1 .AND. part2 !false

test = part1 .OR. part2 !true

test = .NOT. part 1 !false

test = .NOT. part2 !true

test = part1 .EQV. part2 !false

test = part1 .NEQV. part2 !true

...

Character comparison

• String comparison requires strings of the same length. When comparing strings of different lengths, the shorter is padded with blanks (right side).

• The comparison is from left to right.

• Comparison is character by character.

• The comparison ends either when a difference is found or the end of the string is reached.

The collating sequence

• Character comparison follows a collating sequence, typically:
  • 0 < 1 < 2 < ... < 9

  • A < B < ... < Z

  • a < b < ... < z

  • ANSII standard: blank < numerals < upper case < lower case.

• Example:

• The right string becomes `Alex '

• The first four characters are the same...

• character `i' is greater than blank, test=.TRUE.

Portability issues

• The collating sequence is machine dependent!

• Intrinsic functions for string comparison, based on the ASCII standard, are available:

LLE(str1, str2) !str1 <= str2

LGE(str1, str2) !str1 > str2

LGT(str1, str2) !str1 >= str2

These should be used where ever possible.