## 7. Logical expressions

Logical expressions can only have the value `.TRUE.` or
`.FALSE.`. A logical expression can be formed by
comparing arithmetic expressions using the following
*relational operators*:
.LT. meaning < (less than)
.LE. <= (less than or equal to)
.GT. > (greater than)
.GE. >= (greater than or equal to)
.EQ. = (equal to)
.NE. /= (not equal to)

So you *cannot* use symbols like < or = for comparison in
Fortran 77, but you have to use the correct two-letter abbreviation
enclosed by dots! (Such symbols are allowed in Fortran 90, though.)
Logical expressions can be combined by the *logical operators*
`.AND. .OR. .NOT.` which have the obvious meaning.

### Logical variables and assignment

Truth values can be stored in *logical variables*.
The assignment is analagous to the arithmetic assignment.
Example:
logical a, b
a = .TRUE.
b = a .AND. 3 .LT. 5/2

The order of precedence is important, as the last example shows.
The rule is that arithmetic expressions are evaluated first,
then relational operators, and finally logical operators.
Hence b will be assigned `.FALSE.` in the example above.
Logical variables are seldom used in Fortran. But logical expressions
are frequently used in conditional statements like the `if`
statement.

### Exercises

- Exercise A
- Calculate the value of these logical expressions:
.TRUE. .AND. .FALSE. .OR. .TRUE.
2.LT.2 .OR. 5 .EQ. 11/2

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