Language Reference

How Expressions Are Evaluated

Precedence of Operators

An expression can contain more than one kind of operator. When it does, the expression is evaluated from left to right, according to the following precedence among operators:

1. Defined unary
2. Arithmetic
3. Character
4. Relational
5. Logical
6. Defined binary

For example, the logical expression:

L .OR. A + B .GE. C

where L is of type logical, and A, B, and C are of type real, is evaluated the same as the logical expression below:

L .OR. ((A + B) .GE. C)

An extended intrinsic operator maintains its precedence. That is, the operator does not have the precedence of a defined unary operator or a defined binary operator.

Summary of Interpretation Rules

Primaries that contain operators are combined in the following order:

1. Use of parentheses
2. Precedence of the operators
3. Right-to-left interpretation of exponentiations in a factor
4. Left-to-right interpretation of multiplications and divisions in a term
5. Left-to-right interpretation of additions and subtractions in an arithmetic expression
6. Left-to-right interpretation of concatenations in a character expression
7. Left-to-right interpretation of conjunctions in a logical term
8. Left-to-right interpretation of disjunctions in a logical disjunct
9. Left-to-right interpretation of logical equivalences in a logical expression

Evaluation of Expressions

Arithmetic, character, relational, and logical expressions are evaluated according to the following rules:

• A variable or function must be defined at the time it is used. You must define an integer operand with an integer value, not a statement label value. All referenced characters in a character data object or referenced array elements in an array or array section must be defined at the time the reference is made. All components of a structure must be defined when a structure is referenced. A pointer must be associated with a defined target.

  Execution of an array element reference, array section reference, and substring reference requires the evaluation of its subscript, section subscript and substring expressions. Evaluation of any array element subscript, section subscript, substring expression, or the bounds and stride of any array constructor implied-DO does not affect, nor is it affected by, the type of the containing expression. See Expressions Involving Arrays. You cannot use any constant integer operation or floating-point operation whose result is not mathematically defined in an executable program. If such expressions are nonconstant and are executed, they are detected at run time. (Examples are dividing by zero and raising a zero-valued primary to a zero-valued or negative-valued power.) As well, you cannot raise a negative-valued primary of type real to a real power.

• The invocation of a function in a statement must not affect, or be affected by, the evaluation of any other entity within the statement in which the function reference appears. When the value of an expression is true, invocation of a function reference in the expression of a logical IF statement or a WHERE statement can affect entities in the statement that is executed. If a function reference causes definition or undefinition of an actual argument of the function, that argument or any associated entities must not appear elsewhere in the same statement. For example, you cannot use the statements:

      A(I) = FUNC1(I)
      Y = FUNC2(X) + X
  

  if the reference to FUNC1 defines I or the reference to FUNC2 defines X.

  The data type of an expression in which a function reference appears does not affect, nor is it affected by, the evaluation of the actual arguments of the function.

• An argument to a statement function reference must not be altered by evaluating that reference.

Several compiler options affect the data type of the final result:

• When you use the -qintlog compiler option, you can mix integer and logical values in expressions and statements. The data type and kind type parameter of the result depends on the operands and the operator involved. In general:

  • For unary logical operators (.NOT.) and arithmetic unary operators (+,-):
    

    Data Type of OPERAND	Data Type of RESULT of Unary Operation
    BYTE	INTEGER(1)
    INTEGER(n)	INTEGER(n)
    LOGICAL(n)	LOGICAL(n)
    Typeless	Default integer

    where n represents the kind type parameter. In the case of INTEGER and LOGICAL data types, the length of the result is the same as the kind type parameter of the operand.

  • For binary logical operators (.AND., .OR., .XOR., .EQV., .NEQV.) and arithmetic binary operators (**, *, /, +, -), the following table summarizes what data type the result has:
    

    	second operand
    first operand	BYTE	INTEGER(y)	LOGICAL(y)	Typeless
    BYTE	INTEGER(1)	INTEGER(y)	LOGICAL(y)	INTEGER(1)
    INTEGER(x)	INTEGER(x)	INTEGER(z)	INTEGER(z)	INTEGER(x)
    LOGICAL(x)	LOGICAL(x)	INTEGER(z)	LOGICAL(z)	LOGICAL(x)
    Typeless	INTEGER(1)	INTEGER(y)	LOGICAL(y)	Default integer

    Note:

    z is the kind type parameter of the result such that z is equal to the greater of x and y. For example, a logical expression with a LOGICAL(4) operand and an INTEGER(2) operand has a result of INTEGER(4).

    For binary logical operators (.AND., .OR., .XOR., .EQV., .NEQV.), the result of a logical operation between an integer operand and a logical operand or between two integer operands will be integer. The kind type parameter of the result will be the same as the larger kind parameter of the two operands. If the operands have the same kind parameter, the result has the same kind parameter.

• When you use the -qlog4 compiler option and the default integer size is INTEGER(4), logical results of logical operations will have type LOGICAL(4), instead of LOGICAL(n) as specified in the table above. If you specify the -qlog4 option and the default integer size is not INTEGER(4), the results will be as specified in the table above.

• When you specify the -qctyplss compiler option, XL Fortran treats character constant expressions as Hollerith constants. If one or both operands are character constant expressions, the data type and the length of the result are the same as if the character constant expressions were Hollerith constants. See the "Typeless" rows in the previous tables for the data type and length of the result.

See "XL Fortran Compiler-Option Reference" in the User's Guide for information about compiler options.

Using BYTE Data Objects

Data objects of type BYTE can be used wherever a LOGICAL(1), CHARACTER(1), or INTEGER(1) data object can be used.

The data types of BYTE data objects are determined by the context in which you use them. XL Fortran does not convert them before use. For example, the type of a named constant is determined by use, not by the initial value assigned to it.

• When you use a BYTE data object as an operand of an arithmetic, logical, or relational binary operator, the data object assumes:
  • An INTEGER(1) data type if the other operand is arithmetic, BYTE, or a typeless constant
  • A LOGICAL(1) data type if the other operand is logical
  • A CHARACTER(1) data type if the other operand is character

• When you use a BYTE data object as an operand of the concatenation operator, the data object assumes a CHARACTER(1) data type.

• When you use a BYTE data object as an actual argument to a procedure with an explicit interface, the data object assumes the type of the corresponding dummy argument:
  • INTEGER(1) for an INTEGER(1) dummy argument
  • LOGICAL(1) for a LOGICAL(1) dummy argument
  • CHARACTER(1) for a CHARACTER(1) dummy argument

• When you use a BYTE data object as an actual argument passed by reference to an external subprogram with an implicit interface, the data object assumes a length of 1 byte and no data type.

• When you use a BYTE data object as an actual argument passed by value (%VAL), the data object assumes an INTEGER(1) data type.

• When you use a BYTE data object in a context that requires a specific data type, which is arithmetic, logical, or character, the data object assumes an INTEGER(1), LOGICAL(1), or CHARACTER(1) data type, respectively.

• A pointer of type BYTE cannot be associated with a target of type character, nor can a pointer of type character be associated with a target of type BYTE.

• When you use a BYTE data object in any other context, the data object assumes an INTEGER(1) data type.