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As with operators, the programmer can extend the meaning of assignment to types not defined intrinsically and can redefine assignment for two objects of the same derived type. In addition, arrays and pointers each have special forms of assignment statements called masked array assignment and pointer assignment, respectively.These two assignment statement forms are described later in this chapter.
The MIPSpro Fortran Language Reference Manual, Volume 2, describes how any operator can be made generic by the programmer using a generic specifier on an interface block.
The rules for interpretation of an expression are provided by the interpretation of each operator in the expression.
Fortran allows the intrinsic operator symbols to be extended to cases in which the operands are not of the usual intrinsic types defined by the standard.
For example, the (a user-defined type) or for operands of type logical with the interpretation provided by a user-supplied function subprogram.
Thus, a compiler has a great deal of freedom to rearrange or optimize the computation, provided the rearranged expression has the same mathematical meaning.
Arrays and pointers as objects can appear in expressions and assignment statements.Intrinsic operators can be extended further by the programmer to operate on operands of types for which there are no intrinsic operations.Similarly, you can use defined unary and defined binary operators to operate on operands of types for which there are no previous definitions.These are user-defined operators, which are either unary or binary operators.They have the form of a sequence of letters surrounded by periods; are examples of possible user-defined operators.The following example shows an assignment statement: . Fortran has expressions of other types, such as logical, character, and derived type.Values of expressions of these other types can be assigned to variables of these other types.The formal (BNF) rules for forming expressions imply an order for combining operands with operators.These rules specify that expressions enclosed in parentheses are combined first and that, for example, the multiply operator , respectively.The formation rules for expressions imply that the defined unary operators have highest precedence of all operators, and defined binary operators have the lowest precedence of all operators.When they appear in a context where two or more of these operators of the same precedence are adjacent, the operands are combined with their operators in a left-to-right manner, as is the case for the familiar operates on operands of type integer as well as real and complex.