Assignment statements assign values to variables and array elements. Data statements and implied DO lists in data statements are used to initialize variables and array elements.
The following are the different types of Fortran assignment statements:
arithmetic, discussed in “Arithmetic Assignment Statements”
logical, discussed in “Logical Assignment Statements”
character, discussed in “Character Assignment”
aggregate, discussed in “Aggregate Assignment”
This chapter explains how to use each of these statements.
An arithmetic assignment statement assigns the value of an arithmetic expression to a variable or array element of type INTEGER, REAL, DOUBLE PRECISION, COMPLEX, or DOUBLE COMPLEX. The form of an arithmetic statement is the following:
v = e |
where v is the name of an INTEGER, REAL, DOUBLE PRECISION, COMPLEX, or DOUBLE COMPLEX type variable or array element and e is an arithmetic expression.
When an arithmetic assignment statement is executed, the expression e is evaluated and the value obtained replaces the value of the entity to the left of the equal sign.
The values v and e need not be of the same type; the value of the expression is converted to the type of the variable or array element specified. Table 5-1 lists the type conversion rules.
Table 5-1. Type Conversion Rules
Declaration | Function Equivalent |
|---|---|
INTEGER | INT(e) |
REAL | REAL(e) |
DOUBLE PRECISION | DBLE(e) |
COMPLEX | CMPLX(e) |
DOUBLE COMPLEX | DCMPLX(e) |
The following are examples of arithmetic assignment statements:
| I = 4 | Assign the value 4 toI. |
| J = 7 | Assign the value 7 to J. |
| A = I*J+1 | Assign the value 29 to A. |
Table 5-2 gives the detailed conversion rules for arithmetic assignment statements. The functions in the table's second column are intrinsic functions described in Chapter 10, “Statement Functions and Subprograms” and Appendix A, “Intrinsic Functions”.
Table 5-2. Conversion rules for assignment statements
Variable or Array Element (v) | INTEGER or LOGICAL Expression (e) | REAL Expression (e) | COMPLEX Expression (e) |
|---|---|---|---|
INTEGER or LOGICAL | Assign e to v | REAL: Truncate e to integer and assign to v REAL*8: Truncate e to integer and assign to v REAL*16: Truncate e to integer and assign to v | COMPLEX: Truncate real part of e to integer and assign to v COMPLEX*16: Truncate real part of e to integer and assign to v |
REAL | Append fraction (.0) to e and assign to v | REAL: Assign e to v REAL*8: Assign high-order portion of e to v; low-order portion of e is rounded REAL*16: Assign high-order part of e to v; low-order part is rounded | COMPLEX: Assign real part of e to v; imaginary part of e not used COMPLEX*16: Assign high-order part of real part of e to v; low-order portion of real part e is rounded |
REAL *8 | Append fraction (.0) to e and assign to v | REAL: Assign e to high-order portion of v; low-order portion of v is 0 REAL*8: Assign e to v REAL*16: Assign high-order part of e to v; low-order part is rounded | COMPLEX: Assign e to high-order portion of v; low-order portion of v is 0 COMPLEX*16: Assign real part of e to v |
REAL *16 | Append fraction (.0) to e and assign to v | REAL: Assign e to high-order portion of v; low-order portion of v is 0 REAL*8: Assign e to v REAL*16: Assign high-order part of e to v; low-order part is rounded | COMPLEX: Assign e to high-order portion of v; low-order portion of v is 0 COMPLEX*16: Assign real part of e to v |
COMPLEX | Append fraction to e and assign to real part of v; imaginary part of v is 0.0 | REAL: Assign e to real part of v; imaginary part of v is 0.0 REAL*8: Assign high-order portion of e to real part of v; low-order portion of e is rounded; imaginary part of v is 0.0 REAL*16: Assign high-order portion of e to real part of v; low-order part is rounded; imaginary part of v is 0.0 | COMPLEX: Assign e to v COMPLEX*16: High-order parts of real and imaginary components of e are assigned to v; low-order parts are rounded |
COMPLEX *16 | Append fraction to e and assign to v; imaginary part of v is 0.0 | REAL: Assign e to high-order portion of real part of v; imaginary part of v is 0.0 REAL*8: Assign e to real part of v; imaginary part is 0.0 REAL*16: Assign high-order portion of e to real part of v; low-order part is rounded; imaginary part of v is 0.0 | COMPLEX: Assign e to high-order parts of v; low-order parts of v are 0 COMPLEX*16: Assign e to v |
COMPLEX*32 | Append fraction to e and assign to v; imaginary part of v is 0.0 | REAL: Assign e to high-order portion of real part of v; imaginary part of v is 0.0 REAL*8: Assign e to real part of v; imaginary part is 0.0 REAL*16: Assign high-order portion of e to real part of v; low-order part is rounded; imaginary part of v is 0.0 | COMPLEX: Assign e to high-order parts of v; low-order parts of v are 0 COMPLEX*16: Assign e to v |
The logical assignment statement assigns the value of a logical expression to a logical variable or array element. It takes the form
v = e |
where v is the name of a logical variable or logical array element and e is a logical expression.
When a logical assignment statement is executed, the value of the logical expression e is evaluated and replaces the value of the logical entity to the left of the equal sign. The value of the logical expression is either true or false.
The character assignment statement assigns the value of a character expression to a character variable, array element, or substring. The form of a character assignment statement is
v = e |
where v is the name of a character variable, array element, or substring and e is a character expression.
During the execution of a character string assignment statement, the character expression is evaluated and the resultant value replaces the value of the character entity to the left of the equal sign. None of the character positions being defined in v can be referenced in the evaluation of the expression e.
The entity v and character expression e can have different lengths. If the length of v is greater than the length of e, then the value of e is extended on the right with blank characters to the length of v. If the length of e is greater than the length of v, then the value of e is truncated on the right to the length of v.
The following is an example of character assignment:
CHARACTER U*5, V*5, W*7 U = 'HELLO' V = 'THERE' W(6:7) = V(4:5) |
If an assignment is made to a character substring, only the specified character positions are defined. The definition status of character positions not specified by the substring remain unchanged.
An aggregate assignment statement assigns the value of each field of one aggregate to the corresponding field of another aggregate. The aggregates must be declared with the same structure. The form of an aggregate assignment statement is
v = e |
where v and e are aggregate references declared with the same structure.
See Chapter 2, “Constants and Data Structures”, for more information.
The ASSIGN statement assigns a statement label to an integer variable and is used in conjunction with an assigned GOTO statement or an I/O statement. The form of a statement label assignment statement is
ASSIGN s TO e |
where s is a statement label of an executable statement or a FORMAT statement that appears in the same program unit as the ASSIGN statement and e is an integer variable name.
A statement label assignment by the ASSIGN statement is the only way of defining a variable with a statement label value. A variable defined with a statement label value may be used only in an assigned GOTO statement or as a format identifier in an I/O statement. The variable thus defined must not be referenced in any other way until it has been reassigned with an arithmetic value.
An integer variable that has been assigned a statement label value can be redefined with the same statement label, a different statement label, or an arithmetic integer variable.
Examples using the ASSIGN statement are shown below:
Variables, arrays, array elements, and substrings can be initially defined using the DATA statement or an implied DO list in a DATA statement. The BLOCK DATA subprogram is a means of initializing variables and arrays in named common blocks and is discussed in Chapter 4, “Specification Statements”.
Entities not initially defined or associated with an initialized entity are undefined at the beginning of the execution of a program. Uninitialized entities must be defined before they can be referenced in the program.
The implied DO list initializes or assigns initial values to elements of an array.
(dlist, i = e1, e2 [,e3] ) |
The following arguments are available with this statement:
| dlist | a list of array element names and implied DO lists. |
| i | an integer variable name, referred to as the implied DO variable. It is used as a control variable for the iteration count. |
| e1 | an integer constant expression specifying an initial value. |
| e2 | an integer constant expression specifying a limit value. |
| e3 | aan integer constant expression specifying an increment value. |
e1, e2, and e3 are as defined in DO statements.
An iteration count and the values of the implied DO variable are established from e1, e2, and e3 exactly as for a DO loop, except that the iteration count must be positive.
When an implied DO list appears in a DATA statement, the dlist items are specified once for each iteration of the implied DO list with the appropriate substitution of values for any occurrence of the implied DO variable. The appearance of an implied DO variable in an implied DO has no effect on the definition status of that variable name elsewhere in the program unit. For an example of an implied DO list, see “DATA” in Chapter 4.
The range of an implied DO list is dlist.
The integer constant expressions used for e1, e2, and e3 can contain implied DO variables of other implied DO lists.
Any subscript expression in the list dlist must be an integer constant expression. The integer constant expression can contain implied DO variables of implied DO lists that have the subscript expression within their range.