Fortran 77 Language Reference 
Expressions
This chapter discusses Fortran expressions and how they are evaluated.
Expressions, Operators, and Operands
An expression is a combination of one or more operands, zero or more operators, and zero or more pairs of parentheses.
There are three kinds of expressions:
 An arithmetic expression evaluates to a single arithmetic value.
 A character expression evaluates to a single value of type character.
 A logical or relational expression evaluates to a single logical value.
The operators indicate what action or operation to perform.
The operands indicate what items to apply the action to. An operand can be any of the following kinds of data items:
 Constant
 Variable
 Array element
 Function
 Substring
 Structured record field (if it evaluates to a scalar data item)
 An expression
Arithmetic Expressions
An arithmetic expression evaluates to a single arithmetic value, and its operands have the following types. indicates a nonstandard feature.
BYTE
COMPLEX
COMPLEX*32
(SPARC only)DOUBLE COMPLEX
DOUBLE PRECISION
INTEGER
LOGICAL
REAL
REAL*16
(SPARC only)The operators for an arithmetic expression are any of the following:
TABLE 31 Arithmetic Operators Operator Meaning **
*
/

+Exponentiation
Multiplication
Division
Subtraction or Unary Minus
Addition or Unary Plus
If
BYTE
orLOGICAL
operands are combined with arithmetic operators, they are interpreted as integer data.Each of these operators is a binary operator in an expression of the form:
where a and b are operands, and is any one of the
**
,*
,/
,
, or+
operators.The operators + and  are unary operators in an expression of the form:
where b is an operand, and is either of the

or + operators.Basic Arithmetic Expressions
Each arithmetic operator is shown in its basic expression in the following table:
TABLE 32 Arithmetic Expressions Expression Meaning a ** z
a / z
a * z
a  z
z
a + z
+zRaise a
to the powerz
Dividea
byz
Multiplya
byz
Subtractz
froma
Negatez
Addz
toa
Same asz
In the absence of parentheses, if there is more than one operator in an expression, then the operators are applied in the order of precedence. With one exception, if the operators are of equal precedence, they are applied left to right.
TABLE 33 Arithmetic Operator Precedence Operator Precedence **
* /
+ First
Second
Last
For the lefttoright rule, the one exception is shown by the following example:
:
f77
allows two successive operators.Example: Two successive operators:
The above expression is evaluated as follows:
In the above example, the compiler starts to evaluate the
**
, but it needs to know what power to raiseX
to; so it looks at the rest of the expression and must choose between
and*
. It first does the*
, then the
, then the**
.Mixed Mode
Except for BYTE operands, if both operands have the same type, then the resulting value has that type. (The result of an arithmetic operation with both operands type BYTE is default integer.) If operands have different types, then the weaker of two types is promoted to the stronger type, where the weaker type is the one with less precision or fewer storage units. The ranking is summarized in the following table:
Note –REAL*4
,INTEGER*8
, andLOGICAL*8
are of the same rank, but they can be the results of different pairs of operands. For example,INTEGER*8
results if you combineINTEGER*8
and any of the types between 15. Likewise,REAL*4
results if one of the operands isREAL*4
, and the other is any of the types between 15.LOGICAL*8
dictates only the 8byte size of the result.
Example of mixed mode: If
R
is real, andI
is integer, then the expression:has the type real, because first
I
is promoted to real, and then the multiplication is performed.Rules
Note these rules for the data type of an expression:
 If there is more than one operator in an expression, then the type of the last operation performed becomes the type of the final value of the expression.
 Integer operators apply to only integer operands.
 When an
INTEGER*8
operand is mixed withREAL*4
operands, the result isREAL*8
.
 There is one extension to this: a logical or byte operand in an arithmetic context is used as an integer.
 Real operators apply to only real operands, or to combinations of byte, logical, integer, and real operands. An integer operand mixed with a real operand is promoted to real; the fractional part of the new real number is zero. For example, if
R
is real, andI
is integer, thenR+I
is real. However,(2/3)*4.0
is0
. Double precision operators apply to only double precision operands, and any operand of lower precision is promoted to double precision. The new least significant bits of the new double precision number are set to zero. Promoting a real operand does not increase the accuracy of the operand.
 Complex operators apply to only complex operands. Any integer operands are promoted to real, and they are then used as the real part of a complex operand, with the imaginary part set to zero.
 Numeric operations are allowed on logical variables. You can use a logical value any place where the FORTRAN Standard requires a numeric value. The numeric can be
integer
,real
,complex
,double
precision
,double
complex
, orreal*16
(SPARC only). The compiler implicitly converts the logical to the appropriate numeric. If you use these features, your program may not be portable.
 Example: Some combinations of both
integer
andlogical
types:
Resultant Type
For integer operands with a logical operator, the operation is done bit by bit. The result is an integer.
If the operands are mixed integer and logical, then the logicals are converted to integers, and the result is an integer.
Arithmetic Assignment
The arithmetic assignment statement assigns a value to a variable, array element, or record field. The syntax is:
v =
ee Arithmetic expression, a character constant, or a logical expression v Numeric variable, array element, or record field
Assigning logicals to numerics is allowed, but nonstandard, and may not be portable. The resultant data type is, of course, the data type of v.
Execution of an arithmetic assignment statement causes the evaluation of the expression e, and conversion to the type of v (if types differ), and assignment of v with the resulting value typed according to the table below.
Character constants can be assigned to variables of type integer or real. Such a constant can be a Hollerith constant or a string in apostrophes or quotes. The characters are transferred to the variables without any conversion of data. This practice is nonstandard and may not be portable.
Note – Compiling with any of the optionsi2
,dbl
,r8
, orxtypemap
will have an effect on the assumed type of e. This is discussed in Chapter 2. See also the Fortran User's Guide for a description of these options.
Example: Arithmetic assignment:
Character Expressions
A character expression is an expression whose operands have the character type. It evaluates to a single value of type character, with a size of one or more characters. The only character operator is the concatenation operator,
//
.
Expression Meaning a //
zConcatenate a with z.
The result of concatenating two strings is a third string that contains the characters of the left operand followed immediately by the characters of the right operand. The value of a concatenation operation
a//z
is a character string whose value is the value ofa
concatenated on the right with the value ofz
, and whose length is the sum of the lengths ofa
andz
.The operands can be any of the following kinds of data items:
 Character constant
 Character variable
 Character array element
 Character function
 Substring
 Structured record field (if it evaluates to a scalar character data item)
Examples: Character expressions, assuming
C
,S
, andR.C
are characters:
'wxy''AB' // 'wxy'CC // SC(4:7)R.CNote the following (nonstandard) exceptions:
n Control charactersOne way to enter control characters is to hold down the Control key and press another key. Most control characters can be entered this way, but not ControlA, ControlB, ControlC, or ControlJ.
n Multiple byte charactersMultiple byte characters, such as Kanji, are allowed in comments and strings.
Character String Assignment
The form of the character string assignment is:
v =
ee Expression giving the value to be assigned v Variable, array element, substring, or character record field
The meaning of character assignment is to copy characters from the right to the left side.
Execution of a character assignment statement causes evaluation of the character expression and assignment of the resulting value to v.
 If e is longer than v, characters on the right are truncated.
 If e is shorter than v, blank characters are padded on the right.
Example: The following program below displays
joined
:
CHARACTER A*4, B*2, C*8A = 'join'B = 'ed'C = A // BPRINT *, CENDAlso, this program displays the
equal
string:
IF ( ('ab' // 'cd') .EQ. 'abcd' ) PRINT *, 'equal'ENDExample: Character assignment:
CHARACTER BELL*1, C2*2, C3*3, C5*5, C6*6REAL ZC2 = 'z'C3 = 'uvwxyz'C5 = 'vwxyz'C5(1:2) = 'AB'C6 = C5 // C2I = 'abcd'Z = 'wxyz'BELL = CHAR(7) Control Character (^G)
Variable Receiving Value Comment C2
'z '
A trailing blank C3
'uvw'
C5
'ABxyz'
C6
'ABxyzz'
The final 'z'
comes fromC2
I
'abcd'
Z
'wxyz'
BELL
07 hex
ControlG, a bell
Example 4: A Hollerith assignment:
CHARACTER S*4INTEGER I2*2, I4*4REAL RS = 4HwxyzI2 = 2HyzI4 = 4HwxyzR = 4HwxyzRules of Assignment
Here are the rules for character assignments:
 If the left side is longer than the right, it is padded with trailing blanks.
 If the left side is shorter than the right, trailing characters are discarded.
 The left and right sides of a character substring assignment cannot overlap. See the Substrings .
Logical Expressions
A logical expression is a sequence of one or more logical operands and logical operators. It evaluates to a single logical value. The operators can be any of the following.
The period delimiters are necessary.
Two logical operators cannot appear consecutively, unless the second one is the
.NOT.
operator.Logical operators are evaluated according to the following precedence:
TABLE 35 Logical Operator Precedence Operator Precedence .NOT.
.AND.
.OR.
.NEQV.,.XOR., .EQV.Highest
Lowest
If the logical operators are of equal precedence, they are evaluated left to right.
If the logical operators appear along with the various other operators in a logical expression, the precedence is as follows.
TABLE 36 Operator Precedence Operator Precedence Arithmetic
Character
Relational
LogicalHighest
Lowest
The following table shows the meanings of simple expressions:
Logical operations on operands of different sizes result in a logical value promoted to the size of the largest operand. For example
L2.AND.L4
gives aLOGICAL*4
result ifL2
isLOGICAL*2
andL4
isLOGICAL*4.
BYTE
variables are treated the same asLOGICAL*1
.Logical Assignment
This is the syntax for the assignment of the value of a logical expression to a logical variable:
v = e e A logical expression, an integer between 128 and 127, or a single character constant v A logical variable, array element, or record field
Execution of a logical assignment statement causes evaluation of the logical expression e and assignment of the resulting value to v. If e is a logical expression, rather than an integer between 128 and 127, or a single character constant, then e must have a value of either true or false.
Logical expressions of any size can be assigned to logical variables of any size.
Assigning numerics to logicals is allowed. (All nonzero values are treated as
.TRUE.
, and zero is.FALSE.
) This practice is nonstandard, however, and is not portable.Example: A logical assignment:
LOGICAL B1*1, B2*1LOGICAL L3, L4B2 = B1B1 = L3L4 = .TRUE.Relational Operator
A relational operator compares two arithmetic expressions, or two character expressions, and evaluates to a single logical value. The operators can be any of the following:
TABLE 38 Relational Operators Operator Meaning .LT.
.LE.
.EQ.
.NE.
.GT.
.GE.Less than
Less than or equal
Equal
Not equal
Greater than
Greater than or equal
The period delimiters are necessary.
All relational operators have equal precedence. Character and arithmetic operators have higher precedence than relational operators.
For a relational expression, first each of the two operands is evaluated, and then the two values are compared. If the specified relationship holds, then the value is true; otherwise, it is false.
Example: Relational operators:
NODE .GE. 0X .LT. YU*V .GT. UVM+N .GT. UV Mixed mode: integer M+N is promoted to realSTR1 .LT. STR2 STR1 and STR2 are character typeS .EQ. 'a' S is character typeFor character relational expressions:
 "Less than" means "precedes in the ASCII collating sequence."
 If one operand is shorter than the other, the shorter one is padded on the right with blanks to the length of the longer.
Constant Expressions
A constant expression is made up of explicit constants and parameters and the FORTRAN operators. Each operand is either itself another constant expression, a constant, a symbolic name of a constant, or one of the intrinsic functions called with constant arguments.
Examples: Constant expressions:
There are a few restrictions on constant expressions:
 Constant expressions are permitted wherever a constant is allowed, except they are not allowed in
DATA
or standardFORMAT
statements. Constant expressions are permitted in variable format expressions.
 Exponentiation to a floatingpoint power is not allowed; a warning is issued.
 Example: Exponentiation to a floatingpoint power is not allowed:
demo%cat ConstExpr.f
parameter (T=2.0*(3.0**2.5))write(*,*) tenddemo%f77 ConstExpr.f
ConstExpr.f:MAIN:"ConstExpr.f", line 1: Warning:parameter t set to a nonconstantdemo%a.out
31.1769demo%Record Assignment
The general form of record assignment is:
v =
ee A record or record field v A record or record field
Both e and v must have the same structure. That is, each must have the same number of fields, and corresponding fields must be of the same type and size.
Example: A record assignment and a recordfield assignment:
In the above example, the first assignment statement copies one whole record (all five fields) to another record; the second assignment statement copies a whole record into the first element of an array of records; the
WRITE
statement writes a whole record; and the last statement sets theID
of one record to 82.Evaluation of Expressions
The following restrictions apply to all arithmetic, character, relational, and logical expressions:
 If you reference any one of these items in an expression, variable, array element, character substring, record field, pointer, or function, then that item must be defined at the time the reference is executed.
 An integer operand must be defined with an integer value, and not with a statement label value by an
ASSIGN
statement. All the characters of a substring that are referenced must be defined at the time the reference is executed.
 The execution of a function reference must not alter the value of any other entity within the same statement.
 The execution of a function reference must not alter the value of any entity that affects the value of any other function reference in the same statement.
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