Control statements affect the normal sequence of execution in a program unit. The following control statements are described in this chapter:
CALL, “CALL”. This references a subroutine program in a calling program unit.
CONTINUE, “CONTINUE”. This has no operational function; usually serves as the terminal statement of a DO loop.
DO, “DO”. This specifies a controlled loop, called a DO loop, and establishes the control variable, indexing parameters, and range of the loop.
DO WHILE, “DO WHILE”. This specifies a DO loop based on a test for true of a logical expression.
ELSE, “ELSE”. This is used in conjunction with the block IF or ELSE IF statements.
ELSE IF, “ELSE IF”. This is used optionally with the block IF statement.
END, “END”. This indicates the end of a program unit.
END DO, “END DO”. This defines the end of an indexed DO loop or a DO WHILE loop.
END IF, “END IF”. This has no operational function; serves as a point of reference like a CONTINUE statement in a DO loop.
GO TO (Unconditional), “GO TO (Unconditional)”. This transfers program control to the statement identified by the statement label.
GO TO (Computed), “GO TO (Computed)”. This transfers control to one of several statements specified, depending on the value of an integer expression.
GO TO (Assigned), “GO TO (Assigned)”. This is used in conjunction with an ASSIGN statement to transfer control to the statement whose label was last assigned to a variable by an assign statement.
IF (Arithmetic), “IF (Arithmetic)”. This allows conditional branching.
IF (Branch logical), “IF (Branch Logical)”. This allows conditional statement execution.
IF (Test Conditional), “IF (Test Conditional)”. This allows conditional execution of blocks of code. The block IF can contain ELSE IF statements for further conditional execution control. The block IF ends with the END IF.
PAUSE, “PAUSE”. This suspends an executing program.
RETURN, “RETURN”. This returns control to the referencing program unit. It can appear only in a function or subroutine program.
STOP, “STOP”. This terminates an executing program.
The CALL statement references a subroutine subprogram in a calling program unit.
CALL sub[( [a[,a]...] )] |
where sub is the symbolic name of the subroutine and a is an actual argument, an expression, array name, array elements, record elements, record arrays, record array elements, Hollerith constants, or an alternate return specifier of the form *s, where s is a statement label, or &s, where s is a statement label.
A CALL statement evaluates the actual arguments, association of the actual arguments with the corresponding dummy arguments, and execution of the statements in the subroutine. Return of control from the referenced subroutine completes the execution of the CALL statement.
The actual arguments a form an argument list and must agree in order, number, and type with the corresponding dummy arguments in the referenced subroutine.
A subroutine that has been defined without an argument can be referenced by a CALL statement of the following forms:
CALL sub CALL sub()
If a dummy procedure name is specified as a dummy argument in the referenced subroutine, then the actual argument must be an external procedure name, a dummy procedure name, or one of the allowed specific intrinsic names. An intrinsic name or an external procedure name used as an actual argument must appear in an INTRINSIC or EXTERNAL statement, respectively.
If an asterisk is specified as a dummy argument, an alternate return specifier must be supplied in the corresponding position in the argument list of the CALL statement.
If a Hollerith constant is used as an actual argument in a CALL statement, the corresponding dummy argument must not be a dummy array and must be of arithmetic or logical data type. This rule is an exception to the first rule above.
A subroutine can call itself directly or indirectly (recursion).
 | Note: Recursion is an extension to FORTRAN 77. Standard FORTRAN 77 does not allow a subroutine to reference itself. |
In the following example, the main routine calls PAGEREAD, passing the parameters LWORDCOUNT, PAGE, and NSWITCH. After execution of PAGEREAD, control returns to the main program, which stops.
program MakeIndex
character*50 page
dimension page (100)
nswitch = 0
111 lwordcount = inwords1*2
*
call pageread (lwordcount,page,nswitch)
stop
*
subroutine pageread (lwordcount,page,nswitch)
character*50 page
dimension page (100)
icount = 100
.
.
.
end
* |
The CONTINUE statement has no operational function. It usually serves as the terminal statement of a DO loop.
When a CONTINUE statement that closes a DO loop is reached, control transfer depends on the control variable in the DO loop. In this case, control will either go back to the start of the DO loop, or flow through to the statement following the CONTINUE statement. See “DO” for full information about control of DO loops.
In the following example, the DO loop is executed 100 times, and then the program branches to statement 50 (not shown).
iwordcount = 100
do 25, i= 1,lwordcount
read (2, 20, end=45) word
20 format (A50)
25 continue
*
goto 50 |
The DO statement specifies a controlled loop, called a DO loop, and establishes the control variable, indexing parameters, and range of the loop.
DO [s][,]i = e1, e2[, e3] |
The following arguments are used with this statement:
| s | a statement label of the last executable statement in the range of the DO loop. This statement is called the terminal statement of the DO loop. s can be omitted. If s is omitted, the loop must be terminated with an END DO statement. On completion of the loop, execution resumes with the first statement following the END DO statement. |
| i | a name of an integer, real, or double-precision variable, called the DO variable. |
| e1 | an integer, real, or double-precision expression that represents the initial value given to the DO variable. |
| e2 | an integer, real, or double-precision expression that represents the limit value for the DO variable. |
| e3 | an integer, real, or double-precision expression that represents the increment value for the DO variable. |
The range of a DO loop consists of all executable statements following the statement, up to and including the terminal statement of the DO loop. In a DO loop, the executable statements that appear in the DO loop range are executed a number of times as determined by the control parameters specified in the DO statement.
The execution of a DO loop involves the following steps:
Activating the DO loop. The DO loop is activated when the DO statement is executed. The initial parameter m1, the terminal parameter m2, and the incremental parameter m3 are established by evaluating the expressions e1, e2, and e3, respectively. The expressions are converted to the type of the DO variable when the data types are not the same. The DO variable becomes defined with the value of the initial parameter m1. The increment m3 cannot have a value of zero and defaults to the value 1 if e3 is omitted.
Computing the iteration count.The iteration count is established from the following expression:
MAX( INT( (m2 - m1 + m3)/m3), 0)
The iteration count is zero in the following cases:
m1 > m2 and m3 > 0 m1 << m2 and m3 = 0
If the initial value (m1) of the DO exceeds the limit value (m2), as in
DO 10 I = 2,1
the DO loop will not be executed unless the -onetrip compiler option is in effect. This option causes the body of a loop thus initialized to be executed once.
Ordinarily, the compiler-generated code skips any DO loops whose upper or lower limit has already been reached. This action conforms with Fortran standards.
To make FORTRAN 77 compatible with Fortran 66, the compiler allows you to generate code that performs a loop at least once, regardless of whether the upper or lower limit has already been reached. This is accomplished by specifying the -onetrip option. This option is included for older programs written under the assumption that all loops would be performed at least once.
Loop control processing. This step determines if further execution of the range of the DO loop is required. Loop processing begins by testing the iteration count. If the iteration count is positive, the first statement in the range of the DO loop is executed. Normal execution proceeds until the terminal statement is processed. This constitutes one iteration of the loop. Incrementing is then required, unless execution of the terminal statement results in a transfer of control.
If the iteration count is zero, the DO loop becomes inactive. Execution continues with the first executable statement following the terminal statement of the DO loop. If several DO loops share the same terminal statement, incremental processing is continued for the immediately containing DO loop.
Incremental processing. The value of the DO variable is incremented by the value of the incremental parameter m3. The iteration count is then decreased by one, and execution continues with loop control processing as described above.
A DO loop is either active or inactive. A DO loop is initially activated when its DO statement is executed. Once active, a DO loop becomes inactive when one of the following occurs:
The iteration count is zero.
A RETURN statement within the DO loop range is executed.
Control is transferred to a statement outside the range of the DO loop but in the same program unit as the DO loop.
A STOP statement is executed or the program is abnormally terminated.
Reference to a subprogram from within the range of the DO loop does not make the DO loop inactive except when control is returned to a statement outside the range of the DO loop.
When a DO loop becomes inactive, the DO variable of the DO loop retains its last defined value.
You can nest DO loops but do not overlap them.
If a DO statement appears within an IF block, ELSE IF block, or ELSE block, the range of the DO loop must be contained within that block.
If a block IF statement appears within the range of a DO loop, the corresponding END IF statement must appear within the range of the DO loop.
The same statement can serve as the terminal statement in two or more nested DO loops.
Do not use the following statements for the statement labeled s in the DO loop:
Unconditional GO TO END IF
Assigned GO TO RETURN
Arithmetic IF STOP
Block IF END
ELSE IF Another DO statement
ELSE If the statement labeled s is a logical IF statement, it can contain any executable statement in its statement body, except the following:
DO statement END IF
BlockIF END
ELSE IF Another logical IF statement
Except by the incremental process covered above, the DO variable must not be redefined during the execution of the range of the DO loop.
A program must not transfer control into the range of a DO loop from outside the DO loop. When this happens, the result is indeterminate.
When the DO variable is a floating-point variable, especially if the loop increment value e3 cannot be represented exactly in floating-point form, the number of times the loop executes could be off by one due to floating-point arithmetic error.
DO 10, i = 1, 10
D
D
D
10 CONTINUE
E |
In the above example, the statements noted with a D following the DO statement are executed sequentially ten times, then execution resumes at the statement E following CONTINUE.
The DO WHILE statement specifies a controlled loop, called a DO loop, based on a test for true of a logical expression.
DO [s[,]] WHILE (e) |
where s is a statement label of the last executable statement in the range of the DO loop (his statement is called the terminal statement of the DO loop) and e is a logical expression.
If s is omitted, the loop must be terminated with an END DO statement.
The DO WHILE statement tests the specified expression prior to each iteration (including the first iteration) of the statements within the loop. When the logical expression e is found to be true, the body of the loop is executed. If the expression is false, control is transferred to the statement following the loop.
If the label s does not exist, the DO WHILE loop must be terminated with an END DO statement.
Use the ELSE statement in conjunction with the block IF or ELSE IF statements.
An ELSE block is the code that is executed when an ELSE statement is reached. An ELSE block begins after the ELSE statement and ends before the END IF statement at the same IF level as the ELSE statement. (For details about the term IF level, refer to “IF (Test Conditional)”.) As well as containing simple, executable statements, an ELSE block can be empty (contain no statements) or can contain embedded block IF statements. Do not confuse the ELSE block with the ELSE statement.
An ELSE statement is executed when the logical expressions in the corresponding block IF and ELSE IF statements evaluate to false. An ELSE statement does not evaluate a logical expression; the ELSE block is always executed if the ELSE statement is reached. After the last statement in the ELSE block is executed (and provided it does not transfer control), control flows to the END IF statement that closes that whole IF level.
Do not specify ELSE IF or ELSE statements inside an ELSE block at the same IF level.
The IF level of the ELSE statement must be greater than zero; that is, there must be a preceding corresponding block IF statement.
Enter an ELSE block only by executing the ELSE statement. Do not transfer control into the ELSE block from the outside.
If an ELSE statement has a statement label, the label cannot be referenced by any statement.
The following example shows an ELSE block.
IF (R) THEN
A = 0
ELSE IF (Q) THEN
A = 1
ELSE
A = -1
END IF |
The ELSE IF statement is used optionally with the IF block statement.
The following terms are used when defining the ELSE IF statement: ELSE IF block, defined below, and IF level, defined in “Method of Operation”.
An ELSE IF block is the code that is executed when the logical expression of an ELSE IF statement is true. An ELSE IF block begins after the ELSE IF statement and ends before the next ELSE IF, ELSE, or END IF statement at the same IF level as the ELSE IF statement. As well as containing simple, executable statements, an ELSE IF block can be empty (contain no statements) or can contain embedded block IF statements. Do not confuse the ELSE IF block with the ELSE IF statement.
When an ELSE IF statement is reached, the logical expressione is evaluated. If e is true, execution continues with the first statement in the ELSE IF block. If the ELSE IF block is empty, control is passed to the next END IF statement that has the same IF level as the ELSE IF statement. If e is false, program control is transferred to the next ELSE IF, ELSE, or END IF statement that has the same IF level as the ELSE IF statement.
After the last statement of the ELSE IF block is executed (and provided it does not transfer control), control is automatically transferred to the next END IF statement at the same IF level as the ELSE IF statement.
The IF level of the ELSE IF statement must be greater than zero (there must be a preceding corresponding block IF statement).
Do not transfer control into an ELSE IF block from outside the ELSE IF block.
No statement can reference the statement label (if any) of an ELSE IF statement. The only way to reach an ELSE IF statement is through its IF block statement.
The END statement designates the end of a program unit.
An END statement in a main program has the same effect as a STOP statement: it terminates an executing program.
An END statement in a function or subroutine subprogram has the effect of a RETURN statement: it returns control to the referencing program unit.
The END IF statement has no operational function. It serves as a point of reference like a CONTINUE statement in a DO loop.
Every block IF statement requires an END IF statement to close that IF level. (IF level is described in “Method of Operation”).
The IF level of an END IF statement must be greater than zero (there must be a preceding corresponding IF block statement).
The unconditional GO TO statement transfers program control to the statement identified by the statement label.
The computed GO TO statement transfers control to one of several statements specified, depending on the value of an integer expression.
GO TO (s[,s]...)[,]i |
where s is a statement number of an executable statement appearing in the same program unit as the computed GO TO and i is an integer.
A noninteger expression can also be used for i. Non-integer expressions are converted to integers (fractional portions are discarded) before being used to index the list of statement labels.
A computed GO TO statement evaluates the integer expression and then transfers program control to the specified statement. In a computed GO TO statement with the following form
GO TO (s1, s2, ... ,sn),i |
if i<1 or i>n, the program control continues with the next statement following the computed GO TO statement; otherwise, program control is passed to the statement labeled si. Thus, if the value of the integer expression is 1, control of the program is transferred to the statement numbered s1 in the list; if the value of the expression is 2, control is passed to the statement numbered s2 in the list, and so on.
The same statement label can appear more than once in the same computed GO TO statement.
Example 6-6. GO TO (computed) example
In the following example, the fifth list item is chosen because KVAL + 1 = 5. Program control is transferred to the statement labeled 350.
KVAL = 4 GO TO(100,200,300,300,350,9000)KVAL + 1 |
Use the symbolic GO TO statement in conjunction with an ASSIGN statement to transfer control to the statement whose label was last assigned to a variable by an ASSIGN statement.
GO TO i[[,](s[,s]...)] |
where i is an integer variable name and s is a statement label of an executable statement appearing in the same program unit as the assigned GO TO statement.
The variable i is defined with a statement label using the ASSIGN statement in the same program unit as the assigned GO TO statement. When an assigned GO TO is executed, control is passed to the statement identified by that statement label. Normal execution then proceeds from that point.
The same statement label can appear more than once in the same assigned GO TO statement.
If a list in parentheses is present, the statement label assigned to i must be one of those in the list.
Example 6-7. GO TO (assigned) example
GO TO KJUMP,(100,500,72530) |
The value of KJUMP must be one of the statement label values: 100, 500, or 72530.
The arithmetic IF statement allows conditional branching.
IF (e) s1, s2, s3 |
where e is an arithmetic expression of type integer, real, or double-precision (but not complex) and s1, s2, s3 are numbers of executable statements in the same program unit as the arithmetic IF statement.
In the execution of an arithmetic IF statement, the value of the arithmetic expression e is evaluated. Control is then transferred to the statement numbered s1, s2, or s3 if the value of the expression is less than zero, equal to zero, or greater than zero, respectively. Normal program execution proceeds from that point.
You can use the same statement number more than once in an arithmetic IF statement.
Example 6-8. IF (arithmetic) example
Consider the following statement:
IF (A + B*(.5))500,1000,1500 |
If the expression A + B*(.5) is
less than zero, control jumps to statement 500
equal to zero, control jumps to statement 1000
greater than zero, control jumps to statement 1500
The branch logical IF statement allows conditional statement execution.
IF (e) st |
where e is a logical expression and st is any executable statement except DO, block IF, ELSE IF, ELSE, END IF, END, or another logical IF statement.
A logical IF statement causes a Boolean evaluation of the logical expression. If the value of the logical expression is true, statement st is executed. If the value of the expression is false, execution continues with the next sequential statement following the logical IF statement.
Note that a function reference in the expression is allowed but might affect entities in the statement st.
Example 6-9. IF (branch logical) example
The following examples show branch logical IF statements.
IF(A .LE. B) A = 0.0 IF (M .LT. TOC) GOTO 1000 IF (J) CALL OUTSIDE(B,Z,F) |
The test conditional IF statement allows the conditional execution of blocks of code. The block IF can contain ELSE and ELSE IF statements for further conditional execution control. The block IF ends with the END IF statement.
An IF block is the code that is executed when the logical expression of a block IF statement evaluates to true. An IF block begins after the block IF statement and ends before the ELSE IF, ELSE, or END IF statement that corresponds to the block IF statement. As well as containing simple, executable statements, an IF block can be empty (contain no statements) or can contain embedded block IF statements. Do not confuse the term IF block with block IF.
Block IF statements and ELSE IF statements can be embedded, which can make figuring out which statements are in which conditional blocks confusing. The IF level of a statement determines which statements belong to which IF-THEN-ELSE block. Fortunately, the IF level of a statement can be found systematically. The IF level of a statement is the following:
(n1 - n2) |
where (starting the count at the beginning of the program unit): n1 is the number of block IF statements up to and including s, and n2 is the number of END IF statements up to but not including s.
The IF level of every block IF, ELSE IF, ELSE, and END IF statement must be positive because those statements must be part of a block IF statement. The IF level of the END statement of the program unit must be zero because all block IF statements must be properly closed. The IF level of all other statements must either be zero (if they are outside all IF blocks) or positive (if they are inside an I -block).
When a block IF statement is reached, the logical expression e is evaluated. If e is true, execution continues with the first statement in the IF block. If the IF block is empty, control is passed to the next END IF statement that has the same IF level as the block IF statement. If e is false, program control is transferred to the next ELSE IF, ELSE, or END IF statement that has the same IF level as the block IF statement.
After the last statement of the IF block is executed (and provided it does not transfer control), control is automatically transferred to the next END IF statement at the same IF level as the block IF statement.
The PAUSE statement suspends an executing program.
A PAUSE statement without an n specification suspends execution of a program and issues the following message:
PAUSE statement executed To resume execution, type go. Any other input will terminate job. |
A PAUSE statement with an n specification displays the specified character constant or digits and issues the pause message. For example, the following statement:
PAUSE "Console Check" |
results in the following message being displayed:
PAUSE Console Check statement executed To resume execution, type go. Any other input will terminate job. |
If execution is resumed, the execution proceeds as though a CONTINUE statement were in effect.
At the time of program suspension, the optional digit string or character constant becomes accessible to the system as program suspension status information.
RETURN statement returns control to the referencing program unit. It can appear only in a function or subroutine subprogram.The
The syntax in a function subprogram:
|
The syntax in a subroutine subprogram:
|
where e is an integer expression specifying an alternate return.
A noninteger expression can be used for e. Noninteger expressions are converted to integer, and the fractional portions discarded, before control is returned to the alternate return argument.
A RETURN statement terminates the reference of a function or subroutine and transfers control back to the currently referenced program unit. In a function subprogram, the value of the function then becomes available to the referencing unit. In a subroutine, return of control to the referencing program unit completes execution of the CALL statement.
A RETURN statement terminates the association between the dummy arguments of the external procedure and the current actual arguments.
In a subroutine subprogram, if e is not specified in a RETURN statement or if the value of e is less than or greater than the number of asterisks in the SUBROUTINE or ENTRY statement specifying the currently referenced name, then control returns to the CALL statement that initiated the subprogram. Otherwise, the value of e identifies the eth asterisk in the dummy argument list of the currently referenced name. Control returns to the statement identified by the alternate return specifier in the CALL statement that is associated with the eth asterisk in the dummy argument list.
The execution of a RETURN statement causes all entities in an external procedure to become undefined except for entities that are
specified in a SAVE statement
blank
specified in a named common
initialized in a DATA statement that has neither been redefined nor become undefined
The STOP statement terminates an executing program.