GOTO label
When an unconditional GOTO is encountered, control is
immediately transferred to the executable statement labelled with the
label. Whilst it is sometimes necessary to use
unconditional GOTO statements in a program, they should
be used sparingly lest unreadable 'spaghetti' code result.
A GOTO statement may be used to transfer control out
of a block IF or DO loop but not
in to them. The unconditional GOTO statement is
often used in conjunction with logical or block IF statements
to construct loops when a simple DO loop is not appropriate.
...
10 CONTINUE
WRITE(*,*)'Enter a positive value'
READ(*,*)A
IF (A .LE. 0.0) GOTO 10
...
The statement labelled 10 is a CONTINUE statement
which is often used at the beginning or end of a loop structure. This
program fragment executes the CONTINUE statement (which does
nothing) and then prints out the statement
Enter a positive value. The program then
reads in a value from the standard input device and stores it in the
variable A. At this point A is tested to see if
it is positive. If it is, then control passes to the next executable
statement, but if it isn't, then the GOTO 10 statement is
executed and the program returns to the CONTINUE statement.
The program then goes through the WRITE and READ
statements again and tests the new value of A. The program
will not break out of this loop until A is positive.
The indentation of the loop body makes the structure of the program that much more readable.
GOTO (label1, label2, ..., labeln), integer-expression
This form of GOTO statement is obscure and its use in modern
programs is strongly discouraged. It is equivalent to this
block IF statement:
IF (integer-expression .EQ. 1) THEN GOTO label1 ELSE IF (integer-expression .EQ. 2) THEN GOTO label2 ... ELSE IF (integer-expression .EQ. n) THEN GOTO labeln ENDIF
If the integer-expression is less than 1 or greater than
n, then control passes on and no GOTO is executed.
It is permissible for two or more of the labels to be the same.
As with the unconditional GOTO statement, unrestrained use
of the computed GOTO statement rapidly leads to unreadable
code.
STOP 'string'
A STOP statement stops the program and returns control to the
computer's operating system. When a STOP statement is
encountered, the optional string is printed out and the program
ends. This string must be a constant. Although ideally a program
stops only at the end of the program, it may be necessary, perhaps as an
error-trapping device, to have multiple STOP statements in a
program. In this instance, putting a meaningful message in the
string following the STOP can help with debugging.
It is not necessary for a program to contain a STOP statement.
The program will automatically terminate when the END statement
in the main program is encountered.
... IF (X .EQ. 0.0) STOP 'Emergency stop - Denominator is zero' ...
In this program fragment, the program stops if X is zero.
The string
Emergency stop - -Denominator is zero
is printed out to the standard output device as the program terminates.
Besides the DO, IF, GOTO and
STOP statements, sequential execution of statements may be
altered by the END= and ERR= keywords in
input/output statements.
Copyright © 1995-2007 by David Harper and L.M. Stockman