|
CHAPTER 11
MISCELLANEOUS STATEMENTS
This chapter describes statements which were not included
in previous chapters. These statements are of secondary
importance; some are obsolete, and retained only to ensure
compatibility with earlier software; others are highly spe
cialized and likely to be needed only under very unusual
circumstances.
The statements are:
- ASSIGN
- Computed GO TO
- BACKSPACE
- BLOCK DATA
- EQUIVALENCE
- IMPLICIT
- PAUSE, and
- REWIND.
11.1 ASSIGN STATEMENT
ASSIGN is an executable statement that assigns an integer
constant to a special type of integer variable. The integer
constant must be a valid statement label. The integer vari
able can then be used as a transfer destination in an as
signed GO TO statement or a format specifier in a READ
or WRITE statement.
The ASSIGN statement takes the form:
ASSIGN label TO name
where label is a valid statement label and name is an inte
ger variable. Once an ASSIGN statement is executed,
name has the meaning of a statement label and is not a
regular integer variable. Therefore, operations cannot be
performed on name while it has the meaning of label.
The following are valid ASSIGN statements:
ASSIGN 10 TO NUMBER
ASSIGN 20 TO IFORMAT1
These examples assign the value 10 to integer NUM
BER and 20 to integer IFORMAT1. NUMBER is then be
used as a transfer destination in an assigned GO TO state
ment, and IFORMAT1 is used as a format specifier.
The following example program shows the use of the
ASSIGN statement. The program increments a real vari
able X by 3. every time a pass through the IF-GO TO loop
is completed.
Running this program yields:
FOR K= 1, X IS= 3.00
FOR K= 2, X IS= 6.00
FOR K= 3, X IS= 9.00
FOR K= 4, X IS= 12.00
FOR K= 5, X IS= 15.00
Example Program: Assign Statement and
Assigned GO TO
|
C234567890
PROGRAM ASSIGN_STATEMENT
ASSIGN 10 TO NUMBER
ASSIGN 100 TO IFORMAT1
10 K= K + 1
X= X + 3.
WRITE(6,IFORMAT1) 'FOR K= ',K,', X IS= ',X
IF(K.LT.5) THEN
GO TO NUMBER
ENDIF
100 FORMAT(1X,A,I3,A,F8.2)
END
|
11.2 COMPUTED GO TO STATEMENT
The computed GO TO is an executable statement that
transfers control to one of several statement labels in
cluded in its list, depending on the value of an integer ex
pression which follows immediately after the list.
The computed GO TO takes the form:
GO TO (lab1, lab2, ... , labn) integexp
where lab1, lab2, ..., labn are a list of n valid statement la
bels in the program unit, and integexp is an integer expres
sion that yields values from 1 to n. When the computed
GO TO is executed, control is transferred to the statement
label indicated by integexp. For instance, if integexp= 2,
control goes to lab2, which is the second listed label.
The following example illustrates the use of the com
puted GO TO statement.
GO TO (10,20,30,40,50) I
If I= 1 during execution, control is transferred to state
ment 10; if I= 2, control is transferred to statement 20; and
so on.
Typically, the GO TO and computed GO TO state
ments are disruptive, as shown in the example below.
Therefore, their extensive use is discouraged. Usually, the
same objectives can be accomplished in a more orderly
manner with a DO loop/block IF construct combination.
Example 1: Computed GO TO Statement
DATA X,I /0.,0/
5 I= I + 1
IF(I.GT.3) GO TO 40
GO TO(10,20,30) I
10 X= X + 2.5
...
GO TO 5
20 X= X + 4.7
...
GO TO 5
30 X= X + 8.3
...
GO TO 5
40 CONTINUE
This example initializes X = 0. and I = 0 with a DATA
statement. Then, it sets an IF-GO TO loop to vary I from
1 to 3, each time adding a different real number to X. In
this example, ... stands for a block of statements. No
tice that five GO TO's were used in this example.
Example 2: DO Loop and Block IF Construct
This example accomplishes the same task as Example
1, but using a more friendly DO loop and block IF construct.
DO I= 1,3
IF(I.EQ.1) THEN
X= X + 2.5
...
ELSEIF(I.EQ.2) THEN
X= X + 4.7
...
ELSEIF(I.EQ.3) THEN
X= X + 8.3
...
ENDIF
END DO
Notice the absence of GO TO statements in this exam
ple. It is good programming practice to minimize the use
of GO TO and computed GO TO statements to improve
readability.
11.3 BACKSPACE STATEMENT
The BACKSPACE is an executable statement that
moves back to the beginning of the preceding record in an
open input/output file, i.e., it repositions to the start of the
last record transferred, enabling a record to be used more
than once in an input/output operation. An example is:
BACKSPACE(UNIT=5,ERR=90)
where 90 is a statement label to which control is trans
ferred in the event of an error during input/output opera
tion. When no error specifier appears, the BACKSPACE
statement reduces to:
BACKSPACE 5
Example Program: Use of Backspace Statement
|
C234567890
PROGRAM BACKSPACE_EXAMPLE
OPEN(5,FILE='X.DAT',STATUS='UNKNOWN')
OPEN(7,FILE='X.OUT',STATUS='UNKNOWN')
DO J= 6,12
READ(5,100)X
X = X + J
WRITE(6,200) 'J= ',J,' X= ',X
BACKSPACE 5
END DO
100 FORMAT(F8.2)
200 FORMAT(1X,A,I2,3X,A,F8.2)
END
|
This example reads a real number X from an input file
named X.DAT. (Assume that X = 35.) Then, it adds to X
an integer varying from 6 to 12, a total of seven times,
each time backspacing and reading X again from the input
file. Finally, it writes the result to an output file named
X.OUT connected to unit 7. Note that X is being redefined
twice at every pass of the loop, once during the READ
statement and again during the arithmetic assignment
statement (X= X + J).
The input would look like this:
35.00
The output would look like this:
J= 6 X= 41.00
J= 7 X= 42.00
J= 8 X= 43.00
J= 9 X= 44.00
J= 10 X= 45.00
J= 11 X= 46.00
J= 12 X= 47.00
11.4 BLOCK DATA SUBPROGRAM
A BLOCK DATA subprogram is used specifically for the
purpose of initializing COMMON blocks with DATA
statements. It is a special type of subprogram that can
contain only specification statements. Any of the following
statements can appear in a BLOCK DATA subprogram:
- COMMON
- CHARACTER
- DOUBLE PRECISION
- DATA
- DIMENSION
- INTEGER
- LOGICAL
- PARAMETER
- REAL
As with any other program unit, the last statement of a
BLOCK DATA subprogram must be an END statement.
The following example uses a BLOCK DATA subpro
gram to initialize two integer arrays A and B with DATA
statements. Then it adds A and B and writes the result (C)
in the main program. Note the following features of this
program:
- Variables A, B, and C are explicitly declared as integers
in both main program and subprogram.
- The labeled COMMON appears in both main program
and subprogram.
- The BLOCK DATA subprogram is named BLOCK1.
·
- All elements of arrays A and B are being initialized, and
all elements of array C are calculated. Therefore, the
implied DO list in DATA and WRITE statements can
be omitted.
Example Program: Use of Block Data Subprogram
|
C234567890
C------BEGINNING
OF MAIN PROGRAM
PROGRAM BLOCK_DATA_EXAMPLE
INTEGER A,B,C
COMMON /LABEL/ A(20),B(20),C(20)
DO J= 1,20
C(J)= A(J) + B(J)
END
DO WRITE(6,100) C
100 FORMAT(1X,20I3)
END
C------END
OF MAIN PROGRAM
C------BEGINNING
OF BLOCK DATA PROGRAM
BLOCK DATA BLOCK1
INTEGER A,B,C
COMMON /LABEL/ A(20),B(20),C(20)
DATA A /1,3,5,7,9,2,4,6,8,0,1,3,5,7,9,2,4,6,8,0/
DATA B /1,2,3,4,5,6,7,8,9,0,1,2,3,4,5,6,7,8,9,0/
END
C------END
OF BLOCK DATA PROGRAM
|
Things to keep in mind:
- In reference to the ASSIGN statement, do not
perform operations with an assigned name
while it has the meaning of a label.
- Avoid the use of the computed GO TO
construct, since the combined DO loop and
Block IF construct accomplishes the same
objectives with more clarity and less effort.
- Use the BACKSPACE statement sparingly.
- The BLOCK DATA is a subprogram; there
fore, it must end with an END statement.
|
11.5 EQUIVALENCE STATEMENT
The EQUIVALENCE is a nonexecutable statement that
associates two or more variables in the same program unit
with the same storage location. It takes the form:
EQUIVALENCE
(var1,var2,var3)
in which var1, var2, and var3 are variables stored in the
same location in memory. Therefore, an assignment of
var1 causes the same assignment to var2 and var3.
Dummy arguments cannot be specified in EQUIVA
LENCE statements.
With the large storage space of current computers, this
statement has now become obsolete, and it is retained only
for compatibility with older software.
11.6 IMPLICIT STATEMENT
Recall the rules for integer and real type declaration stated
in Chapter 3:
- A variable starting with letters I, J, K, L, M, or N is
implicitly declared as integer.
- A variable starting with letters other than I, J, K, L, M,
and N is implicitly declared as real.
The IMPLICIT statement overrides these rules. When
ever needed, the IMPLICIT statement is placed at the top
of the specification block (see Fig. 1.1). The IMPLICIT
statement has effect on variables only, and not on the
default types of intrinsic functions.
The use of the IMPLICIT statement is illustrated by the
following examples.
IMPLICIT INTEGER (I,J,K,L,M,N)
IMPLICIT REAL (A-H,O-Z)
These statements state the default setting for implicit
declaration of data types. Therefore, they are redundant.
In the absence of an IMPLICIT statement, the processor
assumes that this is the case.
The first example above can also be written as
IMPLICIT INTEGER (I-N)
However, if a range is specified, as in (I-N), the first
letter should precede the second letter in alphabetical order.
The following examples illustrate other rules for im
plicit type declaration.
IMPLICIT REAL (A,B,E-H,M-W)
IMPLICIT INTEGER (I-K)
IMPLICIT DOUBLE PRECISION (D)
IMPLICIT COMPLEX (X-Z)
IMPLICIT LOGICAL (L)
IMPLICIT CHARACTER*20 (C)
These examples state that, in the absence of explicit
type declarations (e.g., INTEGER, REAL, CHARACTER,
and so on), the processor takes variables beginning with A
and B as real, C as character, D as double precision, E
through H as real, I through K as integer, L as logical, M
through W as real, and X through Z as complex.
11.7 PAUSE STATEMENT
The PAUSE statement is an executable statement that tem
porarily suspends program execution and displays a mes
sage on the terminal. It has the form:
PAUSE display
in which the optional argument display is a label that
tracks the particular PAUSE statement that suspended program execution (in case of an executable program having
multiple PAUSE statements). The argument can be either
a character constant or an integer consisting of up to 6 dig
its. If the argument is omitted, the following message is
displayed on the screen:
FORTRAN PAUSE
The following examples illustrate the use of PAUSE:
PAUSE 100
PAUSE 'TAKE ACTION NO. 1'
In these cases, the following messages would be dis
played on the screen, respectively:
100
TAKE ACTION NO. 1
Following a pause, to resume or terminate program
execution, the appropriate command would have to be en
tered at the operating system level.
Example Program: Use of Pause Statement
|
C234567890
PROGRAM PAUSE_EXAMPLE
WRITE(6,*) 'BEFORE FIRST PAUSE'
PAUSE
WRITE(6,*) 'BEFORE SECOND PAUSE'
PAUSE 100
WRITE(6,*) 'BEFORE THIRD PAUSE'
PAUSE 'TAKE ACTION NO. 1'
WRITE(6,*) 'AFTER THREE PAUSES--THE END'
END
|
The output from this program may look like this.
BEFORE FIRST PAUSE
FORTRAN PAUSE
$ CONTINUE
BEFORE SECOND PAUSE
100
$ CONTINUE
BEFORE THIRD PAUSE
TAKE ACTION NO. 1
$ CONTINUE
AFTER THREE PAUSES--THE END
$
In this example, CONTINUE is the appropriate operat
ing system command to resume execution of the program;
$ is the operating system prompt.
11.8 REWIND STATEMENT
The REWIND statement is an executable statement that
repositions an open file to the beginning of the file. An
example is:
m
REWIND(UNIT=5,ERR=95)
where 5 is a logical unit number, and 95 is a statement la
bel to which control is transferred in the event of an error
during input/output operation. When no error specifier
appears, the REWIND statement, like the BACKSPACE
statement, is reduced to:
REWIND 5
|