2-18 DO AND OTHER LOOPS
************************
(Thanks to Dieter Britz for the good comments.
Thanks to Timothy Prince for the important comments)
The subject of most of this chapter is the classical DO loop,
the last section describes possible ways to emulate other
types of loops.
The parallel loops, e.g. FORALL, DOACROSS etc, are discussed
in the chapter on parallel dialects of Fortran.
Syntax of a DO loop
-------------------
The DO loop mechanism is quite complex (see FORTRAN 77 standard 11.10).
DO loop parameters are:
DO [label] I = e1, e2, e3
I Control variable
e1 Begin range
e2 End range
e3 Stride
Number of iterations
--------------------
The formula relating the 3 parameters with the number of iterations
actually performed is not very intuitive:
n = MAX(0, INT((e2 - e1 + e3) / e3))
= MAX(0, INT(((e2 - e1) / e3) + 1))
The MAX(0, ...) is just for making negative values produce zero.
The INT() is not needed for INTEGER parameters, as integer division
will always give an INTEGER result.
The number of iterations is computed upon entering the loop and its
value determines if the loop will be executed or skipped, and the
number of times the loop body will be executed.
The loop is skipped if the number of iterations is zero, that
happens if:
(e1 .GT. e2) .AND. (e3 .GT. 0)
(e1 .LT. e2) .AND. (e3 .LT. 0)
By the way, some compilers (when using a special option), make DO
loops execute at least once, no matter what the parameters values
are, this behaviour is similar to some older FORTRAN 66 compilers.
The FORTRAN 66 standard did not address the issue of zero-trip loops.
There were a few compilers which behaved in the way which eventually
became standard FORTRAN 77.
When e3 equals 1 we get a more intuitive result:
n = MAX(0, INT(e2 - e1 + 1))
If in addition the parameters are INTEGER with e2 > e1:
n = e2 - e1 + 1
When the stride (third parameter) in a DO loop is not equal to 1,
it's easy to make a mistake with the second parameter (end range).
For example:
DO I = 1, 1000, 3 (334 times)
DO I = 5, 1000, 2 (498 times)
+------------------------------------------+
| COMPUTE THE ITERATION COUNT AS A CHECK |
+------------------------------------------+
Loop control variable
---------------------
The value of the control variable is updated on every iteration
of the loop.
Even if the loop is skipped, the loop control variable will be
initialized to e1! If the loop is executed the value of the
control variable upon exiting the loop will be:
I = e1 + ((n - 1)* e3)
On FORTRAN 66 compilers the issue of the value of the loop index
after loop completion was problematic. For example, the Honeywell
f66 compiler, behaved in 3 different ways with the same code with
different levels of optimization!
WARNING:
There are compilers (e.g. DEC FORTRAN), that some of the time do
use the control-variable to test for termination, in this case
changing its value may cause wrong results (DEC FORTRAN issues a
warning when doing so). Anyway, this is a bad programming practice.
+------------------------------------------------------------------+
| NEVER CHANGE THE VALUE OF THE CONTROL VARIABLE INSIDE THE LOOP |
+------------------------------------------------------------------+
An interactive example program:
PROGRAM DOLOOP
C ------------------------------------------------------------------
INTEGER
* COUNT
C ------------------------------------------------------------------
REAL
* X, X1, X2, X3
C ------------------------------------------------------------------
WRITE (*,*) ' ENTER X1, X2, X3 '
READ (*,*) X1, X2, X3
C ------------------------------------------------------------------
COUNT = 0
DO X = X1, X2, X3
COUNT = COUNT + 1
WRITE (*,*) ' X= ', X
ENDDO
C ------------------------------------------------------------------
WRITE (*,*) ' COUNT= ', COUNT
WRITE (*,*) ' N= ', MAX(0, INT((X2 - X1 + X3) / X3))
WRITE (*,*) ' LAST X= ', X
C ------------------------------------------------------------------
END
Note that the control variable DOESN'T equal e2 upon exiting the loop.
Jumping into/outside a DO loop
------------------------------
Jumping from a DO loop outside is allowed but ugly.
Jumping into a DO loop (doing a GOTO to a label inside the loop body)
is prohibited, the loop don't have a chance of getting initialized
properly and will misbehave.
An emulation of a DO loop
-------------------------
The following 'emulation' of a DO loop may make the previous
discussion more clear:
control_variable = e1
iteration_count = MAX(INT((e2 - e1 + e3) / e3), 0)
IF (iteration_count .EQ. 0) GOTO 9999
9998 CONTINUE
....................................
L O O P B O D Y
....................................
iteration_count = iteration_count - 1
IF (iteration_count .GT. 0) THEN {control_variable .LT. e2}
control_variable = control_variable + e3
GOTO 9998
ENDIF
9999 CONTINUE
Testing for loop termination should be done using iteration_count,
that way the loop will be protected against assignments to the
control_variable (the iteration_count is not accessible to the
programmer but the control_variable is).
It seems that some compilers use the control variable test.
Emulating 'advanced' control structures
---------------------------------------
These examples are adapted from Levine's style guide, the idea is to
emulate these very important structures that are missing in FORTRAN 77
in a 'standard' way.
WHILE ( condition ) DO ... ENDDO
================================
10 CONTINUE
IF ( .NOT. condition ) GOTO 20
...................
L O O P B O D Y
...................
GOTO 10
20 CONTINUE
DO ... WHILE ( condition )
==========================
10 CONTINUE
...................
L O O P B O D Y
...................
IF ( condition ) GOTO 10
REPEAT ... UNTIL ( condition )
==============================
10 CONTINUE
...................
L O O P B O D Y
...................
IF ( .NOT. condition ) GOTO 10
Note:
It is very unusual to find a compiler which does not support
DOWHILE()....ENDDO and this enables the use of EXIT and CYCLE
in an Fortran 90 compatible way on several f77 compilers,
including g77 and HPUX.
A much more important factor in the social movement than those already mentioned was the ever-increasing influence of women. This probably stood at the lowest point to which it has ever fallen, during the classic age of Greek life and thought. In the history of Thucydides, so far as it forms a connected series of events, four times only during a period of nearly seventy years does a woman cross the scene. In each instance her apparition only lasts for a moment. In three of the four instances she is a queen or a princess, and belongs either to the half-barbarous kingdoms of northern Hellas or to wholly barbarous Thrace. In the one remaining instance208— that of the woman who helps some of the trapped Thebans to make their escape from Plataea—while her deed of mercy will live for ever, her name is for ever lost.319 But no sooner did philosophy abandon physics for ethics and religion than the importance of those subjects to women was perceived, first by Socrates, and after him by Xenophon and Plato. Women are said to have attended Plato’s lectures disguised as men. Women formed part of the circle which gathered round Epicurus in his suburban retreat. Others aspired not only to learn but to teach. Arêtê, the daughter of Aristippus, handed on the Cyrenaic doctrine to her son, the younger Aristippus. Hipparchia, the wife of Crates the Cynic, earned a place among the representatives of his school. But all these were exceptions; some of them belonged to the class of Hetaerae; and philosophy, although it might address itself to them, remained unaffected by their influence. The case was widely different in Rome, where women were far more highly honoured than in Greece;320 and even if the prominent part assigned to them in the legendary history of the city be a proof, among others, of its untrustworthiness, still that such stories should be thought worth inventing and preserving is an indirect proof of the extent to which feminine influence prevailed. With the loss of political liberty, their importance, as always happens at such a conjuncture, was considerably increased. Under a personal government there is far more scope for intrigue than where law is king; and as intriguers women are at least the209 equals of men. Moreover, they profited fully by the levelling tendencies of the age. One great service of the imperial jurisconsults was to remove some of the disabilities under which women formerly suffered. According to the old law, they were placed under male guardianship through their whole life, but this restraint was first reduced to a legal fiction by compelling the guardian to do what they wished, and at last it was entirely abolished. Their powers both of inheritance and bequest were extended; they frequently possessed immense wealth; and their wealth was sometimes expended for purposes of public munificence. Their social freedom seems to have been unlimited, and they formed combinations among themselves which probably served to increase their general influence.321 The old religions of Greece and Italy were essentially oracular. While inculcating the existence of supernatural beings, and prescribing the modes according to which such beings were to be worshipped, they paid most attention to the interpretation of the signs by which either future events in general, or the consequences of particular actions, were supposed to be divinely revealed. Of these intimations, some were given to the whole world, so that he who ran might read, others were reserved for certain favoured localities, and only communicated through the appointed ministers of the god. The Delphic oracle in particular enjoyed an enormous reputation both among Greeks and barbarians for guidance afforded under the latter conditions; and during a considerable period it may even be said to have directed the course of Hellenic civilisation. It was also under this form that supernatural religion suffered most injury from the great intellectual movement which followed the Persian wars. Men who had learned to study the constant sequences of Nature for themselves, and to shape their conduct according to fixed principles of prudence or of justice, either thought it irreverent to trouble the god about questions on which they were competent to form an opinion for themselves, or did not choose to place a well-considered scheme at the mercy of his possibly interested responses. That such a revolution occurred about the middle of the fifth century B.C., seems proved by the great change of tone in reference to this subject which one perceives on passing from Aeschylus to Sophocles. That anyone should question the veracity of an oracle is a supposition which never crosses the mind of the elder dramatist. A knowledge of augury counts among the greatest benefits222 conferred by Prometheus on mankind, and the Titan brings Zeus himself to terms by his acquaintance with the secrets of destiny. Sophocles, on the other hand, evidently has to deal with a sceptical generation, despising prophecies and needing to be warned of the fearful consequences brought about by neglecting their injunctions. The stranger had a pleasant, round face, with eyes that twinkled in spite of the creases around them that showed worry. No wonder he was worried, Sandy thought: having deserted the craft they had foiled in its attempt to get the gems, the man had returned from some short foray to discover his craft replaced by another. “Thanks,” Dick retorted, without smiling. When they reached him, in the dying glow of the flashlight Dick trained on a body lying in a heap, they identified the man who had been warned by his gypsy fortune teller to “look out for a hidden enemy.” He was lying at full length in the mould and leaves. "But that is sport," she answered carelessly. On the retirement of Townshend, Walpole reigned supreme and without a rival in the Cabinet. Henry Pelham was made Secretary at War; Compton Earl of Wilmington Privy Seal. He left foreign affairs chiefly to Stanhope, now Lord Harrington, and to the Duke of Newcastle, impressing on them by all means to avoid quarrels with foreign Powers, and maintain the blessings of peace. With all the faults of Walpole, this was the praise of his political system, which system, on the meeting of Parliament in the spring of 1731, was violently attacked by Wyndham and Pulteney, on the plea that we were making ruinous treaties, and sacrificing British interests, in order to benefit Hanover, the eternal millstone round the neck of England. Pulteney and Bolingbroke carried the same attack into the pages of The Craftsman, but they failed to move Walpole, or to shake his power. The English Government, instead of treating Wilkes with a dignified indifference, was weak enough to show how deeply it was touched by him, dismissed him from his commission of Colonel of the Buckinghamshire Militia, and treated Lord Temple as an abettor of his, by depriving him of the Lord-Lieutenancy of the same county, and striking his name from the list of Privy Councillors, giving the Lord-Lieutenancy to Dashwood, now Lord Le Despencer. "I tell you what I'll do," said the Deacon, after a little consideration. "I feel as if both Si and you kin stand a little more'n you had yesterday. I'll cook two to-day. We'll send a big cupful over to Capt. McGillicuddy. That'll leave us two for to-morrer. After that we'll have to trust to Providence." "Indeed you won't," said the Surgeon decisively. "You'll go straight home, and stay there until you are well. You won't be fit for duty for at least a month yet, if then. If you went out into camp now you would have a relapse, and be dead inside of a week. The country between here and Chattanooga is dotted with the graves of men who have been sent back to the front too soon." "Adone do wud that—though you sound more as if you wur in a black temper wud me than as if you pitied me." "Wot about this gal he's married?" "Don't come any further." "Davy, it 'ud be cruel of us to go and leave him." "Insolent priest!" interrupted De Boteler, "do you dare to justify what you have done? Now, by my faith, if you had with proper humility acknowledged your fault and sued for pardon—pardon you should have had. But now, you leave this castle instantly. I will teach you that De Boteler will yet be master of his own house, and his own vassals. And here I swear (and the baron of Sudley uttered an imprecation) that, for your meddling knavery, no priest or monk shall ever again abide here. If the varlets want to shrieve, they can go to the Abbey; and if they want to hear mass, a priest can come from Winchcombe. But never shall another of your meddling fraternity abide at Sudley while Roland de Boteler is its lord." "My lord," said Edith, in her defence, "this woman has sworn falsely. The medicine I gave was a sovereign remedy, if given as I ordered. Ten drops would have saved the child's life; but the contents of the phial destroyed it. The words I uttered were prayers for the life of the child. My children, and all who know me, can bear witness that I have a custom of asking His blessing upon all I take in hand. I raised my eyes towards heaven, and muttered words; but, my lord, they were words of prayer—and I looked up as I prayed, to the footstool of the Lord. But it is in vain to contend: the malice of the wicked will triumph, and Edith Holgrave, who even in thought never harmed one of God's creatures, must be sacrificed to cover the guilt, or hide the thoughtlessness of another." "Aye, Sir Treasurer, thou hast reason to sink thy head! Thy odious poll-tax has mingled vengeance—nay, blood—with the cry of the bond." HoME古一级毛片免费观看
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