markh@csd4.milw.wisc.edu (Mark William Hopkins) (05/17/88)
While the issue is "on the table", I feel it appropriate to give a few
procedures to show how the conversions take place when Pascal reads & writes
in a text file.
READING INTEGERS FROM TEXT:
procedure ReadInt(var T : text; var I : integer);
var
Sign : char;
begin
(*** SKIP SPACES ***)
while T^ = ' ' do get(T);
(*** GET THE SIGN ***)
if T^ in ['+','-'] then begin
read(T, Sign)
end
else Sign := '+';
(*** CONVERT THE STRING TO AN INTEGER ***)
I := 0;
while T^ in ['0'..'9'] do begin
I := 10*I + ord(T^) - ord('0');
get(T)
end;
(*** ADJUST FOR THE SIGN ***)
if Sign = '-' then I := -I
end;
READING REAL NUMBERS FROM TEXT:
(The syntax allowed by this procedure is slightly more liberal
than that which standard Pascal allows for.)
procedure ReadReal(var T : text; var R : real);
var
Exponent : integer;
Power : real;
begin
(*** CONVERT THE INTEGER PART ***)
ReadInt(T, R);
(*** CONVERT THE DECIMAL PART ***)
if T^ = '.' then begin
get(T);
Power := 1;
while T^ in ['0'..'9'] do begin
Power := Power / 10;
R := R + ( ord(T^) - ord('0') ) * Power;
get(T)
end
end;
(*** CONVERT THE EXPONENT PART ***)
if T^ in ['e','E'] then begin
get(T);
ReadInt(T, Exponent);
while Exponent > 0 do begin
Exponent := Exponent - 1; R := R * 10
end;
while Exponent < 0 do begin
Exponent := Exponent + 1; R := R / 10
end
end
end;
FORMATTING INTEGERS INTO TEXT:
procedure WriteInt(T : text; I : integer; FieldWidth : integer);
var
Sign : char;
Size, Exponent : integer;
begin
(*** DETERMINE THE SIGN ***)
if I < 0 then begin
Sign := '-'; I := -I
end
else if I > 0 then Sign := '+'
else Sign := '0';
(*** DETERMINE THE SIZE OF THE INTEGER STRING ***)
Exponent := 1; Size := 0;
while (Exponent < I) do begin
Exponent := 10 * Exponent; Size := Size + 1
end;
(*** PAD THE INTEGER TO THE LEFT, IF NECESSARY ***)
if FieldWidth > Size then begin
while FieldWidth > Size + 1 do begin
FieldWidth := FieldWidth - 1; write(T, ' ')
end;
if Sign = '+' then write(T, ' ')
end;
(*** CONVERT THE NUMBER TO A STRING ***)
if Sign = '+' then write(T, ' ')
else if Sign = '-' then write(T, '-')
else if Sign = '0' then write(T, '0');
else while Size > 0 do begin
Size := Size - 1;
Exponent := Exponent / 10;
write(T, chr(I mod Exponent + ord('0')) );
I := I div Exponent
end
end;
FORMATTING REALS TO TEXT:
(F1, F2 are the two field widths used for formatting real numbers and
rounding them to the desired number of decimal places.)
procedure WriteReal(var T : text; R : real; FW1, FW2 : integer);
var
Mantissa : integer;
Decimal : real;
begin
Mantissa := trunc(R); Decimal := R - Mantissa;
(*** The field width FW1 includes the decimal part and decimal ***)
(*** point in it. Therefore the integer part is written out ***)
(*** with a field width of FW1 - FW2 - 1. ***)
WriteInt(T, Mantissa, FW1 - FW2 - 1);
if FW2 > 0 then begin
write(T, '.');
while FW2 > 1 do begin
FW2 := FW2 - 1;
Decimal := Decimal * 10;
write(T, chr( trunc(Decimal) + ord('0') ) );
Decimal := Decimal - trunc(Decimal)
end;
(*** ROUND THE LAST DECIMAL DIGIT ***)
Decimal := Decimal * 10;
write(T, chr( round(Decimal) + ord('0') ) )
end
endfirth@sei.cmu.edu (Robert Firth) (05/17/88)
In article <5817@uwmcsd1.UUCP> markh@csd4.milw.wisc.edu (Mark William Hopkins) writes: > While the issue is "on the table", I feel it appropriate to give a few >procedures to show how the conversions take place when Pascal reads & writes >in a text file. Some comments on these procedures are given thus: [! ... ] READING INTEGERS FROM TEXT: procedure ReadInt(var T : text; var I : integer); var Sign : char; begin (*** SKIP SPACES ***) while T^ = ' ' do get(T); [! may run off the end of the file] (*** GET THE SIGN ***) if T^ in ['+','-'] then begin read(T, Sign) end else Sign := '+'; (*** CONVERT THE STRING TO AN INTEGER ***) I := 0; while T^ in ['0'..'9'] do begin I := 10*I + ord(T^) - ord('0'); [! may cause false overflow on '+' if number large] get(T) end; (*** ADJUST FOR THE SIGN ***) if Sign = '-' then I := -I [! and of course fails on the most negative integer] end; READING REAL NUMBERS FROM TEXT: (The syntax allowed by this procedure is slightly more liberal than that which standard Pascal allows for.) procedure ReadReal(var T : text; var R : real); var Exponent : integer; Power : real; begin (*** CONVERT THE INTEGER PART ***) ReadInt(T, R); [! will overflow on eg 10000000000.0] (*** CONVERT THE DECIMAL PART ***) if T^ = '.' then begin get(T); Power := 1; while T^ in ['0'..'9'] do begin Power := Power / 10; [! cumulative loss of precision since binary 0.1 is approximate] R := R + ( ord(T^) - ord('0') ) * Power; get(T) end end; (*** CONVERT THE EXPONENT PART ***) if T^ in ['e','E'] then begin get(T); ReadInt(T, Exponent); while Exponent 0 do begin Exponent := Exponent - 1; R := R * 10 end; while Exponent < 0 do begin Exponent := Exponent + 1; R := R / 10 [! further loss of precision] end end end; FORMATTING INTEGERS INTO TEXT: procedure WriteInt(T : text; I : integer; FieldWidth : integer); var Sign : char; Size, Exponent : integer; begin (*** DETERMINE THE SIGN ***) if I < 0 then begin Sign := '-'; I := -I [! fails on most negative number] end else if I > 0 then Sign := '+' else Sign := '0'; (*** DETERMINE THE SIZE OF THE INTEGER STRING ***) Exponent := 1; Size := 0; while (Exponent < I) do begin Exponent := 10 * Exponent; [! overflow if I near MAXINT/10 ] Size := Size + 1 end; (*** PAD THE INTEGER TO THE LEFT, IF NECESSARY ***) if FieldWidth Size then begin while FieldWidth > Size + 1 do begin FieldWidth := FieldWidth - 1; write(T, ' ') end; if Sign = '+' then write(T, ' ') end; (*** CONVERT THE NUMBER TO A STRING ***) if Sign = '+' then write(T, ' ') else if Sign = '-' then write(T, '-') else if Sign = '0' then write(T, '0'); else while Size > 0 do begin Size := Size - 1; Exponent := Exponent / 10; write(T, chr(I mod Exponent + ord('0')) ); I := I div Exponent end end; FORMATTING REALS TO TEXT: (F1, F2 are the two field widths used for formatting real numbers and rounding them to the desired number of decimal places.) procedure WriteReal(var T : text; R : real; FW1, FW2 : integer); var Mantissa : integer; Decimal : real; begin Mantissa := trunc(R); [! overflow if R > MAXINT ] Decimal := R - Mantissa; (*** The field width FW1 includes the decimal part and decimal ***) (*** point in it. Therefore the integer part is written out ***) (*** with a field width of FW1 - FW2 - 1. ***) WriteInt(T, Mantissa, FW1 - FW2 - 1); if FW2 > 0 then begin write(T, '.'); while FW2 > 1 do begin FW2 := FW2 - 1; Decimal := Decimal * 10; write(T, chr( trunc(Decimal) + ord('0') ) ); Decimal := Decimal - trunc(Decimal) end; (*** ROUND THE LAST DECIMAL DIGIT ***) [! fails if rounding involves more than one digit ] Decimal := Decimal * 10; write(T, chr( round(Decimal) + ord('0') ) ) end end [ Me again, Robert Firth ] I find it hard to express my feelings in a manner that is both clear and polite. The above code is evidently rubbish; unfortunately it is typical of the stuff produced by amateur software writers - you can find examples galore, as bad or worse, in many packages distributed by major institutes of education. These examples would not have survived any one of: accurate specification, competent design review, code walkthrough, formal analysis, or thorough testing. I conclude that they were subjected to none of these processes. Folks, are we surprised there is a "software crisis"? And what can we, as responsible individuals, do about it?
markh@csd4.milw.wisc.edu (Mark William Hopkins) (05/20/88)
In a previous article firth@bd.sei.cmu.edu.UUCP (Robert Firth) writes: <CONCERNING SOME REASONABLY WRITTEN PROCEDURES THAT HAD BEEN PREVIOUSLY PLACED ON THE NET>: >[ Me again, Robert Firth ] welcome back, we almost lost you there. > >I find it hard to express my feelings in a manner that is both clear and >polite. The above code is evidently rubbish; unfortunately it is typical >of the stuff produced by amateur software writers - you can find examples >galore, as bad or worse, in many packages distributed by major institutes >of education. > >These examples would not have survived any one of: accurate specification, >competent design review, code walkthrough, formal analysis, or thorough >testing. I conclude that they were subjected to none of these processes. > >Folks, are we surprised there is a "software crisis"? And what can we, >as responsible individuals, do about it? Never mind that these comments are innappropriate in this context, I do feel that a suitable response is in order. This story ... from another newsgroup in the past ... most properly expresses my sentiments about the points raised in the poster's article: ******************************************************************************** Subject: A Gown for the Princess: A Parable for Software Engineers Communications of the ACM, September 1987, Volume 30, Number 9, p. 750-751 president's letter by Paul Abrahams, acm president Copyright 1987 by the Association for Computing Machinery, Inc. Copying without fee is permitted provided that the copies are not made or distributed for direct commercial advantage and credit to the source is given. ------------------------------------------------------------ A Gown for the Princess: A Parable for Software Engineers Once upon a time, there lived a king who had a beautiful daughter. The king wished nothing more than to make her happy, and so he resolved that the princess should have a fine new gown that would match her beauty. He summoned the wisest and most silled tailor in all of his kingdom to make a gown for the princess. "My daughter shall have the finest gown in the land," declared the king. "Before I can make such a gown," said the tailor, "I must know what it is that you require." And the tailor discoursed at length about the materials that might be used, the cuts and patterns to be chosen, the method of joining and seaming, the decorations, and other such matters. He spoke of gores and pleats, of hems and stitches, of darts and shirring, of embroidery and lace. The poor king knew nothing of such matters. "I just want a beautiful gown for my daughter!" he exclaimed. "I shall do my best," said the tailor." The next week the tailor returned to the palace. He brought a large sheaf of documents and presented them to the king. "These are the specifications I propose for the gown, your Majesty. I hope it is what you want. It shall be completed in a year's time. And for my labors and the labors of my assistants, I must humbly ask your Majesty for fifty thousand ducats." "These tailors charge the earth and the sky," thought the king, "but there is nothing to do but to pay it and hope for the best." The king nodded sadly, "It shall be as you propose," said the king, and he placed the royal seal upon the specification. The tailor then appointed himself as Chief Tailor and summoned a score of apprentice tailors to assist him. He explained to them how, together, they would create the gown. "Let us decompose the task into its parts: the sleeves, the bodice, the waist, the skirt, and the adornments. Each one of you is to prepare a part of the gown, and my task is to specify the parts and discern how they are to be fitted together. But hasten not to your loom and your table; we first must plan how each part is to be prepared and decompose the task until the smallest detail is known. We must verify beyond the shadow of doubt that the gown we create will meet its specification. To that end we must prove that all its parts will fit together and that each part is as the larger design would have it. Through such verification we may know, before the first stitch is taken, that the gown will render honor to the princess of the realm and be as the king has ordered." Months went by, and the Chief Tailor and his assistants filled sheets of paper with drawings of the gown and of all its parts. Yet the fine fabrics purchased for the gown remained in the storehouse, and not a stitch had been sewn. The king began to worry that the gown would not be completed within the year. He summoned the Chief Tailor to the court and asked why the gown had not yet been started. "The execution shall follow upon the plan, as the night upon the day," said the Chief Tailor. "I shall not take needle and thread to hand until the gown has been completely designed." And so it was. But at last the design was done, and the great enterprise began to take shape. Fabrics were woven, and the team of tailors went busily about their work of cutting, sewing, and shaping. As the different parts of the gown were completed, the tailors brought them to the Chief Tailor for assembly. Here was the most difficult part of the whole effort, for it seemed that, whenever two parts were joined, a piece of fabric remained on one side or the other. Many times the tailors had to take back their work and resew it, and arguments broke out among them as to who was at fault. One day the Chief Tailor came to the king. "Alas, your Majesty, the allotted sum is not sufficient to complete the gown. For the princess has grown in girth since the measurements were made, and it is now necessary to alter the plan. I must ask an additional thousand ducats if it is to be completed on time." Sadly the king nodded. "Go and do it, then," he said. At last, after a year and a half, the gown was completed and brought to the princess for a demonstration. Alas, the poor girl was unable to put it on, for the chief tailor, in his quest for excellence of shape, had forgotten to provide any buttons. Hastily the tailor modified the gown, and the princess then showed it to the king. "Is the gown acceptable?" asked the Chief Tailor. "It is, indeed," answered the king. But the king was troubled at heart, for somehow the gown did not look at all like what he had expected. Several days later the princess was walking in the garden of the palace, wearing her new gown. As she reached up to pluck a grape, the top of the gown slid down, exposing her bosom for all to view. The king was very distressed and angrily summoned the Chief Tailor. "What kind of a gown have you made for my daughter!" he exclaimed. "Not only has it made her look foolish, but I paid you fifty thousand ducats for it!" "Prithee, your Majesty," said the tailor. "The gown was made according to the specifications that you agreed to. Nowhere did we speak of this matter, that the gown should remain in one place only. Indeed, 'tis not a fault but a feature of the gown, that your daughter's beauty should be displayed to all the folk of the land. But, if your Majesty desires to alter the gown, I must humbly request another fifteen hundred ducats for the immense labor necessary to meet your wishes." Sadly the king nodded. "Go and do it, then," he said. A few years later, the Chief Tailor was turned into a frog by a disgruntled witch whose affections he had spurned. Hardly had the Chief Tailor been transformed than strange stains and rips appeared in the gown. The Backup Tailor, who had been appointed by the Chief Tailor to maintain the gown, examined it closely, seeking to repair it. But the documents that described the design were not to be found, and the Backup Tailor finally had to give up. The princess put the gown in the back of her closet, and there it remains to this very day. ------------------------------------------------------------
markh@csd4.milw.wisc.edu (Mark William Hopkins) (05/21/88)
This is a followup from the previous article and addresses more directly the response of R. Firth several articles back concerning the nature of several number-to-string procedures listed previously: <MY INTENTIONS>: > While the issue is "on the table", I feel it appropriate to give a few >procedures to show how the conversions take place when Pascal reads & writes >in a text file. ... <FOLLOWED BY LISTINGS FOR ReadInt, ReadReal, WriteInt and WriteReal> ... <THE INAPPROPRIATE CRITICISM>: >I find it hard to express my feelings in a manner that is both clear and >polite. The above code is evidently rubbish; unfortunately it is typical >of the stuff produced by amateur software writers - you can find examples >galore, as bad or worse, in many packages distributed by major institutes >of education. > >These examples would not have survived any one of: accurate specification, >competent design review, code walkthrough, formal analysis, or thorough >testing. I conclude that they were subjected to none of these processes. > >Folks, are we surprised there is a "software crisis"? And what can we, >as responsible individuals, do about it? <THE REASONS FOR BEING INAPPROPRIATE>: (1) The procedure listings are for show only, they are not meant to be run. This much is obvious, since Pascal ALREADY has the facilities built-in. (2) The procedures are meant to SIMULATE Pascal ... for better or for worse ... flaws and all ... in order to demonstrate just how Pascal carries out the special built-in read procedures and to make explicit the limitations and flaws of these built-in procedures. But in any case, I feel that the criticism should be more directly addressed: So let us scrutinize the ReadInt procedure to see if it actually does simulate Pascal's integer read: THE LISTINGS: P A S C A L : program Pascal(input, output); var I : integer; begin read(I); write(I:1) end. R e a d I n t : program Simulate(input, output); var I : integer; ... (*** Procedure ReadInt inserted here ***) begin ReadInt(I); write(I:1) end. where the procedure listing is: procedure ReadInt(var T : text; var I : integer); var Sign : char; begin (*** SKIP SPACES ***) while T^ = ' ' do get(T); {! may run off the end of the file} (*** GET THE SIGN ***) if T^ in ['+','-'] then begin read(T, Sign) end else Sign := '+'; (*** CONVERT THE STRING TO AN INTEGER ***) I := 0; while T^ in ['0'..'9'] do begin I := 10*I + ord(T^) - ord('0'); {! may cause false overflow on '+' if number large} get(T) end; (*** ADJUST FOR THE SIGN ***) if Sign = '-' then I := -I {! and of course fails on the most negative integer} end; Let us look into some of the supposed flaws of the ReadInt procedure: (1) O V E R F L O W S In all fairness, I should mention that the way a compiler handles overflow is implementation dependent. The ReadInt procedure was promised to simulate how Pascal reads integers ... for better or for worse. Script started on Fri May 20 14:13:34 1988 T H E I N P U T F I L E : (csd4) 1: more In 123456789012345 H O W S T A N D A R D P A S C A L M A Y H A N D L E O V E R F L O W : (csd4) 2: pix Pascal.p <In Execution begins... -2045911175 Execution terminated. H O W R e a d I n t H A N D L E S O V E R F L O W : (csd4) 3: pix ReadInt.p <In Execution begins... -2045911175 Execution terminated. T H E R E L E V A N T C O D E : (csd4) 4: more ReadInt.p ... procedure ReadInt(var T : text; var I : integer); ... I := 0; while T^ in ['0'..'9'] do begin <THE MISPLACED CRITICISM>: I := 10*I + ord(T^) - ord('0'); {! may cause false overflow on '+' if number large} get(T) end; ... So much for overflow ... (2) O V E R R E A D I N G T H E E N D - O F - F I L E : The question asked here is whether ReadInt successfully simulates the way Pascal reads integers when there are no integers to be read ... Script started on Fri May 20 14:17:01 1988 T H E I N P U T F I L E : (csd4) 1: more In <EMPTY> H O W S T A N D A R D P A S C A L H A N D L E S I T : (csd4) 2: pix Pascal.p <In Execution begins... standard input: Tried to read past end of file H O W R e a d I n t H A N D L E S I T : (csd4) 3: pix ReadInt.p <In Execution begins... standard input: Tried to read past end of file ... Error in "ReadInt"+2 near line 9. ... T H E R E L E V A N T C O D E : (csd4) 4: more ReadInt.p ... procedure ReadInt(var T : text; var I : integer); ... begin (*** SKIP SPACES ***) <THE MISPLACED CRITICISM>: 9 ---> while T^ = ' ' do get(T); {! may run off the end of the file} ... "Pray tell, your majesty ... excuse me, I mean 'Your Majesty' ... a simulation so perfect that it even simulates the flaws!" Now a few words about the procedures themselves. Nobody in their right mind would actually set up and run text read procedures that simulated what was already built-in. If I were actually going to write procedures to do text I/O they would have had ERROR CORRECTION facilities built in them.
jfjr@mitre-bedford.ARPA (Jerome Freedman) (05/23/88)
The impending quarrel over number-string conversions will
be uninteresting to most of us but..
This is a sidelight to this dispute. I don't intend to get
involved but it perked my curiosity. Can any body point me
to literature in which these problems are discussed (and solved)
Jerry Freedman, Jr
jfjr@mitre-bedford.arpa
Jerry Freedman, Jr "Does Unix have
jfjr@mitre-bedford.arpa a Buddha nature?"
(617)271-4563