Pascal Programming/Records
The key to successful programming is finding the "right" structure of data and program.
—Niklaus Wirth[1]
After you have learned to use an array
, this chapter introduces you to another data type structure concept called record
.
Like an array
, the use of record
s primarily serves the purposes of allowing you to write clean, structured programs.
It is otherwise optional.
Concept
[edit | edit source]You briefly saw a record
in the first chapter.
While an array
is a homogenous aggregation of data, that means all members have to have the same base data type, a record
is potentially, but not necessarily an aggregation of data having various different data types.[2]
Declaration
[edit | edit source]A record
data type declaration looks pretty much like a collection of variable declarations:
program recordDemo;
type
(* a standard line on a text console *)
line = string(80);
(* 1st grade through 12th grade *)
grade = 1..12;
(* encapsulate all administrative data in one structure *)
student = record
firstname: line;
lastname: line;
level: grade;
end;
The declaration begins with the word record
and ends with end
.
Inbetween you declare fields, or members, member elements of the entire record
.
Here again the semicolon has the function of separating members. The keyword end will actually terminate a record declaration. Note, how in the following correct example there is no semicolon after the last member’s declaration:
program recordSemicolonDemo;
type
sphere = record
radius: real;
volume: real;
surface: real
end;
|
|
All record
members have to bear distinct names within the record
declaration itself.
For instance in the example above, declaring two “variables”, member elements of the name level
will be rejected.
There is no requirement on how many fields you have to declare.
An “empty” record
is also possible:[fn 1]
type
emptyRecord = record
end;
Many fields of the same data type
[edit | edit source]Similar to the declaration of variables you can define multiple fields of the same data type at once by separating identifiers with a comma.
The previous declaration of sphere
could also be written as:
type
sphere = record
radius, volume, surface: real;
end;
Most Pascal veterans and style guides, however, discourage the use of this shorthand notation (both for variable as well as record
declarations, but also in formal parameter lists).
It is only reasonable if all declared identifiers absolutely always have same data type;
it is virtually guaranteed you will never want to change the data type of just one field in the comma-separated list.
If in doubt, use the longhand.
In programming, convenience plays a tangential role.
Use
[edit | edit source]By declaring a record
variable you immediately have the entire set of “sub”‑variables at your hand.
Accessing them is done by specifying the record
variable’s name, plus a dot (.
), followed by the record
field’s name:
var
posterStudent: student;
begin
posterStudent.firstname := 'Holden';
posterStudent.lastname := 'Caulfield';
posterStudent.level := 10;
end.
You already saw the dot notation in the previous chapter on strings, where appending .capacity
on a name of a string(…)
variable refers to the respective variable’s character capacity.
This is not a coincidence.
program dotNoGo(output); { This program does not compile. }
type
line = string(80);
quizItem = record
question: line;
answer: line;
end;
var
response: line;
challenge: quizItem;
begin
writeLn(line.capacity); { ↯ `line` is not a variable }
writeLn(response.capacity); { ✔ correct }
writeLn(quizItem.question); { ↯ `quizItem` refers to a data type }
{ Data type declarations (as per definition) do not reserve any memory }
{ thus you cannot “read/write” from/to a data type. }
writeLn(challenge.question); { ✔ correct }
end.
.
) notation is only valid if there is memory.[fn 2]
Advantages
[edit | edit source]But why and when do we want to use a record
?
At first glance and in the given examples so far it may seem like a troublesome way to declare and use multiple variables.
Yet the fact that a record
is handled as one unit entails one big advantage:
- You can copy entire
record
values via a simple assignment (:=
). - This means you can pass much data at once: A
record
can be a parameter of routines, and in EP functions can return them as well.[fn 3]
Evidently you want to group data together that always appear together. It does not make sense to group unrelated data, just because we can. Another quite useful advantage is presented below in the section on variant records.
Routing override
[edit | edit source]As you saw earlier, referring to members of a record
can get a little tedious, because we are repeating the variable name over and over again.
Fortunately, Pascal allows us abbreviate things a bit.
With
-clause
[edit | edit source]The with
-clause allows us to eliminate repeating a common prefix, specifically the name of a record
variable.[3]
begin
with posterStudent do
begin
firstname := 'Holden';
lastname := 'Caulfield';
level := 10;
end;
end.
All identifiers that identify values are first looked for in the record
scope of posterStudent
.
If there is no match, all variable identifiers outside of the given record
are considered too.
Of course it is still possible to denote a record
member by its full name.
E. g. in the source code above it would be perfectly legal to still write posterStudent.level
within the with
-clause.
Concededly, this would defeat the purpose of the with
-clause, but sometimes it may still be beneficial to emphasize the specific record
variable just for documentation.
It is nevertheless important to understand that the FQI, the fully-qualified identifier, the one with a dot in it, does not lose its “validity”.
In principle, all components of structured values “containing dots” can be abbreviated with with
.
This is also true for the data type string
you have learned in the previous chapter.
program withDemo(input, output);
type
{ Deutsche Post „Maxi-Telegramm“ }
telegram = string(480);
var
post: telegram;
begin
with post do
begin
writeLn('Enter your telegram. ',
'Maximum length = ',
capacity, ' characters.');
readLn(post);
{ … }
end;
end.
Here, within the with
-clause capacity
, and for that matter post.capacity
, refer to post.capacity
.
Multiple levels
[edit | edit source]If multiple with
-clauses ought to be nested, there is the short notation:
with snakeOil, sharpTools do
begin
…
end;
which is equivalent to:
with snakeOil do
begin
with sharpTools do
begin
…
end;
end;
It is important to bear in mind, first identifiers in sharpTools
are searched, and if there is no match, secondly, identifiers in snakeOil
are considered.
Variant records
[edit | edit source]In Pascal a record
is the only data type structure concept that allows you to, so to speak, alter its structure during run-time, while a program
is running.
This super practical property of record
permits us to write versatile code covering many cases.
Declaration
[edit | edit source]Let’s take a look at an example:
type
centimeter = 10..199;
// order of female, male has been chosen, so `ord(sex)`
// returns the [minimum] number of non-defective Y chromosomes
sex = (female, male)
// measurements according EN 13402 size designation of clothes [incomplete]
clothingSize = record
shoulderWidth: centimeter;
armLength: centimeter;
bustGirth: centimeter;
waistSize: centimeter;
hipMeasurement: centimeter;
case body: sex of
female: (
underbustMeasure: centimeter;
);
male: (
);
end;
The variant part of a record
starts with the keyword case
, which you already know from selections.
After that follows a record
member declaration, the variant selector, but instead of a semicolon you put the keyword of
thereafter.
Below that follow all possible variants.
Each variant is marked by a value out of the variant selector’s domain, here female
and male
.
Separated by a colon (:
) follows a variant denoter surrounded by parentheses.
Here you can list additional record
members that are only available if a certain variant is “active”.
Note that all identifiers across all alternatives must be unique.
The individual variants are separated by a semicolons, and there can be at most one variant part which has to appear at the end.
Because you will need to be able to list all possible variants, the variant selector has to be an ordinal data type.
Use
[edit | edit source]Using variant records requires you to first select a variant.
Variants are “activated” by assigning a value to the variant selector.
Note, variants are not “created”; they all already exist at program
startup.
You merely need to make a choice.
boobarella.body := female;
boobarella.underbustMeasure := 69;
Only after assigning a value to the variant selector and as long as this value remains unchanged, you are allowed to access any fields of the respective variant.
It is illegal to reverse the previous two lines of code and attempt accessing the underbustMeasure
field even though body
is not defined yet and, more importantly, does not bear the value female
.
It is certainly permissible to change the variant selector later in your program
and then use a different variant, but all previously stored values in the variant part relinquish their validity and you cannot restore them.
If you switch back the variant to a previous, original value, you will need to assign all values in that variant anew.
Application
[edit | edit source]This concept opens up new horizons:
You can design your programs more interactively in a neat fashion.
You can now choose a variant based on run-time data (data that is read while the program
is running).
Because at any time (after the first assignment of a value to the variant selector) only one variant is “active”, your program
will crash if it attempts reading/writing values of an “inactive” variant.
This is a desirable behavior, because that is the whole idea of having distinct variants.
It guarantees your programs overall integrity.
Anonymous variants
[edit | edit source]Pascal also permits having anonymous variant selectors, that is selectors not bearing any name. The implications are
- you cannot explicitly select (nor query) any variant, so
- in turn all variants are considered “active” at the same time.
“But wasn’t this the object of the exercise?” you might ask.
Yes, indeed, since there is no named selector your program
cannot keep track which variant is supposed to work and which one is “defective”.
You are responsible to determine which variant you can sensibly read/write at present.
program anonymousVariantsDemo(output);
type
bitIndex = 0..(sizeOf(integer) * 8 - 1);
exposedInteger = record
case Boolean of
false: (
value: integer;
);
true: (
bit: set of bitIndex;
);
end;
var
i: exposedInteger;
begin
i.bit := [4];
writeLn(i.value);
end.
16
16
is (and this should be considered “a coincidence”) . We stress that all Pascal standards do not make any statement regarding internal memory structure. A high-level programming language is not concerned about how data is stored, it even does not know the notion of “bits”, “voltage high”/“voltage low”.
This concept exists in many other programming languages too. In the programming language C, for instance, it is called a union.
Conditional loops
[edit | edit source]So far we have been exclusively using counting loops. This is great if you can predict in advance the number of iterations, how many times the loop’s body needs to be executed. Yet every so often it is not possible to formulate a proper expression determining the number of iterations in advance.
Conditional loops allow you to make the execution of the next iteration dependent on a Boolean
expression.
They come in two flavors:
- Head-controlled loop, and
- tail-controlled loop.
The difference is, the loop’s body of a tail-controlled loop is executed at least once in any case, whereas a head-controlled loop might never execute the loop body at all. In either case, a condition is evaluated over and over again and must uphold for the loop to continue.
Head-controlled loop
[edit | edit source]A head-controlled loop is frequently called while
-loop because of its syntax.
program characterCount(input, output);
type
integerNonNegative = 0..maxInt;
var
c: char;
n: integerNonNegative;
begin
n := 0;
while not EOF do
begin
read(c);
n := n + 1;
end;
writeLn('There are ', n:1, ' characters.');
end.
$ cat ./characterCount.pas | ./characterCount
There are 240 characters.
$ printf '' '' | ./characterCount
There are 0 characters.
Boolean
expression framed by the words while
and do
. The condition must evaluate to true
for any (subsequent) iteration to occur.
As you can see from the output, in the second case, it may even be zero times: Evidently for empty input n := n + 1
was never executed.EOF
is shorthand for EOF(input)
.
This standard function
returns true
if there is no further data available to read, commonly called end of file.
It is illegal, and will horribly fail, to read
from a file if the respective EOF
function call returns true
.
Unlike a counting loop, you are allowed to modify data the conditional loop’s condition depends on.
const
(* instead of a hard-coded length `64` *)
(* you can write `sizeOf(integer) * 8` in Delphi, FPC, GPC *)
wordWidth = 64;
type
integerNonNegative = 0..maxInt;
wordStringIndex = 1..wordWidth;
wordString = array[wordStringIndex] of char;
function binaryString(n: integerNonNegative): wordString;
var
(* temporary result *)
binary: wordString;
i: wordStringIndex;
begin
(* initialize `binary` with blanks *)
for i := 1 to wordWidth do
begin
binary[i] := ' ';
end;
(* if n _is_ zero, the loop's body won't be executed *)
binary[i] := '0';
(* reverse Horner's scheme *)
while n >= 1 do
begin
binary[i] := chr(ord('0') + n mod 2);
n := n div 2;
i := i - 1;
end;
binaryString := binary;
end;
The n
the loop’s condition depends on will be repeatedly divided by two.
Because the division operator is an integer division (div
), at some point the value 1
will be divided by two and the arithmetically correct result 0.5
is truncated (trunc
) toward zero.
Yet the value 0
does not satisfy the loop’s condition anymore, thus there will not be any subsequent iterations.
Tail-controlled loop
[edit | edit source]In a tail-controlled loop the condition appears below the loop’s body, at the foot. The loop’s body is always run once before even the condition is evaluated at all.
program repeatDemo(input, output);
var
i: integer;
begin
repeat
begin
write('Enter a positive number: ');
readLn(i);
end
until i > 0;
writeLn('Wow! ', i:1, ' is a quite positive number.');
end.
The loop’s body is encapsulated by the keywords repeat
and until
.[fn 4]
After until
follows a Boolean
expression.
In contrast to a while
loop, the tail-controlled loop always continues, always keeps going, until
the specified condition becomes true
.
A true
condition marks the end.
In the above example the user will be prompted again and again until he eventually complies and enters a positive number.
Date and time
[edit | edit source]This section introduces you to features of Extended Pascal as defined in the ISO standard 10206. You will need an EP‑compliant compiler to use those features.
Time stamp
[edit | edit source]In EP there is a standard data type called timeStamp
.
It is declared as follows:[fn 5]
type
timeStamp = record
dateValid: Boolean;
timeValid: Boolean;
year: integer;
month: 1..12;
day: 1..31;
hour: 0..23;
minute: 0..59;
second: 0..59;
end;
As you can see from the declaration, timeStamp
also contains data fields for a calendar date, not just the time as indicated by a standard clock.
Getting a time stamp
[edit | edit source]EP also defines a unary procedure
that populates a timeStamp
variable with values.
GetTimeStamp
assigns values to all members of a timeStamp record
passed in the first (and only) parameter.
These values represent the “current date” and “current time” as at the invocation of this procedure.
However, in the 1980’s not all (personal/home) computers did have a built-in “real time” clock.
Therefore, the ISO standard 10206 devised prior 21st century stated that the word “current” was “implementation-defined”.
The dateValid
and timeValid
fields were specifically inserted to address the issue that some computers simply do not know the current date and/or time.
When reading values from a timeStamp
variable, it is still advisable to check their validity first after having getTimeStamp
fill them out.
If getTimeStamp
was unable to obtain a “valid” value, it will set
day
,month
andyear
to a value representing January 1, 1 CE, but alsodateValid
tofalse
.- In the case of time,
hour
,minute
andsecond
become all0
, a value representing midnight. ThetimeValid
field becomesfalse
.
Both are independent from each other, so it may certainly be the case that just the time could be determined, but the date is invalid.
Note that the Gregorian calendar was introduced during the year 1582 CE, so the timeStamp
data type is generally useless for any dates before 1583 CE.
Printable dates and times
[edit | edit source]Having obtained a timeStamp
, EP furthermore supplies two unary functions:
date
returns a human-readablestring
representation ofday
,month
andyear
, andtime
returns a human-readablestring
representation ofhour
,minute
andsecond
.
Both functions will fail and terminate the program
if dateValid
or timeValid
indicate an invalid datum respectively.
Note, the exact format of string
representation is not defined by the ISO standard 10206.
program
:
program dateAndTimeFun(output);
var
ts: timeStamp;
begin
getTimeStamp(ts);
if ts.dateValid then
begin
writeLn('Today is ', date(ts), '.');
end;
if ts.timeValid then
begin
writeLn('Now it is ', time(ts), '.');
end;
end.
Today is 12 Dec 2024. Now it is 03:51:42.
dateValid
and timeValid
are false
.Summary on loops
[edit | edit source]This is a good time to take inventory and reiterate all kinds of loops.
Conditional loops
[edit | edit source]Conditional loops are the tools of choice if you cannot predict the total number of iterations.
head-controlled loop | tail-controlled loop |
---|---|
while condition do
begin
…
end;
|
repeat
begin
…
end
until condition;
|
condition must evaluate to true for any (including subsequent) iterations to occur.
|
condition must be false for any subsequent iteration to occur.
|
It is possible to formulate either loop as the other one, but usually one of them is more suitable.
A tail-controlled loop is particularly suitable if you do not have any data yet to make a judgment, to evaluate a proper condition
prior the first iteration.
Counting loops
[edit | edit source]Counting loops are good if you can predict the total number of iterations before entering the loop.
counting up loop | counting down loop |
---|---|
for controlVariable := initialValue to finalValue do
begin
…
end;
|
for controlVariable := initialValue downto finalValue do
begin
…
end;
|
After each non-final iteration controlVariable becomes succ(controlVariable) . controlVariable must be less than or equal to finalValue for another iteration to occur.
|
After each non-final iteration controlVariable becomes pred(controlVariable) . controlVariable must be greater than or equal to finalValue for another iteration to occur.
|
Both, the initialValue and finalValue expressions, are evaluated exactly once.[4] This is very different to conditional loops.
|
Inside counting loops’ bodies you cannot modify the counting variable, only read it. This prevents you from any accidental manipulations and ensures the calculated predicted total number of iterations will indeed occur.
Loops on aggregations
[edit | edit source]If you are using an EP-compliant compiler, you furthermore have the option to use a for … in
loop on sets.
program forInDemo(output);
type
characters = set of char;
var
c: char;
parties: characters;
begin
parties := ['R', 'D'];
for c in parties do
begin
write(c:2);
end;
writeLn;
end.
Tasks
[edit | edit source]You have made it this far, and it is quite impressive how much you already know.
Since this chapter’s concept of a record
should not be too difficult to grasp, the following exercises mainly focus on training.
A professional computer programmer spends most of his time on thinking what kind of implementation, using which tools (e. g. array
“vs.” set
), is the most useful/reasonable.
You are encouraged to think first, before you even start typing anything.
Nonetheless, sometimes (esp. due to your lack of experience) you need to just try things out, which is fine if it is intentional.
Aimlessly finding a solution does not discern an actual programmer.
record
contain another record
?array
to contain another array
, this is quite possible for a record
too. Write a test program
to see for yourself. The important thing is to note that the dot-notation can be expanded indefinitely (myRecordVariable.topRecordFieldName.nestedRecordFieldName.doubleNestedRecordFieldName
). Evidently at some point it becomes too difficult to read so use this wisely.
while true do
begin
…
end;
The condidition needs to be negated in a repeat … until
loop:
repeat
begin
…
end
until false;
true
, or expressions that can never be fulfilled (in the case of a repeat … until
loop), are not. For instance, given that i
was an integer
the loop while i <= maxInt do
will run indefinitely, because i
can never exceed maxInt
[fn 6] and thus break the loop’s condition. Therefore be reminded to carefully formulate expressions for conditional loops and ensure it will eventually reach a terminating state. Otherwise it can be frustrating for the user of your program
.
while
-loop:
repeat
begin
imagineJumpingSheep;
sheepCount := sheepCount + 1;
waitTwoSeconds;
end
until asleep;
while
-loop even begins:
imagineJumpingSheep;
sheepCount := sheepCount + 1;
waitTwoSeconds;
while not asleep do
begin
imagineJumpingSheep;
sheepCount := sheepCount + 1;
waitTwoSeconds;
end;
repeat … until
-loop is more suitable in this case.
program
that takes the output of the command getent passwd
as input and only prints the first field/column of every line. In a file, fields are separated by a colon (:
). Your program
will list all known user names.getent passwd | ./cut1
(the file name of your executable program may differ).
program cut1(input, output);
const
separator = ':';
var
line: string(80);
begin
while not EOF(input) do
begin
{ This reads the _complete_ line, but at most}
{ line.capacity characters are actually saved. }
readLn(line);
writeLn(line[1..index(line, separator)-1]);
end;
end.
index
will return the index of the colon character which you do not want to print, thus you will need to subtract 1
from its result. This program
will evidently fail if a line does not contain a colon.
program
so only user names whose UID is greater than or equal to 1000
. The UID is stored in the third field.program cut2(input, output);
const
separator = ':';
minimumID = 1000;
var
line: string(80);
nameFinalCharacter: integer;
uid: integer;
begin
while not EOF do
begin
readLn(line);
nameFinalCharacter := index(line, separator) - 1;
{ username:encryptedpassword:usernumber:… }
{ ↑ `nameFinalCharacter + 1` }
{ ↑ `… + 2` is the index of the 1st password character }
uid := index(subStr(line, nameFinalCharacter + 2), separator);
{ Note that the preceding `index` did not operate on `line` }
{ but an altered/different/independent “copy” of it. }
{ This means, we’ll need to offset the returned index once again. }
readStr(subStr(line, nameFinalCharacter + 2 + uid), uid);
{ Read/readLn/readStr automatically terminate reading an integer }
{ number from the source if a non-digit character is encountered. }
{ (Preceding blanks/space characters are ignored and }
{ the _first_ character still may be a sign, that is `+` or `-`.)}
if uid >= minimumID then
begin
writeLn(line[1..nameFinalCharacter]);
end;
end;
end.
subStr
can be omitted effectively meaning “give me the rest of a string
.” Note that this programming task mimics (some of) the behavior of . Use programs/source code that has already been programmed for you whenever possible. Reinventing the wheel is not necessary. Nonetheless, this basic task is a good exercise. On a RHEL system you may rather want to set minimumID
to 500
.
program
meets all requirements. Note, an implementation using an array[1..limit] of Boolean
would have been perfectly fine as well, although the shown set of natural
implementation is in principle preferred.
program eratosthenes(output);
type
{ in Delphi or FPC you will need to write 1..255 }
natural = 1..4095;
{$setLimit 4096}{ only in GPC }
naturals = set of natural;
const
{ `high` is a Borland Pascal (BP) extension. }
{ It is available in Delphi, FPC and GPC. }
limit = high(natural);
{ Note: It is important that `primes` is declared }
{ in front of `sieve` and `list`, so both of these }
{ routines can access the _same_ variable. }
var
primes: naturals;
{ This procedure sieves the `primes` set. }
{ The `primes` set needs to be fully populated }
{ _before_ calling this routine. }
procedure sieve;
var
n: natural;
i: integer;
multiples: naturals;
begin
{ `1` is by definition not a prime number }
primes := primes - [1];
{ find the next non-crossed number }
for n := 2 to limit do
begin
if n in primes then
begin
multiples := [];
{ We do _not_ want to remove 1 * n. }
i := 2 * n;
while i in [n..limit] do
begin
multiples := multiples + [i];
i := i + n;
end;
primes := primes - multiples;
end;
end;
end;
{ This procedures lists all numbers in `primes` }
{ and enumerates them. }
procedure list;
var
count, n: natural;
begin
count := 1;
for n := 2 to limit do
begin
if n in primes then
begin
writeLn(count:8, '.:', n:22);
count := count + 1;
end;
end;
end;
{ === MAIN program === }
begin
primes := [1..limit];
sieve;
list;
end.
sieve
task from the list
task, both routine definitions and the main part of the program
at the bottom remain quite short and are thus easier to understand.
program
that reads an infinite number of numerical values from input
and at the end prints on output
the arithmetic mean.program arithmeticMean(input, output);
type
integerNonNegative = 0..maxInt;
var
i, sum: real;
count: integerNonNegative;
begin
sum := 0.0;
count := 0;
while not eof(input) do
begin
readLn(i);
sum := sum + i;
count := count + 1;
end;
{ count > 0: do not do division by zero. }
if count > 0 then
begin
writeLn(sum / count);
end;
end.
Note that using a data type
excluding negative numbers (here we named it integerNonNegative
) mitigates the issue that count
may flip the sign, a condition known as overflow. This would cause the program
to fail if count := count + 1
became too large, and effectively falls out of the range 0..maxInt
.
maxReal
, no programmatic way to tell that sum
became too large or too small rendering it severely inaccurate, because any value of sum
may be legit nevertheless.
time function
that returns a string
in the “American” time format 9:04 PM
. This may look easy at first, but it can become quite a challenge. Have fun!time
itself. However, the output of time
itself is not standardized, so we will need to define everything by ourselves:
type
timePrint = string(8);
function timeAmerican(ts: timeStamp): timePrint;
const
hourMinuteSeparator = ':';
anteMeridiemAbbreviation = 'AM';
postMeridiemAbbreviation = 'PM';
type
noonRelation = (beforeNoon, afterNoon);
letterPair = string(2);
var
{ contains 'AM' and 'PM' accessible via an index }
m: array[noonRelation] of letterPair;
{ contains a leading zero accessible via a Boolean expression }
z: array[Boolean] of letterPair;
{ holds temporary result }
t: timePrint;
begin
{ fill `t` with spaces }
writeStr(t, '':t.capacity);
This fallback value (in the case ts.timeValid
is false
) allows the programmer/“user” of this function
to “blindly” print its return value. There will be a noticeable gap in the output. Another sensible “fallback” value would be an empty string
.
with ts do
begin
if timeValid then
begin
m[beforeNoon] := anteMeridiemAbbreviation;
m[afterNoon] := postMeridiemAbbreviation;
z[false] := '';
z[true] := '0';
writeStr(t,
((hour + 12 * ord(hour = 0) - 12 * ord(hour > 12)) mod 13):1,
hourMinuteSeparator,
z[minute < 10], minute:1, ' ',
m[succ(beforeNoon, hour div 12)]);
This is the most complicated part of this problem. First of all, all number parameters to writeStr
are explicitly suffixed with :1
as the minimum-width specification, because there are some compilers that would otherwise assume, for example, :20
as a default value. Since we know that timeStamp.hour
is in the range 0..23
we can use the div
and mod
operations as demonstrated. However, we will need account of an hour
value of 0
, which is usually denoted as 12:00 AM (and not zero). A conditional “shift” by 12 using the shown Boolean
expression and ord
“fixes” this. Furthermore, here is a brief reminder that in EP the succ
function accepts a second parameter.
end;
end;
timeAmerican := t;
end;
Sources:
- ↑ Wirth, Niklaus (1979). "The Module: a system structuring facility in high-level programming languages". proceedings of the symposium on language design and programming methodology. Berlin, Heidelberg: Springer. Abstract. doi:10.1007/3-540-09745-7_1. ISBN 978-3-540-09745-7. https://link.springer.com/content/pdf/10.1007%2F3-540-09745-7_1.pdf. Retrieved 2021-10-26.
- ↑ Cooper, Doug. "Chapter 11. The
record
Type". Oh! Pascal! (third edition ed.). p. 374. ISBN 0-393-96077-3.[…] records have two unique aspects:
First, the stored values can have different types. This makes records potentially heterogeneous—composed of values of different kinds. Arrays, in contrast, hold values of just one type, so they're said to be homogeneous.
[…]{{cite book}}
:|edition=
has extra text (help); line feed character in|quote=
at position 269 (help); syntaxhighlight stripmarker in|chapter=
at position 17 (help) - ↑ Wirth, Niklaus (1973-07-00). The Programming Language Pascal (Revised Report ed.). p. 30.
Within the component statement of the with statement, the components (fields) of the record variable specified by the with clause can be denoted by their field identifier only, i.e. without preceding them with the denotation of the entire record variable.
{{cite book}}
: Check date values in:|date=
(help) - ↑ Jensen, Kathleen; Wirth, Niklaus. Pascal – user manual and report (4th revised ed.). p. 39. doi:10.1007/978-1-4612-4450-9. ISBN 978-0-387-97649-5.
The initial and final values are evaluated only once.
Notes:
- ↑ This kind of
record
will not be able to store anything. In the next chapter you will learn a (and the only) instance it could be useful. - ↑ Indeed most compilers consider the dot as a dereferencing indicator and the field name denotes a static offset from a base memory address.
- ↑ In Standard (“unextended”) Pascal, ISO standard 7185, a
function
can only return “simple data type” and “pointer data type” values. - ↑ Actually the shown
begin … end
is redundant sincerepeat … until
constitute a frame in their own right. For pedagogical reasons we teach you to always usebegin … end
nevertheless wherever a sequence of statements usually appears. Otherwise you might change yourrepeat … until
loop to awhile … do
loop forgetting to surround the loop’s body statements with a properbegin … end
frame. - ↑ The
packed
designation has been omitted for simplicity. - ↑ According to most compilers’ definition of
maxInt
. The ISO standards merely require, that all arithmetic operations in the interval-maxInt..maxInt
work absolutely correct, but it is thinkable (although unlikely) that more values are supported.