Pascal Programming/Sets
This chapter introduces you to a new custom data type. Sets are one of the basic structured data types. When programming you will frequently find that some logic can be modeled with sets. Learning and mastering usage of sets is a key skill, since you will encounter them a lot in Pascal.
Basics
editNotion
editSets are (possibly empty) aggregations of distinguishable objects. Either a set contains an object, or it does not. An object being part of a set is also referred to as element of that set.
Let's say we know the objects “apple”, “banana” and “pencil”. The set Fruit ≔ {“apple”, “banana”} contains the objects “apple” and “banana”. “Pencil” is not a member of the set Fruit.
Digitization
editWhen a computer is supposed to store and process a set, it actually handles a series of Boolean
values.^{[fn 1]}
Every one of those Boolean
values tells us whether a certain element is part of a set.
A computer does also store a Boolean value for every object that is not part of a certain set.

Sets in Pascal
editThe computer needs to know how many Boolean
values it needs to set aside.
In order to achieve this, a set in Pascal requires an ordinal type as a set’s base type.
An ordinal type always has a finite range of permissible discrete values, thus the computer knows beforehand how many Boolean
values to reserve, how many elements we can expect a set contain at most.
In consequence, a valid set type
declaration is:
type
characters = set of char;
A variable of the data type characters
can only contain char
values.
This set cannot contain, for instance, 42
, that is an integer
value, nor is this information stored in any way.
A data type declaration for a set of real is illegal, since the set’s base type, the data type real , is not an ordinal data type.
Remember, in order to qualify as an ordinal data type there must be a means to assign every legal value an integer value.^{[fn 2]} 
Sets are particularly useful in conjunction with enumeration data types, which you just learned in previous chapter. Let’s consider an example in Pascal:
program setDemo;
type
skill = (cooking, cleaning, driving, videogames, eating);
skills = set of skill;
var
slob: skills;
begin
slob := [videogames, eating];
end.
Here, we have declared a variable slob
, which represents a set of the skill
enumeration data type values.
In the penultimate line we populate our set slob
with two objects, videogames
and eating
.
The brackets indicate a set literal.
[videogames, eating]
is a set expression which we are assigning to the slob
variable.
The set variable slob
contains no other objects.
However, the computer still stores five Boolean
values for every potential member of that set.
The number five is number of elements in skill
, the set’s base type.
The information that cooking
, cleaning
and driving
are not part of the set slob
is stored explicitly (by the proper Boolean
value false
).
Inspecting a set
editIf we want to learn, whether a certain object is part of a set, the set operator in
yields the corresponding Boolean
value the computer uses to store that information.
program setInspection(output);
type
skill = (cooking, cleaning, driving, videogames, eating);
skills = set of skill;
var
slob: skills;
begin
slob := [videogames, eating];
if videogames in slob then
begin
writeLn('Is she a level 45 dungeon master?');
end;
end.
The in
operator is one of Pascal’s noncommutative operators.
This means, you cannot swap the operands.
On the RHS you always need to write an expression evaluating to a set
value, whereas the LHS has to be an expression evaluating to this set’s base type.
Even though we, as humans, can say that 42 in slob
is wrong, i. e. false
, such a comparison is illegal.
Per definition, the slob
set can only contain skill
values.
Operations
editSo far, sets probably seemed like a really complicated way for using Boolean
values.
The true power of sets lies in a number of distinct operations, making sets an easier, and thus better alternative to handling two or more individual (but related) Boolean
values directly.
Combinations
editIn Pascal, two sets of the same kind, the same data type, can be combined forming a new set of the respective data type. Following operators are available:
name  mathematical symbol  source code symbol 

union  ∪  +

difference  ∖  

intersection  ∩  *

symmetric difference  △  >< ^{†}

_{† The symmetric difference operator is only defined in EP.}
Union
editThe result of unifying two sets into one is called union. Let’s say, recently our slob has learned how to drive and does that now too. This can be written as:
slob := slob + [driving];
Now, slob
contains all objects it previously held, plus all objects from the other set, [driving]
.
Difference
editOf course sets can be deprived of a set of elements by using the difference operator, in source code written as 
.
slob := slob  [];
This removes all objects present in the second set from the first set.
Here, the empty set ([]
) does not contain any objects, thus removing no objects has virtually no effect on slob
.
Intersection
editFurthermore you can intersect sets. The intersection of two sets is defined as the set of elements both operands contain.
program intersectionDemo;
type
skill = (cooking, cleaning, driving, videogames, eating);
skills = set of skill;
var
slob, blueCollar, common: skills;
begin
blueCollar := [cooking, cleaning, driving, eating];
slob := [driving, videogames, eating];
common := blueCollar * slob;
end.
The set common
now (only) contains driving
and eating
, because those are the objects member of both operands, of both given sets.
Symmetric difference
editA disjunct result to the intersection gives the symmetric difference. It is the union of the operands without the elements contained in both sets.
unique := blueCollar >< slob;
Now unique
is [cooking, cleaning, videogames]
, because those are the values from either set, but not both.
Comparisons
editTwo sets of the same kind, the same data type, can be compared by looking at each element in both sets.
name  mathematical symbol  source code symbol 

equality  =  =

inequality  ≠  <>

inclusion  ⊆  <=

inclusion  ⊇  >=

element  ∈  in

All comparison operators, as before, evaluate to a Boolean
expression.
Inclusion
editThe inclusion of a two sets means that all objects one set contains are present in another set.
If the expression A <= B
evaluates to true
, all objects present in the set A
are also present B
.
In a Venn diagram you will notice that one circle’s area is completely surrounded by another circle, if not identical to the other circle.
Equality and inequality
editThe equality of two sets is defined as A <= B and B <= A
.
All objects contained in the lefthand set are present in the righthand set and vice versa.
In other words, there is not a single object that is present in just one of the sets.
The inequality is just the negation thereof.
Element of
editThe in
operator is the only set operator that does not act on two sets but on one potential set member candidate and a set.
It has been introduced above.
With respect to Venn diagrams, though, you can say that the in
operator is “like” pointing with your index finger to a point inside a circle, or outside of it.
Predefined set routines
editCardinality
edit(After initialization) at any time a set contains a certain number of elements.
In mathematics the number of objects being part of a set is called cardinality.
The cardinality of a set can be retrieved using the function card
, an EP extension.
emptySet := [];
writeLn(card(emptySet));
This will print 0
as there are no elements in an empty set.
Unfortunately, not all compilers implement the card
function.
The FPC does not have none.
The GPC does supply one, though.
Universe
editOriginally, Wirth proposed a function all
:
^{[1]}
all(T)
is the set of all values of type T
For example:
superwoman := all(skill);
The set superwoman
would contain all available skill
values, cooking
, cleaning
, driving
, videogames
, eating
.
Unfortunately, this proposal never made it into the ISO standards, nor do the FPC or GPC support that function, or provide an equivalent.
The only alternative is to use an appropriate set constructor (an EP extension):
[cooking..eating]
is equivalent to all(skill)
, provided that cooking
is the first skill
and eating
the last skill
value (referring to the order these items were listed during the data type declaration of skill
).
Inclusion and exclusion
editNot standardized, but convenient is BP’s definition of include
and exclude
procedures.
These are shorthand for very frequent set manipulations.
The procedures allow you to quickly add or remove one object from one set.
include(recognizedLetters, 'Z');
is identical to
recognizedLetters := recognizedLetters + ['Z'];
but you do not need to type out the set name twice and everything, thus reducing the chance of typing mistakes. Likewise,
exclude(primeSieve, 4);
will do the same as
primeSieve := primeSieve  [4];
Both, the FPC and GPC, support these handy routines, which are in fact in all cases implemented as compiler intrinsics, not actual procedure
s.
Intermediate usage
editSet literals
editEffectively stating sets is a required skill when handling sets.
It is important to understand that sets merely store the information that an object is a member of a set, or not.
The set ['A', 'A', 'A']
is identical to ['A']
.
Specifying 'A'
multiple times does not make it “more” part of that set.
Also, it is not necessary to list all members in any particular order.
['X', 'Z', 'Y']
is just as acceptable as ['X', 'Y', 'Z']
is.
Mathematically speaking, sets are not ordered.
Pascal’s requirement that a set’s base data type has to be an ordinal type is purely a technical requirement.
For readability reasons it is usually sensible, though, to list elements in ascending order.
The EP standard gives you nice short notation for set
literals containing a continuous series of values.
Instead of writing [7, 8, 9, 11, 12, 13]
you can also write ranges like [7..9, 11..13]
evaluating to the very same value.
Of course, all numbers could also be variables, or expressions in general.
Set literals are always a positive statement which objects are in a set.
If we wanted a set of integer
values between 0
and 10
without 3
, 5
and 7
, but do not want to write this set out entirely (i. e. as [0, 1, 2, 4, 6, 8, 9, 10]
), you can either write [0..2, 4, 6, 8..10]
or the expression [0..10]  [3, 5, 7]
.
The latter is probably a little easier to grasp what objects are and which are not in the final set.
Memory restrictions
editAlthough a set of integer
is legal and complies with all Pascal standards, many compilers do not support such large sets.
Per definition, a set of integer
can contain (at most) all values in the range maxInt..maxInt
.
That is a lot (try writeLn(maxInt)
or read your compiler’s documentation to find out this value).
On a 64‑bit platform this value (usually) is 2^{63}−1, i. e. 9,223,372,036,854,775,808.
As of the year 2020 many computers will quickly run out of main memory if they attempted to hold that many Boolean
values.
As a consequence, BP restricts permissible set’s base types.
In BP the base type’s largest and smallest values’ ordinal values have to be in the range 0..255
.
The value 255
is 2^{8}−1.
As of version 3.2.0, the FPC sets the same limitations.
The GPC allows set
definitions beyond 2^{8} elements, although some configuration is required:
You need to specify the ‑‑setlimit
commandline parameter or a specially crafted comment in your source code:
program largeSetDemo;
{$setLimit=2147483647} // this is 2³¹ − 1
type
// 1073741823 is 2³⁰ − 1
characteristicInteger = 1073741823..1073741823;
integers = set of characteristicInteger;
var
manyManyIntegers: integers;
begin
manyManyIntegers := [];
include(manyManyIntegers, 1073741823);
end.
This will instruct the GPC that a set of characteristicInteger
can only store up to this many characteristicInteger
values.
Loops
editNow that you have made the acquaintance of enumeration data types and sets, you see yourself faced with dealing a growing number of data. Pascal, like many other programming languages, support a language construct called loops.
Characteristics
editLoops are (possibly empty) sequences of statements that are repeated over and over again, or even never, based on a Boolean
value.
The sequence of statements is termed loop body.
The loop head contains (possibly implicitly) a Boolean
expression determining whether the loop body is executed.
Every time the loop body is run, an iteration is in progress.
The term loop originates from the circumstance that some early models of computers required programs to be fed (“loaded”) via punched paper tape. If a portion of that paper tape was meant to be processed multiple times, that piece of paper tape was cut, bent and temporarily fixated so it formed a physical loop. Thankfully, advancements in computer technology has made it far more convenient to handle repeating code.
Pascal (and many other programming languages) differentiate between two groups of loops:
 counting loops, presented here, and
 conditional loops, presented in a chapter to come.
Counting loops have in common that, before running the first iteration it can already be determined how many times the loop body will be executed just by evaluating the loop head.^{[fn 3]}
Conditional loops on the other hand are based on an abort condition, i. e. a Boolean
expression.
Except for infinite loops, there is no way to tell in advance how many times, how many iterations a conditional loop will have without thoroughly (mathematically) analyzing the loop body and loop head, and possibly even considering circumstances beyond the loop.
Counting loops
editCounting loops do not necessarily count a quantity. They are named after the fact that they employ a variable, a counting variable. This variable of any ordinal data type (de facto) assigns every iteration a number.
A counting loop is introduced by the reserved word for
:
program forLoopDemo(output);
var
i: integer;
begin
for i := 1 to 10 do // loop head
begin // ┐
writeLn(i:3); // ┝ loop body
end; // ┘
end.
After for
follows a specially crafted assignment to the counting variable.
Range of counting variable
edit1 to 10
(with the auxiliary reserved word to
) denotes a range of values the counting variable i
will assume while executing the loop body.
1
and 10
are both expressions possessing the counting variable’s data type, that means there could also appear variables or more complex expressions, not just constant literals as shown.
This range is like a set
.
It may possibly be empty:
The range 5 to 4
is an empty range, since there are no values between 5
up to and including 4
.
In consequence, the counting variable will not be assigned any value out of this empty range, as there simply are none available, and the loop body is never executed.
Nevertheless, the range 8 to 8
contains exactly one value, i. e. 8
.
During the first iteration the corresponding counting variable, here i
, will have the first value out of the given range, the start value, in the example above this is the value 1
.
In the successive iteration the variable i
has the value 2
, and so forth up to and including the final value of the given range, here 10
.
Immutability of counting variable
editIt is not necessary to actually utilize the counting variable inside the loop body, but you can use it if you are just obtaining its current value:
Inside the loop body of for
‑loops it is forbidden to assign any values to the counting variable.
Forbidden assignments include, but are not limited to putting the counting the variable on the LHS of :=
, but also read
/readLn
may not use the counting variable.
Tampering with the counting variable is forbidden, because the loop head will effectively employ succ(i)
to obtain the next iteration’s value.
The loop head implicitly contains the Boolean
expression counting variable ≠ final value.
If the counting variable was manipulated this condition might never be met, thus destroying the characteristics of a for
‑loop.
Preventing the programmer to do any assignments preemptively ensures such an infinite loop is not, accidentally as well as deliberately, created.
Reverse direction
editPascal also allows for
‑loops in a reversed direction using the reserved word downto
instead of to
:
program downtoDemo(output);
var
c: char;
begin
for c := 'Z' downto 'B' do
begin
write(c, ' ');
end;
writeLn('A');
end.
Here, the range is 'Z'
down and including to 'B'
.
The loop’s terminating condition is still counting variable ≠ final value, but in this case the counting variable c
becomes pred(c)
(not succ
) at the end of each iteration, after the loop body has been executed.
Loops on collections
editEP allows to iterate over discrete aggregations, such as sets. This is particularly useful if you have a routine that needs to be applied to every value of an aggregation. Here is an example to demonstrate the principle:
program forInDemo(output);
var
c: char;
vowels: set of char;
begin
vowels := ['A', 'E', 'I', 'O', 'U'];
for c in vowels do
begin
writeLn(c);
end;
end.
Now you see the word in
again, but in this context c in vowels
is not an expression.
The data type restrictions for in
are still in effect:
On the RHS an aggregation expression is given, whereas the LHS is in this case a variable that has the aggregation’s data type.
This variable will be assigned every value out of the given aggregation every time an iteration is processed.
Since the RHS just needs an expression, not necessarily a variable, so you can shorten the example even further to:
for c in ['A', 'E', 'I', 'O', 'U'] do
Note, unlike the counting loops above, you are not supposed to make any assumptions about the order the loop variable is assigned values to.
It may be in ascending, descending, or completely mixed up “order”, but the specific order is “implementation defined”, i. e. it depends on the used compiler.
Accompanying documents of the compiler explain in which order the for … in
loop is processed.
Tasks
editfruit  fruit
evaluate to?Sources:
 ↑ Wirth, Niklaus. The Programming Language Pascal (Revised Report ed.). p. 38.
{{cite book}}
: Unknown parametermonth =
ignored (help); Unknown parameteryear =
ignored (help)
Notes:
 ↑ This is an analogy for explanation. The ISO standards do not set this requirement. Compiler developers are free to choose whatever implementation of the set data type they think is suitable. It would be perfectly OK to just store a list of elements that are present in a set and nothing else. Nevertheless, all, Delphi, FPC, as well as the GPC do store sets as a series of bits (“
Boolean
values”) as it is explained here.  ↑ The data type
real
does not qualify as an ordinal data type. Although it stores a finite subset of ℚ, the set of rational numbers, so there is a map ℚ_{T} ↦ ℕ, this depends on thereal
data type’s precision (T), thus there is no one standard way of definingord
forreal
values, but many.  ↑ This statement ignores many dialects’ extensions
break
/leave
/continue
/cycle
that neither the ISO standards 7185 (Standard Pascal) or 10206 (Extended Pascal) define, or thegoto
statement.