Chapter 5: Data Structures

Data structures organize and manage the data in a program. Pascal’s built-in data structures let you store and work with data efficiently, and this chapter covers three of them: arrays, records, and sets.

Arrays

An array is a collection of elements of the same type, stored in contiguous memory locations. It holds several values in a single variable, and you reach each value through its index.

Declaring arrays

var
  arrayName: array[indexRange] of dataType;

Example

program ArrayExample;

var
  numbers: array[1..5] of integer;
  i: integer;

begin
  { Initializing array elements }
  for i := 1 to 5 do
    numbers[i] := i * 2;

  { Accessing array elements }
  for i := 1 to 5 do
    writeln('numbers[', i, '] = ', numbers[i]);
end.

Here numbers is an array of integers with indices from 1 to 5. The program sets each element in one loop and reads them back in another.

Multidimensional arrays

Pascal also supports multidimensional arrays. A common one is the two-dimensional array, which you can picture as a table of rows and columns.

Example

program MultiArrayExample;

var
  matrix: array[1..3, 1..3] of integer;
  i, j: integer;

begin
  { Initializing matrix elements }
  for i := 1 to 3 do
    for j := 1 to 3 do
      matrix[i, j] := i * j;

  { Accessing matrix elements }
  for i := 1 to 3 do
  begin
    for j := 1 to 3 do
      write(matrix[i, j]:4);
    writeln;
  end;
end.

matrix is a two-dimensional array with 3 rows and 3 columns. Nested loops set its elements and then read them back.

Records

A record is a composite data type that groups data of different types together, which makes records a good fit for complex data structures.

Declaring records

type
  RecordName = record
    field1: dataType1;
    field2: dataType2;
    ...
  end;

Example

program RecordExample;

type
  Person = record
    name: string;
    age: integer;
    salary: real;
  end;

var
  employee: Person;

begin
  { Initializing record fields }
  employee.name := 'Alice';
  employee.age := 30;
  employee.salary := 50000.0;

  { Accessing record fields }
  writeln('Name: ', employee.name);
  writeln('Age: ', employee.age);
  writeln('Salary: ', employee.salary:0:2);
end.

Person is a record type with three fields: name, age, and salary. The employee variable is an instance of Person.

Sets

A set is a collection of elements of the same type, with no duplicates. Sets are handy whenever you need membership tests, union, intersection, or difference.

Declaring sets

var
  setName: set of baseType;

Example

program SetExample;

type
  Day = (Mon, Tue, Wed, Thu, Fri, Sat, Sun);

var
  weekdays, weekend: set of Day;
  d: Day;

begin
  weekdays := [Mon, Tue, Wed, Thu, Fri];
  weekend := [Sat, Sun];

  { Set operations }
  if Tue in weekdays then
    writeln('Tuesday is a weekday');

  writeln('Weekend days: ');
  for d in weekend do
    write(d, ' ');
  writeln;
end.

Day is an enumerated type for the days of the week, and weekdays and weekend are sets of type Day. The program checks whether a day is in a set, then iterates over a set’s elements.

Set operations

Pascal’s set operations include:

• Union: +
• Intersection: *
• Difference: -
• Symmetric difference: ><

To add or remove a single element, use the procedures Include(s, x) and Exclude(s, x).

Example

program SetOperationsExample;

type
  Day = (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
  DaySet = set of Day;

procedure PrintSet(const title: string; s: DaySet);
var
  d: Day;
begin
  write(title, ': ');
  for d in s do
    write(d, ' ');
  writeln;
end;

var
  setA, setB, resultSet: DaySet;

begin
  setA := [Mon, Wed, Fri];
  setB := [Wed, Thu, Fri, Sat];

  resultSet := setA + setB;  { Union }
  PrintSet('Union', resultSet);

  resultSet := setA * setB;  { Intersection }
  PrintSet('Intersection', resultSet);

  resultSet := setA - setB;  { Difference }
  PrintSet('Difference', resultSet);

  resultSet := setA >< setB;  { Symmetric Difference }
  PrintSet('Symmetric Difference', resultSet);
end.

writeln can’t print a whole set, so PrintSet loops over the elements and prints each one.