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Pascal arrays allow you to define type of variables that can hold several data items of the same kind but a record is another user-defined data type available in Pascal which allows you to combine data items of different kinds.
Records consist of different fields. Suppose you want to keep track of your books in a library, you might want to track the following attributes about each book −
- Title
- Author
- Subject
- Book ID
Defining a Record
To define a record type, you may use the type declaration statement. The record type is defined as −
type record-name = record field-1: field-type1; field-2: field-type2; ... field-n: field-typen; end;
Here is the way you would declare the Book record −
type Books = record title: packed array [1..50] of char; author: packed array [1..50] of char; subject: packed array [1..100] of char; book_id: integer; end;
The record variables are defined in the usual way as
var r1, r2, ... : record-name;
Alternatively, you can directly define a record type variable as −
var Books : record title: packed array [1..50] of char; author: packed array [1..50] of char; subject: packed array [1..100] of char; book_id: integer; end;
Accessing Fields of a Record
To access any field of a record, we use the member access operator (.). The member access operator is coded as a period between the record variable name and the field that we wish to access. Following is the example to explain usage of structure −
program exRecords;
type
Books = record
title: packed array [1..50] of char;
author: packed array [1..50] of char;
subject: packed array [1..100] of char;
book_id: longint;
end;
var
Book1, Book2: Books; (* Declare Book1 and Book2 of type Books *)
begin
(* book 1 specification *)
Book1.title := ''C Programming'';
Book1.author := ''Nuha Ali '';
Book1.subject := ''C Programming Tutorial'';
Book1.book_id := 6495407;
(* book 2 specification *)
Book2.title := ''Telecom Billing'';
Book2.author := ''Zara Ali'';
Book2.subject := ''Telecom Billing Tutorial'';
Book2.book_id := 6495700;
(* print Book1 info *)
writeln (''Book 1 title : '', Book1.title);
writeln(''Book 1 author : '', Book1.author);
writeln( ''Book 1 subject : '', Book1.subject);
writeln( ''Book 1 book_id : '', Book1.book_id);
writeln;
(* print Book2 info *)
writeln (''Book 2 title : '', Book2.title);
writeln(''Book 2 author : '', Book2.author);
writeln( ''Book 2 subject : '', Book2.subject);
writeln( ''Book 2 book_id : '', Book2.book_id);
end.
When the above code is compiled and executed, it produces the following result −
Book 1 title : C Programming Book 1 author : Nuha Ali Book 1 subject : C Programming Tutorial Book 1 book_id : 6495407 Book 2 title : Telecom Billing Book 2 author : Zara Ali Book 2 subject : Telecom Billing Tutorial Book 2 book_id : 6495700
Records as Subprogram Arguments
You can pass a record as a subprogram argument in very similar way as you pass any other variable or pointer. You would access the record fields in the similar way as you have accessed in the above example −
program exRecords;
type
Books = record
title: packed array [1..50] of char;
author: packed array [1..50] of char;
subject: packed array [1..100] of char;
book_id: longint;
end;
var
Book1, Book2: Books; (* Declare Book1 and Book2 of type Books *)
(* procedure declaration *)
procedure printBook( var book: Books );
begin
(* print Book info *)
writeln (''Book title : '', book.title);
writeln(''Book author : '', book.author);
writeln( ''Book subject : '', book.subject);
writeln( ''Book book_id : '', book.book_id);
end;
begin
(* book 1 specification *)
Book1.title := ''C Programming'';
Book1.author := ''Nuha Ali '';
Book1.subject := ''C Programming Tutorial'';
Book1.book_id := 6495407;
(* book 2 specification *)
Book2.title := ''Telecom Billing'';
Book2.author := ''Zara Ali'';
Book2.subject := ''Telecom Billing Tutorial'';
Book2.book_id := 6495700;
(* print Book1 info *)
printbook(Book1);
writeln;
(* print Book2 info *)
printbook(Book2);
end.
When the above code is compiled and executed, it produces the following result −
Book 1 title : C Programming Book 1 author : Nuha Ali Book 1 subject : C Programming Tutorial Book 1 book_id : 6495407 Book 2 title : Telecom Billing Book 2 author : Zara Ali Book 2 subject : Telecom Billing Tutorial Book 2 book_id : 6495700
Pointers to Records
You can define pointers to records in very similar way as you define pointer to any other variable as follows −
type record-ptr = ^ record-name; record-name = record field-1: field-type1; field-2: field-type2; ... field-n: field-typen; end;
Now, you can store the address of a record type variable in the above-defined pointer variable. To declare a variable of the created pointer type, you use the var keyword −
var r1, r2, ... : record-ptr;
Before using these pointers, you must create storage for a record-name type variable, which will be manipulated by these pointers.
new(r1); new(r2);
To access the members of a record using a pointer to that record, you must use the ^. operator as follows −
r1^.feild1 := value1; r1^.feild2 := value2; ... r1^fieldn := valuen;
Finally, don”t forget to dispose the used storage, when it is no longer in use −
dispose(r1); dispose(r2);
Let us re-write the first example using a pointer to the Books record. Hope this will be easy for you to understand the concept −
program exRecords;
type
BooksPtr = ^ Books;
Books = record
title: packed array [1..50] of char;
author: packed array [1..50] of char;
subject: packed array [1..100] of char;
book_id: longint;
end;
var
(* Declare Book1 and Book2 of pointer type that refers to Book type *)
Book1, Book2: BooksPtr;
begin
new(Book1);
new(book2);
(* book 1 specification *)
Book1^.title := ''C Programming'';
Book1^.author := ''Nuha Ali '';
Book1^.subject := ''C Programming Tutorial'';
Book1^.book_id := 6495407;
(* book 2 specification *)
Book2^.title := ''Telecom Billing'';
Book2^.author := ''Zara Ali'';
Book2^.subject := ''Telecom Billing Tutorial'';
Book2^.book_id := 6495700;
(* print Book1 info *)
writeln (''Book 1 title : '', Book1^.title);
writeln(''Book 1 author : '', Book1^.author);
writeln( ''Book 1 subject : '', Book1^.subject);
writeln( ''Book 1 book_id : '', Book1^.book_id);
(* print Book2 info *)
writeln (''Book 2 title : '', Book2^.title);
writeln(''Book 2 author : '', Book2^.author);
writeln( ''Book 2 subject : '', Book2^.subject);
writeln( ''Book 2 book_id : '', Book2^.book_id);
dispose(Book1);
dispose(Book2);
end.
When the above code is compiled and executed, it produces the following result −
Book 1 title : C Programming Book 1 author : Nuha Ali Book 1 subject : C Programming Tutorial Book 1 book_id : 6495407 Book 2 title : Telecom Billing Book 2 author : Zara Ali Book 2 subject : Telecom Billing Tutorial Book 2 book_id : 6495700
The With Statement
We have discussed that the members of a record can be accessed using the member access operator (.). This way the name of the record variable has to be written every time. The With statement provides an alternative way to do that.
Look at the following code snippet taken from our first example −
(* book 1 specification *) Book1.title := ''C Programming''; Book1.author := ''Nuha Ali ''; Book1.subject := ''C Programming Tutorial''; Book1.book_id := 6495407;
The same assignment could be written using the With statement as −
(* book 1 specification *) With Book1 do begin title := ''C Programming''; author := ''Nuha Ali ''; subject := ''C Programming Tutorial''; book_id := 6495407; end;
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