Pascal Const Array

Delphi passes the length as a hidden parameter to the subroutine. An open array may also be defined with const value type. This is called a Variant open array - it. What does const mean in C? Const is a keyword. More specifically, const is a type qualifier.Type qualifiers are part of C types. In the type int const, const is a type qualifier, and int is a type specifier.Here, const qualifies the type int.Qualifiers change the semantics of the type in some way.

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1C - Constants (author: Tao Yue, state: changed)

(update 2019-12-22 Paul Robinson to add record, array constants)

Constants are referenced by identifiers, and can be assigned one value at the beginning of the program. The value stored in a constant cannot be changed.

Constants come in three flavors: Scalars, records, and arrays. A scalar constant is a single identifier which is assigned a single value. A record constant is a single identifier holding one or more separate values in a structured form. An array constant holds multiple values. They will be explained in greater detail in separate sections below.

A constant, whether a scalar, a record field, or an array subscript, may be used in place of a literal of the same type, or may be used anywhere a variable may be used with two exceptions. They cannot be used as the target of an assignment statement, and they cannot be passed as a VAR parameter in a call to a procedure, function, method or property.

• 3Array constants
  • 3.1One-dimensional arrays

Scalar constants

Scalar constants are defined in the Const (constant) section of the program:

For example, let's define some constants of various data types: strings, characters, integers, reals, and Booleans. These data types will be further explained in the next section.

Note that in Pascal, single characters are enclosed in single quotes, or apostrophes (')! This contrasts with newer languages which often use or allow double quotes or Heredoc notation. Standard Pascal does not use or allow double quotes to mark characters or strings.

Constants are useful for defining a value which is used throughout your program but may change in the future. Instead of changing every instance of the value, you can change just the constant definition.

Typed constants force a constant to be of a particular data type. For example,

would yield an identifier a which contains a real value 12.0 instead of the integer value 12.

Record constants

Record constants are created by creating a record type with one or more fields, then creating a constant that references that record type, filling the field(s) with values.

For record constants, the general format is

const

identifier: record_type = ( field_values );

identifier …

Where

• identifier is the name of the record constant;
• record_type is the name of the record type used to describe this record constant and
• field_values is a list consisting of a field name from the record_type referenced earlier, a colon, and the value to be assigned to that field, separated from the next field:value pair by a semi colon.

Lets try a simple example, a complex number constant.

Note that fields must be initialized in the same order that they are declared (or the compiler will flag an error). If a record value is missing the compiler will warn of an uninitialized value. The values assigned can be literals or may be a scalar constant.

For how to describe a constant array of record, see the section below.

Array constants

Array constants work much the same as scalar constants, except multiple values can be specified. All of the values must be the same type, whether it is a number (byte, word, integer. real, etc.) or character based (char, string, ansistring, etc.)

One-dimensional arrays

For one-dimensional array constants. the general format is:

const

identifier: array[low_value . high_value] of type = ( values );

identifier …

Where

• identifier is the name of the array;
• low_value is the lower bound of the array;
• high_value is the upper bound of the array;
• type is the type of value stored in the elements of the array (char, integer, real, string, etc.) and
• values is a list of values with each item in the list separated from the next item by a comma.

Here's a simple one, the alphabet:

There are two rules to remember. As was said before, all the values given have to be of the same type, and you have to specify as many values as there are elements in the array. While the example given above is okay, there will be other parts of the program that reference this array, such as for loops. etc. Instead of using the literal 26 for the size of the array, let's use a scalar constant instead:

Now, any place you need to reference the size of the array, you can just use LetterCount.

Here's a more interesting example, in which several data types are used. This example, which is probably from a calendar program, has all sorts of types: char, string, and integer.

Notice that for the items indicated as type string, while every value had to be a string, they were not required to all be the same size.

One-dimensional array of record

An array constant may consist of records. To describe them, we use the parentheses ( ) around each record. Using the complex type defined in the section on record constants, we can create a constant array of complex records, as follows:

As before, values need not be literals, they can be other scalar constants. Also, values must be given in the same order as defined in the record.

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Now for a more (pun unintentional) complex example, we use an enumerated type and a set to define a record constant array:

Take notice of how the items are delimited (separated). Elements in the record are delimited by semi colons, while elements of the array are delimited by commas.

Two-dimensional array constant

As used in the example of array of record, two-dimensional arrays are described by encapsulating each of the second dimension in parentheses. To make it easier to understand, the entries in the array have been identified by number, e.g. 13 is the first row, third column, while 21 is the second row, first column. Here is the format:

Three-dimensional array constant

Three-dimensional arrays are rare, and you might never use one, but you should know how to construct a three-dimensional constant array if you need it. Again, each level of array is encapsulated by parentheses. Here is a three-dimensional constant array, and the values represent their position in the array.

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Pointers in Pascal are easy and fun to learn. Some Pascal programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect Pascal programmer. Let's start learning them in simple and easy steps.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using the name of the pointer variable, which denotes an address in memory. Ewa keygen 1.26.59.15.

What Are Pointers?

A pointer is a dynamic variable, whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is −

The pointer type is defined by prefixing the up-arrow of caret symbol (^) with the base type. The base-type defines the types of the data items. Once a pointer variable is defined to be of certain type, it can point data items of that type only. Once a pointer type has been defined, we can use the var declaration to declare pointer variables.

Following are some valid pointer declarations −

The pointer variables are dereferenced by using the same caret symbol (^). For example, the associated variable referred by a pointer rptr, is rptr^. It can be accessed as −

The following example will illustrate this concept −

When the above code is compiled and executed, it produces the following result −

Printing a Memory Address in Pascal

In Pascal, we can assign the address of a variable to a pointer variable using the address operator (@). We use this pointer to manipulate and access the data item. However, if for some reason, we need to work with the memory address itself, we need to store it in a word type variable.

Let us extend the above example to print the memory address stored in the pointer iptr −

When the above code is compiled and executed, it produces the following result −

NIL Pointers

It is always a good practice to assign a NIL value to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NIL points to nowhere. Consider the following program −

When the above code is compiled and executed, it produces the following result −

To check for a nil pointer you can use an if statement as follows −

Pascal Pointers in Detail

Pointers have many but easy concepts and they are very important to Pascal programming. There are following few important pointer concepts, which should be clear to a Pascal programmer −