The data type conception. Variables and constants. Assignment operation

program, intended for the realization on the COMPUTER, represents the

formalized description of any task solution algorithm. The actions which are carried out by the program according to this algorithm are directed on the transformation of some objects which determine the current condition of the task solution process. Such internal objects of the program are called the data. For the storage of any data element the observant system allocates necessary space in the operative computer memory, the size of which we’ll call the length of this element. The main characteristic of any data element is its type.

the data element:
an allowable set of values which the

object of this type can accept during the work of the program (the set of all specified values we’ll call the range of the type definition);

structure of operations which are allowed to be executed on the objects of the given type;

a way to represent the data element of the considered type in the machine memory;

the rules of any operation performance from the set of allowable for this type operations.

of the following basic types:
integers of various length with the

sign and without it;

real numbers of various length;

symbols, representable in a standard format ASCII.

during all operating time of the program are called constants.

Other objects that are changing during the algorithm performance are called variables. From the point of view of the language С, any variable is identified with its name, or the identifier. From the position of the COMPUTER it is considered to be the changing in the time contents of some operative memory’s area.

and to change the values of variables is called the

operation of assignment. For its designation in С the symbol "=" is used. For example:

a = 2; b = 3 * (a+4);

After performance of the specified actions the variable a receives the value equal to 2, and variable b - 18.

the data in the program
As it was already spoken

in lecture 1, the descriptions in the language С are the lines of the program defining the names and the characteristics of the data elements which participate in the work of the algorithm.
In the simplest case the instruction of the data description in the С-program has the following format:
type-specifier identifier <, identifier...>;

of the memory class;
type-specifier – the type descriptor; identifier

– the name (identifier) of the variable.

The description of the data in the program

descriptors of the type specified in the following table are

allowable. Here in parentheses the length of the data element of each type is specified in bytes (1 byte = 8 bits) and the area of allowable values for personal computer IBM PC AT.

Types of the data in language C

be used as modifiers of the data basic types at

the construction of the derivatives. For example, the following descriptions are considered by the compiler as equivalent:
short int.................. short long int................... long unsigned int............... unsigned unsigned short int......... unsigned short unsigned long int.......... unsigned long signed char................ char signed int................. int signed short............... short signed long................ long

program.
int a, b, c;/* Variables a, b, c are

declared */
/* to have the int type */

float alpha, beta;/* Variables alpha and beta are declared */
/* to have the float type*/

for various COMPUTERS may be different than the certain difficulties

at providing the appropriate software mobility arise.
To overcome the arising difficulties it is possible to use, where it is necessary, the operation of the definition of the memory quantity required for some variable or type representation. Generally this operation looks as follows:
sizeof (name)
where “name” is either the identifier of a variable, or the data type name.

С is any expression, coming to an end a semicolon.

In particular, separately worth semicolon is interpreted by the compiler as the zero - operator. The most significant group of operators is formed with instructions (operators) of management of process of performance of the program. To them concern:

has been already considered one of the major operations of

the language С - operation of the assignment, which allows to assign and change values of variables. Here we’ll consider a group of arithmetic operations and we’ll define the concept of arithmetic expression. The traditional way of setting the arithmetic operations is the use of two-place arithmetic operators of multiplicate and additive groups. Operands of any two-place operation can be both constants, and variables, and names of the latter should be predefined in one of the instructions of the type description.

for representation of operations of multiplication (*), divisions (/) operands

and reception of the remainder (%) from the division of the first operand by the second. In the latter case both operands should be the whole values. Operation of multiplication is possible on the operands of any type (may be various!). The operation of division applied to two integer operands, can lead to the loss of the result’s fractional part. The attempt of division by zero gives the mistake at the stage of the compilation or program execution. The order of the multiplicate group operation performance is from left to right. The group of additive operators includes two operators: additions (+) and subtraction (-). Both these operations are feasible on the operands of any type (may be various!). The order of the additive group operations performance is from left to right. Their priority is lower than the multiplicate operations priority, but higher, than the assignment operation priority.

+ 2.7 In this example both operands are to the right

of the assignment operator and are numerical constants. Arithmetic expressions of a similar kind are called the constant expressions.
2. b = (c + d) % 4 Here the traditional order of performance of arithmetic operations is broken by use of the parentheses allocating subexpression in structure of more complex arithmetic expression. According to definition of operation of % operands c and d should be variables of the whole type.
3. e = ++ f / (g + h) In the given example the form of the operator ++ is used prefix and consequently operation of division will be executed after actual increase in value of a variable f on unit.

of any arithmetic expression and generally has a format
expression1

= expression2
However, not pressing while in semantic details of such record, we shall use its simplified form
identifier = expression
where identifier there is a predefined name of a variable, and expression - any arithmetic expression. But even in such simplified circuit are stacked difficult enough in the semantic relation of expression of a kind
p = 2 * (q + s) / (t = u * ++ v)

group of two-place operations of the relation) carries out comparison

of the first operand with the second, checking, whether that parity between operands which is determined by a symbol of operation is true. The full set of operations of the relation is set by the following group of operators:
<-it is less > - it is more <= - it is less than or equal >= -more or equally == - it is equal ! = - it is not equal

such as int, equal to unit if comparison is true,

and to zero otherwise. Thus, operations of the relation in internal machine representation result in arithmetic result.
Strictly speaking, logic value "true" should answer any numerical value which is distinct from zero. Further we shall see, what exactly such agreement is accepted in language С. It enables to unit concepts of arithmetic, conditional and logic expressions in uniform concept "expression" that is very important from the point of view of flexibility and "symmetry" of language. The expressions designed by means of operations of the relation, it is accepted to name conditional expressions.

the relation are processed from left to right. The established

order can be changed by the conclusion of a part of expression in parentheses. For example, record
x it is completely equivalent to record
(x but expression of a kind
x <(y == z)
differs from previous the order of performance of operations

unit two or more conditional expressions. It can be made,

using a set of two-place logic operations:
&& - logic And
|| - logic OR
! - logic NOT (denying)

Logic operations

expressions. Then:
1. value expressions exp1 and exp2 are

true;
2. Value exp1 || exp2 is true, if even one of expressions - operands matters "true";
3. value! exp1 it is true, if expression exp1 is false, and vice versa.
The expressions constructed with use of logic operations, we shall name logic expressions. We shall notice, that logic expressions are direct generalization of simple conditional expressions.

right. The priority of logic operations && and || is

lower than a priority of any operation of the relation and consequently logic expressions:
a are completely equivalent, though second of them is more preferable because of presentation inherent in it. However operation of logic denying (!) possesses very high priority (it is the same as priority of arithmetic operations) and only parentheses have a higher priority.