The data type conception. Variables and constants. Assignment operation

1. The data type conception. Variables and constants. Assignment operation

Any 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.

2. The concept of the type includes the following information about 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.

3.

Language С provides an opportunity of
representation and data processing 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.

4.

Those elements of the data that keep the
constant values 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.

5. The instruction of the programming language that allows to assign and to change the values of variables is called the operation of assignment. For its designation in С the symbol "=" is used. For example:

The instruction of the programming language that
allows to assign 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.

6. Types of the data in language C. The description of 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:
<sc-specifier> type-specifier identifier <, identifier...>;

7. <sc-specifier> type-specifier identifier <, identifier...>;

The description of the data in the
program
<sc-specifier> type-specifier identifier <, identifier...>;
where sc-specifier is the descriptor of the
memory class;
type-specifier – the type descriptor; identifier
– the name (identifier) of the variable.

8. Types of the data in language C

For the majority of compilers from the
language С the 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.

9.

The remark. Key words “unsigned”, “signed”,
“short” and “long” can 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

10. Let's consider some examples of the data description in the 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*/

11.

As the internal machine data presentations of
the same 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.

12. Elements of language C

Operators
The elementary operator of language 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:

13. Arithmetic operations and arithmetic expressions

In the first paragraph it 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.

14. Arithmetic operations and arithmetic expressions

Operators of multiplicate group serve 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.

15. Let’s show some simple examples of arithmetic expressions:

a = 4.3 + 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.

16.

In language С operation of giving is a full part 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)

17. Operations of the relation, logic operations and logic expressions

The 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 less than or equal
== - it is equal
> - it is more
>= -more or equally
! = - it is not equal

18.

Result of any operation of the relation is numerical
value 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.

19.

During calculation of value of any conditional
expression, operation of 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 <y == z
it is completely equivalent to record
(x <y) == z
but expression of a kind
x <(y == z)
differs from previous the order of performance of
operations <and ==.

20. Logic operations

By development of real programs frequently it
appears necessary to 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)

21. Let's assume, that exp1 and exp2 - two simple conditional 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.

22.

The standard order of their processing - from
left to right. The priority of logic operations
&& and || is lower than a priority of any
operation of the relation and consequently
logic expressions:
a <b && b <c and (a <b) && (b <c)
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.

23.

Generally operands of logic operations can be not only
conditional expressions, but also any arithmetic
expressions. It is easy for understanding, if to zero
value of arithmetic expression to put in conformity logic
value “false" and, on the contrary, any numerical value
distinct from zero to identify with logic value "true".
The elementary instruction of language С using logic
expressions, the conditional operator is:
expression1? expression2: expression3
where expression1 there is a logic expression, and
expression2 and expression3 essence any arithmetic
expressions. If expression1 accepts value "true" result
of conditional operation will be value expression2,
otherwise it is equal to value expression3.

24. For example, the instruction:

abs_a = (a> 0)? a:-a
gives a variable abs_a absolute value of a
variable a.

25. Automatic transformation of the types and operation of reduction

If the structure of arithmetic or conditional
expression includes operands of various
types the compiler automatically carries out
their reduction the common type. In spite of
the fact that in some cases character of
transformation depends on a kind of
concrete operation and such as operands,
there is a common set of standard rules of
transformation:

26.

1. If operation is carried out above the data of two
various types, both sizes are resulted in the
"maximum" type;
2. In the operator of giving the end result of
calculation of expression in the right part is resulted
in type of a variable by which value should be given
(left type).
The sequence of names of the types ordered from
the "maximum" type to "lowest", looks as follows:
double, float, long, int, short and char. Application of
a key word unsigned raises a rank of corresponding
type of the data with is familiar.

27.

This opportunity is realized in operation of
reduction of types as follows: Before the
given size in parentheses the name of
required type enters the name. Let, for
example, the variable res has type int.
Then value of arithmetic expression
res = 2.7 + 1.5
according to the general rules of
transformation of types, it is equal 4.

28.

However having applied obvious image
operation of reduction of type to both operands
in the right part
res = (int) 2.7 + (int) 1.5
let's receive the result equal 3. This example
speaks about necessity with extra care to
concern to every possible transformations of
types of operands.

29. The elementary operators of language C. The compound operator

The arithmetic above arithmetic, conditional and logic
expressions in aggregate form objects of the program
which can be named simply expressions. This
terminology emphasizes equality of operations of
various groups from the point of view of their role in
formation of expressions. Moreover, to any
expression in С there meets some numerical value
(behind exception, certainly, those cases when the
result of operation is considered uncertain) which can
be used further. However in themselves expressions
of any kind yet are not the finished instructions of
language С.

30. The elementary operators of language C. The compound operator

For a designation of the completed actions the
concept of the operator is used. The elementary
operator of language is any expression, coming to an
end a semicolon (;). In particular, separately worth
semicolon is interpreted by the compiler as the zero operator. For example, instructions of a kind
a = c * (d + e)/4;
f = alpha ++;
being operators - expressions, play a role of the
finished instructions for observant systems.

31. The elementary operators of language C. The compound operator

The group of one or greater number of the operators
made in braces, forms the compound operator. We
shall notice, that by rules of language the semicolon
after a closing brace is not put. For example, group of
three operators
{x = 7; y = x + 10; z = (x + y)/13;}
it is considered by the compiler as one compound
operator. In the program in language С compound
operators can be used everywhere level with simple
operators.