variabel c++
The usefulness of the
"Hello World" programs shown in the
previous section is quite questionable. We had to write several lines of
code, compile them, and then execute the resulting program just to
obtain a simple sentence written on the screen as result. It certainly
would have been much faster to type the output sentence by ourselves.
However, programming is not limited only to printing simple texts on the
screen. In order to go a little further on and to become able to write
programs that perform useful tasks that really save us work we need to
introduce the concept of
variable.
Let us think that I ask you to retain the number 5 in your mental
memory, and then I ask you to memorize also the number 2 at the same
time. You have just stored two different values in your memory. Now, if I
ask you to add 1 to the first number I said, you should be retaining
the numbers 6 (that is 5+1) and 2 in your memory. Values that we could
now -for example- subtract and obtain 4 as result.
The whole process that you have just done with your mental memory is a
simile of what a computer can do with two variables. The same process
can be expressed in C++ with the following instruction set:
1
2
3
4
|
a = 5;
b = 2;
a = a + 1;
result = a - b;
|
Obviously, this is a very simple example since we have only used two
small integer values, but consider that your computer can store millions
of numbers like these at the same time and conduct sophisticated
mathematical operations with them.
Therefore, we can define a
variable as a portion of memory to store a determined value.
Each variable needs an identifier that distinguishes it from the others. For example, in the previous code the
variable identifiers were
a,
b and
result, but we could have called the variables any names we wanted to invent, as long as they were valid identifiers.
Identifiers
A
valid identifier is a sequence of one or more letters, digits or underscore characters (
_).
Neither spaces nor punctuation marks or symbols can be part of an
identifier. Only letters, digits and single underscore characters are
valid. In addition, variable identifiers always have to begin with a
letter. They can also begin with an underline character (
_
), but in some cases these may be reserved for compiler specific
keywords or external identifiers, as well as identifiers containing two
successive underscore characters anywhere. In no case can they begin
with a digit.
Another rule that you have to consider when inventing your own
identifiers is that they cannot match any keyword of the C++ language
nor your compiler's specific ones, which are
reserved keywords. The standard reserved keywords are:
asm, auto, bool, break, case, catch, char, class, const, const_cast,
continue, default, delete, do, double, dynamic_cast, else, enum,
explicit, export, extern, false, float, for, friend, goto, if, inline,
int, long, mutable, namespace, new, operator, private, protected,
public, register, reinterpret_cast, return, short, signed, sizeof,
static, static_cast, struct, switch, template, this, throw, true, try,
typedef, typeid, typename, union, unsigned, using, virtual, void,
volatile, wchar_t, while
Additionally, alternative representations for some operators cannot be
used as identifiers since they are reserved words under some
circumstances:
and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq
Your compiler may also include some additional specific reserved keywords.
Very important: The C++ language is a
"case sensitive"
language. That means that an identifier written in capital letters is
not equivalent to another one with the same name but written in small
letters. Thus, for example, the
RESULT variable is not the same as the
result variable or the
Result variable. These are three different variable identifiers.
Fundamental data types
When programming, we store the variables in our computer's memory, but
the computer has to know what kind of data we want to store in them,
since it is not going to occupy the same amount of memory to store a
simple number than to store a single letter or a large number, and they
are not going to be interpreted the same way.
The memory in our computers is organized in bytes. A byte is the minimum
amount of memory that we can manage in C++. A byte can store a
relatively small amount of data: one single character or a small integer
(generally an integer between 0 and 255). In addition, the computer can
manipulate more complex data types that come from grouping several
bytes, such as long numbers or non-integer numbers.
Next you have a summary of the basic fundamental data types in C++, as
well as the range of values that can be represented with each one:
| Name | Description | Size* | Range* |
|---|
char |
Character or small integer. |
1byte |
signed: -128 to 127
unsigned: 0 to 255 |
short int
(short) |
Short Integer. |
2bytes |
signed: -32768 to 32767
unsigned: 0 to 65535 |
int |
Integer. |
4bytes |
signed: -2147483648 to 2147483647
unsigned: 0 to 4294967295 |
long int
(long) |
Long integer. |
4bytes |
signed: -2147483648 to 2147483647
unsigned: 0 to 4294967295 |
bool |
Boolean value. It can take one of two values: true or false. |
1byte |
true or false |
float |
Floating point number. |
4bytes |
+/- 3.4e +/- 38 (~7 digits) |
double |
Double precision floating point number. |
8bytes |
+/- 1.7e +/- 308 (~15 digits) |
long double |
Long double precision floating point number. |
8bytes |
+/- 1.7e +/- 308 (~15 digits) |
wchar_t |
Wide character. |
2 or 4 bytes |
1 wide character |
* The values of the columns
Size and
Range depend on
the system the program is compiled for. The values shown above are those
found on most 32-bit systems. But for other systems, the general
specification is that
int has the natural size suggested by the system architecture (one
"word") and the four integer types
char,
short,
int and
long must each one be at least as large as the one preceding it, with
char being always one byte in size. The same applies to the floating point types
float,
double and
long double, where each one must provide at least as much precision as the preceding one.
Declaration of variables
In order to use a variable in C++, we must first declare it specifying
which data type we want it to be. The syntax to declare a new variable
is to write the specifier of the desired data type (like int, bool,
float...) followed by a valid variable identifier. For example:
1
2
|
int a;
float mynumber;
|
These are two valid declarations of variables. The first one declares a variable of type
int with the identifier
a. The second one declares a variable of type
float with the identifier
mynumber. Once declared, the variables
a and
mynumber can be used within the rest of their scope in the program.
If you are going to declare more than one variable of the same type, you
can declare all of them in a single statement by separating their
identifiers with commas. For example:
This declares three variables (
a,
b and
c), all of them of type
int, and has exactly the same meaning as:
1
2
3
|
int a;
int b;
int c;
|
The integer data types
char,
short,
long and
int
can be either signed or unsigned depending on the range of numbers
needed to be represented. Signed types can represent both positive and
negative values, whereas unsigned types can only represent positive
values (and zero). This can be specified by using either the specifier
signed or the specifier
unsigned before the type name. For example:
1
2
|
unsigned short int NumberOfSisters;
signed int MyAccountBalance;
|
By default, if we do not specify either
signed or
unsigned
most compiler settings will assume the type to be signed, therefore
instead of the second declaration above we could have written:
with exactly the same meaning (with or without the keyword
signed)
An exception to this general rule is the
char type, which exists by itself and is considered a different fundamental data type from
signed char and
unsigned char, thought to store characters. You should use either
signed or
unsigned if you intend to store numerical values in a
char-sized variable.
short and
long can be used alone as type specifiers. In this case, they refer to their respective integer fundamental types:
short is equivalent to
short int and
long is equivalent to
long int. The following two variable declarations are equivalent:
1
2
|
short Year;
short int Year;
|
Finally,
signed and
unsigned may also be used as standalone type specifiers, meaning the same as
signed int and
unsigned int respectively. The following two declarations are equivalent:
1
2
|
unsigned NextYear;
unsigned int NextYear;
|
To see what variable declarations look like in action within a program,
we are going to see the C++ code of the example about your mental memory
proposed at the beginning of this section:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
|
// operating with variables
#include <iostream>
using namespace std;
int main ()
{
// declaring variables:
int a, b;
int result;
// process:
a = 5;
b = 2;
a = a + 1;
result = a - b;
// print out the result:
cout << result;
// terminate the program:
return 0;
}
|
4
|
Do not worry if something else than the variable declarations themselves
looks a bit strange to you. You will see the rest in detail in coming
sections.
Scope of variables
All the variables that we intend to use in a program must have been
declared with its type specifier in an earlier point in the code, like
we did in the previous code at the beginning of the body of the function
main when we declared that
a,
b, and
result were of type
int.
A variable can be either of global or local scope. A global variable is a
variable declared in the main body of the source code, outside all
functions, while a local variable is one declared within the body of a
function or a block.
Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration.
The scope of local variables is limited to the block enclosed in braces (
{}) where they are declared. For example, if they are declared at the beginning of the body of a function (like in function
main)
their scope is between its declaration point and the end of that
function. In the example above, this means that if another function
existed in addition to
main, the local variables declared in
main could not be accessed from the other function and vice versa.
Initialization of variables
When declaring a regular local variable, its value is by default
undetermined. But you may want a variable to store a concrete value at
the same moment that it is declared. In order to do that, you can
initialize the variable. There are two ways to do this in C++:
The first one, known as
c-like initialization, is done by appending an equal sign followed by the value to which the variable will be initialized:
type identifier = initial_value ;
For example, if we want to declare an
int variable called
a initialized with a value of 0 at the moment in which it is declared, we could write:
The other way to initialize variables, known as
constructor initialization, is done by enclosing the initial value between parentheses (
()):
type identifier (initial_value) ;
For example:
Both ways of initializing variables are valid and equivalent in C++.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
|
// initialization of variables
#include <iostream>
using namespace std;
int main ()
{
int a=5; // initial value = 5
int b(2); // initial value = 2
int result; // initial value undetermined
a = a + 3;
result = a - b;
cout << result;
return 0;
}
|
6
|
Introduction to strings
Variables that can store non-numerical values that are longer than one single character are known as strings.
The C++ language library provides support for strings through the standard
string class. This is not a fundamental type, but it behaves in a similar way as fundamental types do in its most basic usage.
A first difference with fundamental data types is that in order to
declare and use objects (variables) of this type we need to include an
additional header file in our source code:
<string> and have access to the
std namespace (which we already had in all our previous programs thanks to the
using namespace statement).
1
2
3
4
5
6
7
8
9
10
11
|
// my first string
#include <iostream>
#include <string>
using namespace std;
int main ()
{
string mystring = "This is a string";
cout << mystring;
return 0;
}
|
This is a string
|
As you may see in the previous example, strings can be initialized with
any valid string literal just like numerical type variables can be
initialized to any valid numerical literal. Both initialization formats
are valid with strings:
1
2
|
string mystring = "This is a string";
string mystring ("This is a string");
|
Strings can also perform all the other basic operations that fundamental
data types can, like being declared without an initial value and being
assigned values during execution:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
// my first string
#include <iostream>
#include <string>
using namespace std;
int main ()
{
string mystring;
mystring = "This is the initial string content";
cout << mystring << endl;
mystring = "This is a different string content";
cout << mystring << endl;
return 0;
}
|
This is the initial string content
This is a different string content
|
For more details on C++ strings, you can have a look at the
string class reference.
0 komentar: