Section 1.2 Variables. Data types. Constants.

The usefulness of the "Hello World" programs shown in the previous section are something more than questionable. We had to write several lines of code, compile them, and then execute the resulting program just to obtain a sentence on the screen as the result. It is true that it would have been much faster to simply write the output sentence by ourselves, but programming is not limited only to printing texts on 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 the variable.

Let's think that I ask you to retain the number 5 in your mental memory, and then I ask you to also memorize the number 2. You have just stored two 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 subtract and obtain 4 as the result.

All this process that you have made is a simile of what a computer can do with two variables. This same process can be expressed in C++ with the following instruction set:

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

Neither spaces nor marked letters can be part of an identifier. Only letters, digits and underline characters are valid. In addition, variable identifiers should always begin with a letter. They can also begin with an underline character ( _ ), but this is usually reserved for external links. They can never begin with a digit.

Another rule that you have to consider when inventing your own identifiers is that they cannot match any key word of the C++ language nor your compiler's specific ones since they could be confused with these. For example, the following expressions are always considered key words according to the ANSI-C++ standard and therefore they must not be used as identifiers:

asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, 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

and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq

Very important: The C++ language is "case sensitive", 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 variable RESULT is not the same as the variable result nor the variable Result.

Data types

Our computer's memory is organized in bytes. A byte is the minimum amount of memory that we can manage. A byte can store a relatively small amount of data, usually an integer between 0 and 255 or one single character. But in addition, the computer can manipulate more complex data types that come from grouping several bytes, such as long numbers or numbers with decimals. Next you have a list of the existing fundamental data types in C++, as well as the range of values that can be represented with each one of them:

In addition to these fundamental data types there also exist the pointers and the void parameter type specification, that we will see later.

Declaration of variables

int a;
float mynumber;

If you need to declare several variables of the same type and you want to save some writing work you can declare all of them in the same line separating the identifiers with commas. For example:

int a, b, c;

int a;
int b;
int c;

Integer data types (char, short, long and int) can be signed or unsigned according to the range of numbers that we need to represent. Thus to specify an integer data type we do it by putting the keyword signed or unsigned before the data type itself. For example:

unsigned short NumberOfSons;
signed int MyAccountBalance;

int MyAccountBalance;

The only exception to this rule is the char type that exists by itself and it is considered a diferent type than signed char and unsigned char.

Finally, signed and unsigned may also be used as a simple types, meaning the same as signed int and unsigned int respectivelly. The following two declarations are equivalent:

unsigned MyBirthYear;
unsigned int MyBirthYear;

To see what variable declaration looks like in action in a program, we are going to show the C++ code of the example about your mental memory proposed at the beginning of this section:

// operating with variables #include <iostream.h> 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 about the variable declarations looks a bit strange to you. You will see the rest in detail in coming sections.

Initialization of variables

type identifier = initial_value ;

int a = 0;

Additionally to this way of initializating variables (known as c-like), C++ has added a new way to initialize a variable: by enclosing the initial value between parenthesis ():

type identifier (initial_value) ;

int a (0);

Scope of variables

Anyway, it is recommended under some circumstances to follow the indications of the C language when declaring variables, since it can be useful when debugging a program to have all the declarations grouped together. Therefore, the traditional C-like way to declare variables is to include their declaration at the beginning of each function (for local variables) or directly in the body of the program outside any function (for global variables).

Global variables can be referred to anywhere in the code, within any function, whenever it is after its declaration. The scope of the local variables is limited to the code level in which they are declared. If they are declared at the beginning of a function (like in main) their scope is the whole main 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 used in the other function and vice versa. In C++, the scope of a local variable is given by the block in which it is declared (a block is a group of instructions grouped together within curly brackets {} signs). If it is declared within a function it will be a variable with function scope, if it is declared in a loop its scope will be only the loop, etc... In addition to local and global scopes there exists external scope, that causes a variable to be visible not only in the same source file but in all other files that will be linked together.

Constants: Literals.

Integer Numbers

1776
707
-273

In addition to decimal numbers (those that all of us already know) C++ allows the use as literal constants of octal numbers (base 8) and hexadecimal numbers (base 16). If we want to express an octal number we must precede it with a 0 character (zero character). And to express a hexadecimal number we have to precede it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:

75         // decimal
0113       // octal
0x4b       // hexadecimal

[ Note: You can find more information on hexadecimal and octal representations in the document Numerical radixes]

Floating Point Numbers
They express numbers with decimals and/or exponents. They can include a decimal point, an e character (that expresses "by ten at the Xth height", where X is the following integer value) or both.

3.14159    // 3.14159
6.02e23    // 6.02 x 1023
1.6e-19    // 1.6 x 10-19
3.0        // 3.0

Characters and strings
There also exist non-numerical constants, like:

'z'
'p'
"Hello world"
"How do you do?"

When writing both single characters and strings of characters in a constant way, it is necessary to put the quotation marks to distinguish them from possible variable identifiers or reserved words. Notice this:

x
'x'

Character constants and string constants have certain peculiarities, like the escape codes. These are special characters that cannot be expressed otherwise in the sourcecode of a program, like newline (\n) or tab (\t). All of them are preceded by an inverted slash (\). Here you have a list of such escape codes:

\n	newline
\r	carriage return
\t	tabulation
\v	vertical tabulation
\b	backspace
\f	page feed
\a	alert (beep)
\'	single quotes (')
\"	double quotes (")
\?	question (?)
\\	inverted slash (\)

'\n'
'\t'
"Left \t Right"
"one\ntwo\nthree"

coonstants of string of characters can be extended by more than a single code line if each code line ends with an inverted slash (\):

"string expressed in \
two lines"

"we form" "a single" "string" "of characters"

Defined constants (#define)

#define identifier value

#define PI 3.14159265
#define NEWLINE '\n'
#define WIDTH 100

circle = 2 * PI * r;
cout << NEWLINE;

The #define directive is not a code instruction, it is a directive for the preprocessor, therefore it assumes the whole line as the directive and does not require a semicolon (;) at the end of it. If you include a semicolon character (;) at the end, it will also be added when the preprocessor will substitute any occurence of the defined constant within the body of the program.

declared constants (const)

const int width = 100;
const char tab = '\t';
const zip = 12440;

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