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  C++ Programming Tutorials
Basics of C++

Structure of a program
Variables Data types
Constants
Operators
Basic Input/output

Control Structures
Control Structures
Functions (I)
Functions (II)

Compound Data Types
Arrays
Character Sequences
Pointers
Dynamic Memory
Data Structures
Other Data Types

Object Oriented Programming
Classes [I]
Classes [II]
Friendship & Inheritance
Polymorphism

Advanced Concepts
Templates
Namespaces
Exceptions
Type Casting
Preprocessor Directives

C++ Standard Library
Input/output with Files

Soft Skills
Communication Skills
Leadership Skills
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C++ Programming Tutorials


Variables. Data Types.
The usefulness of the "Hello TechPreparation" 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:

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 they can 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.

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. 1byte 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. 2bytes 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 1 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.

NEXT >> Declaration of variables

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