C++ Variables, Identifiers and Data Types

Variable:

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 t

hat 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.

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

Note: 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:

* 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.