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Basics of C++ |
| Variables, Data Types. |
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| 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. |
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| 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. |
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| 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: |
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a = 5;
b = 2;
a = a + 1;
result = a - b; |
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| 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. |
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| 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 |
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| 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. |
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| 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: |
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| 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 |
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| Additionally, alternative representations for some operators cannot be used as identifiers since they are reserved
words under some circumstances: |
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| and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq |
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| Your compiler may also include some additional specific reserved keywords. |
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| 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. |
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| Fundamental data types |
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| 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. |
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| 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. |
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| 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: |
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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 |
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| * 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. |
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