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, 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:
int a = 0;
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:
int a (0);
Both ways of initializing variables are valid and equivalent in C++.

// 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;
}

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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.

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Declaration of variables

Today we're gonna learn how to declare 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:
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
int a, b, c;

This declares three variables (a, b and c), all of them of type int, and has exactly the same meaning as:
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:
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:

int MyAccountBalance;
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:
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:
unsigned NextYear;
unsigned int NextYear;

To see what variable declarations look like in action within a program here is an example:
The blue section is input the grey section is output

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 posts.

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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. Next we shall learn about declaration of variables.

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Variables. Data Types.

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:
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.

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Lesson 2 : Structure of a program (Part II)

In C++, the separation between statements is specified with an ending semicolon (;) at the end of each one, so the separation in different code lines does not matter at all for this purpose. We can write many statements per line or write a single statement that takes many code lines. The division of code in different lines serves only to make it more legible and schematic for the humans that may read it. Let us add an additional instruction to our first program.

This is input:                                                                      This is output:

// my second program in C++
#include <iostream>                                                           Hello World! I'm a C++ program
using namespace std;
int main ()
{
cout << "Hello World! ";
cout << "I'm a C++ program";
return 0;
}

 In this case, we performed two insertions into cout in two different statements. Once again, the separation in different lines of code has been done just to give greater readability to the program, since main could have been perfectly valid defined this way 
EXAMPLE:
int main () { cout << " Hello World! "; cout << " I'm a C++ program "; return 0; }

We were also free to divide the code into more lines if we considered it more convenient
EXAMPLE:
 int main ()
{
cout <<
"Hello World!";
cout
<< "I'm a C++ program";
return 0;
}
And the result would again have been exactly the same as in the previous examples.
Pre-processor directives (those that begin by #) are out of this general rule since they are not statements. They are lines read and processed by the pre-processor and do not produce any code by themselves. Pre-processor directives must be specified in their own line and do not have to end with a semicolon (;).

Comments

Comments are parts of the source code disregarded by the compiler. They simply do nothing. Their purpose is only
to allow the programmer to insert notes or descriptions embedded within the source code.
C++ supports two ways to insert comments.
Which are:
// line comment
/* block comment */

The first of them, known as line comment, discards everything from where the pair of slash signs (//) is found up
to the end of that same line. The second one, known as block comment, discards everything between the /*
characters and the first appearance of the */ characters, with the possibility of including more than one line.
We are going to add comments to our second program:

This is input:                                                                          This is output:

/* my second program in C++
with more comments */
#include <iostream>                                                                Hello World! I'm a C++ program
using namespace std;
int main ()
{
cout << "Hello World! "; // prints Hello
World!
cout << "I'm a C++ program"; // prints I'm a
C++ program
return 0;
}

If you include comments within the source code of your programs without using the comment characters combinations //, /* or */, the compiler will take them as if they were C++ expressions, most likely causing one or several error messages when you compile it.

Thats it for today we will start a new topic tomorrow anyone that wants to ask any questions or make recommendations please feel free to do so.
             

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Lesson 1:Structure of a program

The best way to start learning a programming language is probably by learning to write a program. Writing a program is the most basic and also crucial part of programming. Why don't we start by learning about the way a program works, it's structure and basics.
Therefore, here is our first program:

This will be the input                                        This will be output 

      #include <iostream>                                                   Hello World!

using namespace std;

int main ()

{

cout << "Hello World!";

return 0;

}

                                                                                     

The input is the very first program a programmer will ever see and same can be said for the output. Now lets start with the learning. 

#include <iostream>

The "#include" is a pre-processor function that tells the compiler to include the <iostream> library. They are no regular code lines. This specific file "iostream" includes the declarations of the basic standard input-output library in C++. Keep it in mind.

using namespace std;

All the elements of the standard C++ library are declared within what is called a namespace, the

namespace with the name std (standard) . So in order to access its functionality we declare with this expression that we will be using these entities (std) . This line is very frequent in C++ programs that use the standard library. Most programs do use the std library so it is an important point to remember.

int main ()

This line corresponds to the beginning of the definition of the main function. The main function is the point by where all C++ programs start their execution, independently of its location within the source code. It does not matter whether there are other functions with other names defined before or after it - the instructions contained within this function's definition will always be the first ones to be executed in any C++ program. For that same reason, it is essential that all C++ programs have a main function.

The word main is followed in the code by a pair of parentheses (()). That is because it is a function

declaration: In C++, what differentiates a function declaration from other types of expressions are these parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within them.

Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is contained within these braces is what the function does when it is executed.

cout << "Hello World!";

This line is a C++ statement. A statement is a simple or compound expression that can actually produce some effect. In fact, this statement performs the only action that generates a visible effect in our first program.

cout represents the standard output stream in C++, and the meaning of the entire statement is to insert

a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (which usually is the screen).

cout is declared in the iostream standard file within the std namespace, so that's why we needed to

include that specific file and to declare that we were going to use this specific namespace earlier in our code.

Notice that the statement ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C++ programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement).

return 0;

The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code 0). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C++ console program.

                                            (FIN)

This code could also be written on a single line keep in mind like this :
int main () { cout << "Hello World!"; return 0; }

This would mean exactly the same as the above code and would have same meaning. 

See you on the next lesson and feel free to ask me any questions you might want.

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WE START TODAY

From today onwards we will be learning C++.  You may ask any questions regarding C++. We have just taken a start, so there is a lot to come. We are going to learn programming in C++ from scratch. We'll be covering the basics, provide summaries and help in any and every possible way. We hope you become a part of this experience.

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First Post

Anyone in the future ever decides to see what the first post of this site this was

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