Variables are an extremely core concept to most object orientated programming languages. I like to visualize a variable much like a box. We can put things in the box, we can take things out of the box, and at any point we can see what is inside the box. Each box also has a name to which we can refer to it by, and in C++, each box can only hold a certain type of data.
When we create variables we call this the variable declaration, and then when we set them for the first time, we call this the initialization. To declare a variable in C++, we write the function. To declare a basic integer variable called 'age', we could write the following:
From this point we can then refer to the variable by its name, so in this case, we can just write 'age' whenever we want to refer to the variable. To initialise the variable we can write its name, followed by the equals sign, followed by the value we want to set the variable to (followed by a semicolon). The value we set it to can be a constant (a value that doesn't change), or another variable of the same type. An operator is a symbol which has a certain meaning in the programming language, in this case, the equals operator, represented by the
= symbol, is an operator which sets whatever is on the left of the operator to whatever is on the right.
The constant value we set the variable to depends on the to 5 with something like the following:
Few Points to Note regarding functions in C: 1) main in C program is also a function. 2) Each C program must have at least one function, which is main. 3) There is no limit on number of functions; A C program can have any number of functions. 4) A function can call itself and it is known as “Recursion“. I have written a separate guide. The int returned by main is a way for a program to return a value to “the system” that invokes it. On systems that doesn’t provide such a facility the return value is ignored, but that doesn’t make “void main” legal C or legal C.
We can actually combine the variable declaration and initialization into one more-compact line, like the following:
The 'age' variable now contains the number '5', and we can refer to this '5' by writing 'age' anywhere in our program. We can also change the value of the variable at any point by using the equals operator as we did for the first initialization:
Although this seems purely for convenience at the moment (as we could just write '5', '3', or '21' in place of 'age'), trust me when I say that these become extremely useful and powerful when you start dealing with dynamic logic and user input (the latter of which we'll be covering later in this tutorial).
Just to give an example of accessing the contents of variables by using their names, we could create a new variable called 'age_two' which is set to the value of 'age', and then we can also try outputting one or both of these variables:
To be clear, all this code should be going into the basic program structure which we learnt how to create in the last tutorial. So we want our 'iostream' include for
cout , cin , and some other stuff, we want the std namespace, and we want the majority of our code to be going in our 'main' function. So our full code to demonstrate variables so far, which you can compile and run at any point to test the functionality, is as follows:
Some number variables can handle positive and negative numbers, whereas 'unsigned' number variables can only handle positive numbers, although because of this restriction, can hold larger numbers. You can write the
signed or unsigned keywords before the and 'short' - numbers with a decimal place in. Floats are accurate to around 6 or 7 digits and are declared using the float type. Float constants can be defined by simply writing a number with a decimal point followed by the 'f' notation. An example of a simple float declaration and initialization to a float constant is as follows:
Care must be taken, however, with float (and other decimal) operations, as rounding and precision problems to do with how the numbers are stored can trip you up (we don't have infinite memory for recurring decimals like
1/3 for example) -- I recommend reading this article for more information on this if you're interested.
Doubles
The 'double' or 'e'. Character variables are declared by using the
char type, and character constants are defined by using single quotes (apostrophes) around the character. An example of character declaration and initialization to a character constant is as follows:
Strings
The lastve talked about string variables in relation to
cout before, and as such you should know that string constants are defined by using double quotes. String variables are declared by using the string type, however as strings aren't actually 'primitive' types in C++ (and are instead defined by the standard library of stuff that comes bundled with C++), you are required to #include <string> to use thist strings aren't massively useful, but this is just because we don't really know how to utilize all the functionality of different data-types yet - for example, we don't know how to perform simple mathematics on number types, or how to check the value of booleans to change the logic of the program. All will be revealed in future tutorials.
C Language Main Function
A scope is a region of the program and broadly speaking there are three places, where variables can be declared −
We will learn what is a function and it's parameter in subsequent chapters. Here let us explain what are local and global variables.
Local Variables
Variables that are declared inside a function or block are local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own. Following is the example using local variables −
Global Variables
Global variables are defined outside of all the functions, usually on top of the program. The global variables will hold their value throughout the life-time of your program.
A global variable can be accessed by any function. That is, a global variable is available for use throughout your entire program after its declaration. Following is the example using global and local variables −
A program can have same name for local and global variables but value of local variable inside a function will take preference. For example −
C++ Int Return Function
When the above code is compiled and executed, it produces the following result −
In Function Int Main Dev C SystemInitializing Local and Global VariablesC Main Function Definition
When a local variable is defined, it is not initialized by the system, you must initialize it yourself. Global variables are initialized automatically by the system when you define them as follows −
C++ Main Function Example
It is a good programming practice to initialize variables properly, otherwise sometimes program would produce unexpected result.
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