Creating Variabel

A variable in C++ is a name for a piece of memory that can be used to store information. You can think of a variable as a mailbox, or a cubbyhole, where we can put and retrieve information. All computers have memory, called RAM (random access memory), that is available for programs to use. When a variable is declared, a piece of that memory is set aside for that variable.

In order to declare a variable, we generally use a declaration statement. Here’s an example of declaring variable x as an integer variable (one that can hold integer values):

char name_unit [20];

In C++, variables are also known as l-values (pronounced ell-values). An l-value is a value that has an address (in memory). Since all variables have addresses, all variables are l-values. They were originally named l-values because they are the only values that can be on the left side of an assignment statement. When we do an assignment, the left hand side of the assignment operator must be an l-value. Consequently, a statement like 5 = 6; will cause a compile error, because 5 is not an l-value. The value of 5 has no memory, and thus nothing can be assigned to it. 5 means 5, and it’s value can not be reassigned. When an l-value has a value assigned to it, the current value is overwritten. The opposite of l-values are r-values. An r-value refers to any value that can be assigned to an l-value. r-values are always evaluated to produce a single value. Examples of r-values are single numbers (such as 5, which evaluates to 5), variables (such as x, which evaluates to whatever number was last assigned to it), or expressions (such as 2+x, which evaluates to the last value of x plus 2).

Here is an example of some assignment statements, showing how the r-values evaluate:

int y;      // declare y as an integer variable
y = 4;      // 4 evaluates to 4, which is then assigned to y
y = 2 + 5;  // 2 + 5 evaluates to 7, which is then assigned to y

int x;      // declare x as an integer variable
x = y;      // y evaluates to 7, which is then assigned to x.
x = x;      // x evaluates to 7, which is then assigned to x (useless!)
x = x + 1;  // x + 1 evaluates to 8, which is then assigned to x.