The Fundamentals of Programming #2: Constants and Variables

Today we are going to discuss about constants and variables, which are the “things” that can hold data for us. This is part of The Fundamentals of Programming Using C series.

Look at this code:

variable character letter <- "T"

In that piece of code, we declare a variable of the type character, whose name is letter, with the value “T”.

The declaration of variables, or constants, is the process in which the compiler is “warned” about its existence. In this way, every time the name of the variable/constant is mentioned in the code, the previously stated value can be used.

When we declare the variable letter, we are reserving an address and space in RAM memory to store that value. In this case, the value was assigned at the same time of its declaration using the operator <-.

Since the moment we declare a variable or a constant, its reserved space in RAM memory will be accessible using its name. Whenever the word letter is mentioned in the code, the value of the variable will be returned.

Constants

Let's talk first about constants.

Constants, as the name suggests, allow us to store unchangeable values, in other words, the value of a constant can't be changed after its declaration.

Let's check an example:

constant string BLOG <- "Bits N Me"    
BLOG <- "Bits N Me" // This would produce an error

In the first line, we declare the constant BLOG and assign to it the value “Bits N Me”. Then, in the second line, we try to change it's value. But since it is a constant, an error will be produced because its content cannot be changed during the run time of the program.

Nomenclature

Conventionally, the name of a constant is all written with capital letters so we can easily distinguish constants from variables inside the source code. Using non capital letters will not produce any error. This is only a convention.

Although, following this rule, it will be easier for you, and for any other developer that sees your code, to distinguish between a variable and a constant without looking for its declaration.

Variables

On the other hand, we have variables.

Variables, as opposite to constants, allow us to store values that can be changed during the execution of the code. They're heavily used to store states and they're fundamental in programming.

Let's see an example:

variable string subject <- "Constants"      
subject <- "Variables"

In the first line we declare a variable, whose name is subject, with the value “Constants”. Then, we change its value to “Variables”. No error will be produced because variables allow us to change its value along the execution of the program.

Nomenclature

There are two main nomenclatures when we talk about variables names and it depends on the language. There are languages that follow the variation loweCamelCase of the pattern CamelCase. But there are other languages that prefer the name of the variables to be separated_by_lines.

Naming rules

There are some must-follow rules when we talk about the name of variables and constants. If we don't follow this rules, an error will be produced.

The name of variables and constants must:

  • not begin with numbers (e.g.: 9food is not allowed, but food9 is);
  • not be equal to a reserved word (e.g.: if is not allowed, but maria is); and
  • not contain special characters (there are some exceptions depending on the language).

Reserved words are those words that are inserted into the language own syntax. If the word “for” is part of a language's syntax, it can't be used as a variable (or constant) name. If you try to do it, you will receive an error.

Variables and constants can be of different data types. There are some basic data types that are available in almost every language. But it can vary from language to language. Later, we will talk about the available data types in C.