Data representation in programming

A data type is a classification that tells the computer how to interpret a value. It defines what kind of data a variable can hold (e.g., whole numbers, text, true/false values) and what operations can be performed on that data. Choosing the correct data type is crucial for writing efficient and error-free programs. The most common fundamental types are Integer, Real, Character, String, and Boolean.

Computers do not understand letters or symbols; they only work with binary numbers. To represent text, we use character sets, which are standardized tables that map each character to a unique binary code. The two most important character sets are ASCII and Unicode.

ASCII (American Standard Code for Information Interchange) traditionally uses 7 bits, allowing it to represent 128 different characters. This is enough for all uppercase and lowercase English letters, numbers 0-9, and common punctuation. Extended ASCII uses 8 bits (1 byte) to represent 256 characters.

Unicode was created to overcome the limitations of ASCII. It aims to include every character from every language in the world. Unicode characters can be represented using different encodings, such as UTF-8 and UTF-16. A single Unicode character can take up 1 to 4 bytes, allowing for over a million possible characters.

In programming, we use named storage locations to hold data. A 'variable' is a named location in memory whose value can be changed as the program runs. Think of it as a labeled box where you can store something, and later replace it with something else. A 'constant' is a named location whose value is set once and cannot be changed during program execution. Constants are useful for values that should never change, like the value of Pi or the maximum number of attempts in a game.

Before you can use a variable, you must 'declare' it, which means giving it a name and a data type. 'Assignment' is the process of putting a value into a variable or constant, typically using an assignment operator like `=` or `<-`.

A string is a sequence of characters, and it is one of the most common data types you will work with. Programming languages provide a rich set of built-in functions to manipulate strings. Common operations include:

• Concatenation: Joining two or more strings end-to-end to form a new string. The `+` operator is often used for this.
• Length: Finding the number of characters in a string.
• Substring: Extracting a portion of a string. This is often done with functions like LEFT, RIGHT, or MID (or slicing).
• Conversion: Changing a string of digits into a number (e.g., "123" to 123) or a number into a string (e.g., 45 to "45") to perform the correct operations.

An array is a data structure used to store a collection of elements, all of the same data type, in a contiguous block of memory. A one-dimensional (1D) array can be visualized as a single row or column of numbered slots, where each slot holds one element. To access a specific element in the array, you use its 'index' or 'subscript' – a number that indicates its position. Indexing often starts at 0 in many languages (0-based indexing) or at 1 (1-based indexing). Arrays are static, meaning their size is fixed when they are created.

When your program ends, all the data stored in its variables and arrays (which are held in RAM) is lost. This is because RAM is 'volatile' storage. To save data permanently, we must write it to a 'file' on a secondary storage device like a hard drive or SSD. This is called 'persistent storage'.

The basic process for using a file involves three steps:

1. Open: You open a file and specify a mode. The main modes are READ (to get data from the file), WRITE (to put data into the file, overwriting existing content), and APPEND (to add new data to the end of the file).
2. Process: You either read data from the file into your program's variables or write data from your variables into the file.
3. Close: You must close the file. This ensures all data is correctly saved and makes the file available to other programs.