1.2 Scr0320: memory, variables, and literals in scratch 2.0  (Page 3/11)

Sequential memory or combination random/sequential memory isn't very useful for most computer programs at the inner working level because access to eachparticular piece of data is slow. As a result, most modern computers have random access memory that is used for storage and retrieval of data by programs whilethey are running and disks that are used for long-term storage and retrieval of data that needs to be saved over longer periods of time.

Variables

As the computer program performs its operations in the prescribed order, it is often necessary for it to store intermediate results somewhere in itsmemory and to retrieve those results later for use in subsequent operations. The intermediate results are often stored in little chunks of memory that werefer to as variables .

Brief definition of a variable

The following definition is paraphrased from Wikipedia .

In computer programming, a variable is a storage location and an associated symbolic name (an identifier) that contains some known or unknown quantity or information -- a value.

The variable name is the usual way to reference the stored value; this separation of name and content allows the name to be used independently of the exact information it represents.

The value stored in the variable may change during program execution.

A physical analogy

We can think of random access memory as being analogous to a metal rack containing a large number of compartments. The compartments are all the samesize and are arranged in a regular grid of rows and columns.

Each compartment has a numeric address printed above it. No two compartments have the same numeric address. Each compartment also has a little slot intowhich you can insert a name or a label for the compartment. No two compartments can have the same name.

Although the analogy is not perfect, we can think of one of those compartments as being analogous to a variable.

Pretend that you are a computer program

Think of yourself as a computer program. You have the ability to create labels for each compartment (variable) . You have the ability to write values on little slips of paper and to put them into the compartments. You also have the abilityto read the values written on the little slips of paper and to use those values for some purpose. However, there are four rules that you must observe:

• You may not remove a slip of paper from a compartment without replacing it by another slip of paper on which you have written a value.
• You may not put a slip of paper in a compartment without removing the one already there.
• You may not give the same label or name to two or more compartments.
• You may not change a label once you have assigned it to a compartment.

Names versus addresses

Although each compartment in the physical memory in the computer has a numeric address, as a programmer using a high-level programming language such as Scratch, you usually don't need to be concerned about the numeric addresses of the compartments. (The compartments are often referred to as locations in memory.)