2.2 Scr0120: memory, variables, and literals  (Page 4/10)

Instead, you can think about the compartments and refer to them in terms of their names. (Names are easier for most people to remember and understand than numeric addresses.) Keep in mind, however, that computer memory locations don't really have names. Deep inside the computer program, the namesthat you use to identify compartments in memory are cross-referenced to memory addresses. At the lowest level, the program works with memory addresses insteadof names.

Execute an algorithm

A computer program always executes some sort of procedure or algorithm. The algorithm may be very simple (such as how to make a peanut butter sandwich) , or it may be very complex as would be the case for a spreadsheet program. As theprogram executes the algorithm, it uses the random access memory to store and retrieve the data that is needed to execute the algorithm.

Declaring a variable

In Scratch, it is necessary to establish the name of a variable before you can use it. (That is not the case in some programming languages such as Python.)

In programmer jargon, this is referred to as declaring a variable . The process of declaring a variable causes memory to be set aside to contain avalue, and causes that chunk of memory to be given a name. That name can be used later to refer to the value stored in that chunk of memory.

Some programming languages, (not including Scratch), require that the variable declaration also specify the type of data that will be storedin a variable.

Case sensitivity

The name of a variable is case sensitive in Scratch (but the name may not be case sensitive in other programming languages) . In other words, the name avariable is not the same as the name aVariable in case-sensitive programming languages. If both names were used in the sameprogram, they would refer to two different variables.

Use names to access values

Variables have names like price , cost , and markup . The names of the variables give us easy access to the values stored in the variables.

Some operations cause the value of a variable to change while other operations simply access the value without changing it.

Storing and retrieving

As the program executes, the values stored in variables can be replaced by other values. In that way, the values of the variables can change asthe program executes. (In other words, the value of a variable can vary over time.)

It is also possible to retrieve the value stored in a variable without modifying it. In that way, the values of variables can be used to evaluateexpressions without modifying those values.

Literals

Where does data originate ?

The data used by a computer program can originate from a variety of sources. For example, it can be entered from the keyboard. It can be read from a diskfile. It can be downloaded from a web site, etc.

Hard coded data

The data values can also be coded into the program when the program is written. In this case, we would call it a literal or a literal value .

A value coded into the program

That is just about all there is to understanding literals. There are some special rules that come into play in certain situations, and I will discussthose rules at the appropriate points in time. For now, just remember that a literal is a value that is coded into the program when the program is written.