“MSX BASIC” COLUMN by Orazio Cacciola (part 12)

THE MATRIXES. THE DIM/ERASE COMMANDS and THE CLEAR/FRE FUNCTIONS

It is very inconvenient when we have to make a program that makes use of many variables that will be used several times. Think of a sequence of constants that are called from different points in the program or an archive with names, addresses, etc. Let's take the first mentioned topic, constants, as an example, and let's pretend that our program will use 20 of them, so we will have to create them. Since these variables will have the same functionality, to create them we could use a variable that we will call C, followed by a number. For example:
C1,C2, C3, C4, and so on. However, we know that MSX BASIC only stores the first 2 characters of the variables, therefore since the variables must be 20, using the method described above allows us to use up to the variable C9 because the C10 will be 'seen' by Basic as C1 and it will store the data on the last one, deleting the previous one. So to continue we have to use another letter. Imagine what a mess. But fortunately there is a solution to all this. This is called ARRAY OR MATRIX. Arrays are almost always created at the beginning of the program, they reserve drawers in the memory where all the data will be stored. We say how much of these assets we want to reserve, using the DIM command. Let's take the example above: DIM C(19). I put 19 because MSX BASIC starts counting from 0. This also applies to alphanumeric variables. To continue, already knowing the data, since it is constant, we can load it into a DATA function and recall it with a FOR/NEXT loop.
I'll make a small example program.

10 DIM C(19) 20 FOR I=0 TO 19:READ C 30 C(I)=C: NEXT I 40 DATA ..........

We can also create 2-dimensional, three-dimensional and even larger matrices, but given the limited memory of our MSX it will be prohibitive to use matrices that are too large, the memory would run out very quickly. To create a 2-dimensional matrix just add a second data followed by a comma: DIM C(19.3), by doing so we have reserved 80 (20X4) drawers in memory. While for a three-dimensional one, it will be: DIM C(19,3,2). This time there will be 240 (20x4x3). The memory in bytes that the so-called drawers occupy (this is my way of calling them) will vary depending on the variable stored. If we need to use a matrix that does not exceed 10, i.e. eleven insertions, it is not mandatory to initialize it with the DIM command, even if it is two-dimensional or higher. We cannot dimension a variable twice, if this happens it will be reported as an error, unless it has been eliminated with the command that I will explain below.

After creating a matrix and using it up to a certain point in our program, we know that from now on it will no longer be needed or some data will no longer be useful. We can delete it, freeing up memory space, using the ERASE command. The latter will be deleted together with all its data. In the case of the second argument mentioned above, we will resize the variable with the correct parameters and re-enter the data we will use. After the ERASE command you will only have to enter the name of the variable without its sizing. The correct syntax is: ERASE Variable,[Variable],[Variable] ……

CLEAR
Clears all variables, even those in arrays, closes files and eliminates functions created with DEF. Change the space reserved for strings, by default it is 200 (characters). Sets the maximum address that can be used by Basic MSX. It can be used without arguments to clean up memory.