I'm currently trying to understand this BASIC program. I especially have issues with this part:
DIM C(52),D(52)
FOR D=D TO 1 STEP -1
C=C-1
C(C)=D(D)
NEXT D
I guess that it is a for-loop which starts at D where the last executed iteration is D=1 (hence inclusive?)
What does C(C) do? C is an array with 52 elements and I assumed C(X) is an access to the X-th element of the array C. But what does it do when the parameter is C itself?
In the original BASIC program, there is a GOTO 1500 on line 90, which comes before lines 16-19, that you’ve reproduced here. Line 1500 is the start of the program’s main loop. This particular programmer uses the (not uncommon) pattern of placing subroutines at the beginning of their BASIC program, using a GOTO to jump to the main code.
The code you’ve reproduced from the Creative Computing program you’ve linked is a subroutine to “get a card”, as indicated by the comment above that section of code:
100 REM--SUBROUTINE TO GET A CARD. RESULT IS PUT IN X.
REM is a BASIC statement; it stands for “remark”. In modern parlance, it’s a comment.
In BASIC, arrays, strings, and numbers are in separate namespaces. This means that you can (and commonly do) have the same variable name for arrays as for the integer that you use to access the array. The following variables would all be separate in BASIC and not overwrite each other:
C = 12
C(5) = 33
C$ = "Jack of Spades"
C$(5) = "Five of Hearts"
A single program could contain all four of those variables without conflict. This is not unknown in modern programming languages; Perl, for example, has very similar behavior. A Perl script can have a number, a string, an array, and a hash all with the same name without conflicting.
If you look at line 1500 of the program you linked and follow through, you’ll see that the variable C is initialized to 53. This means that the first time this subroutine is called, C starts at 53, and gets immediately decremented to 52, which is the number of cards. After the program has run a bit, the value of C will vary.
Basically, this bit of code copies to the array C some values in the array D. It chooses which values of D() to copy to C() using the (most likely integer) numeric variables C and D. As the code steps through D from D’s initial value down to 1, C is also decremented by 1.
If D begins with value 3, and C begins with value 10, this happens:
C(9) = D(3)
C(8) = D(2)
C(7) = D(1)
Note that this example is purely hypothetical; I have not examined the code closely enough to verify that this combination of values is one that can occur in a program run.
A couple of caveats. There are many variations of BASIC, and few absolutes among them. For example, some BASIC dialects will use what looks like a string array as a means of accessing substrings and sometimes even modifying substrings within a string. In these dialects, C$(2) will be the second (or third, if zero-based) character in the string C$. The BASIC program you’ve linked does not appear to be one of those variants, since it uses LEFT$ and MID$ to access substrings.
Second, many BASIC dialects include a DEFSTR command, which defines a variable as a string variable without having to use the “$” marker. If a variable were defined in this manner as a string, it is no longer available as a number. This will often be true of both the scalar and the array forms. For example, consider this transcript using TRS-80 Model III BASIC:
READY
>10 DEFSTR C
>20 C = "HELLO, WORLD"
>30 PRINT C
>40 C(3) = 5
>RUN
HELLO, WORLD
?TM Error IN 40
READY
>
The program successfully accepts a string into the variable C, and prints it; it displays a “Type Mismatch Error” on attempting to assign a number to element 3 of the array C. That’s because DEFSTR C defines both C and C() as strings, and it becomes an error to attempt to assign a number to either of them.
The program you’ve linked likely (but not definitely) runs on a BASIC that supports DEFSTR. However, the program does not make use of it.
Finally, many variants will have a third type of variable for integers, which will not conflict with the others; often, this variable is identified by a “%” in the same way that a string is identified by a “$”:
C = 3.5
C% = 4
C$ = "FOUR"
In such variants, all three of these are separate variables and do not conflict with each other. You’ll often see a DEFINT C at the top of code that uses integers, to define that variable (and the array with the same name) as an integer, to save memory and to make the program run more quickly. BASICs of the era often performed integer calculations significantly faster than floating point/real calculations.