Table-Driven Programming

CS 301 Lecture, Dr. Lawlor

The C++ "Array Initializer"

So C++ allows this handy "array initializer syntax" for setting up a 1D array:
    const int arr[]={7,9,23};

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This makes an array, "arr", containing three ints: seven, nine, and twenty-three. 

It's the same as declaring:
    int arr[3];
    arr[0]=7;
    arr[1]=9;
    arr[2]=23;
But clearly the initializer is shorter and simpler!

Whitespace inside an initializer doesn't matter, and you can even add comments and stuff in there:
    const int arr[]={
       7, /* dwarves, in decimal */
       0x9, /* square of three, expressed in hexadecimal */
       027, /* number of angels that can dance on the head of a pin, in octal */
   };
It's still the same three ints as before.

If you want to save memory, you can make a 1D array of unsigned char (which are like int, but only 8 bits wide):
    const unsigned char arr[]={7,9,23};

Table-Driven Programming

A little array that tells the program what to do is called a "table", and programs written this way are called "table-driven".  Here's a simple table-driven program that prints out a certain number of "#" signs on each line, with the exact number determined from a little table:
const unsigned char table[]={
	20,
	20,
	2,
	2,
	2,
	20,
	20,
	0 /* end of the table */
};

int foo(void) {
	int i=0; /* location in the table */
	while (table[i]!=0) { /* print one entry in the table */
		int  n=table[i++]; /* number of times to print */
		char c='#'; /* character to print */
		for (int repeat=0;repeat<n;repeat++) /* print it n times */
			std::cout<<c<<" ";
		std::cout<<std::endl;
	}
	return 0;
}

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Here's a similar table-driven program that reads two entries in the table each time around the loop.  The first table entry is treated as a repetition count, and the second table entry is the letter to repeat.  Again, the program stops when it hits a repetition count of zero:
const unsigned char table[]={
/* ---  n, character to print n times ---- */
	3,'q',
	2,'@',
	4,'~',
	1,'z',
	0 /* end of the table */
};

int foo(void) {
	int i=0; /* location in the table */
	while (table[i]!=0) { /* print one entry in the table */
		int  n=table[i++]; /* number of times to print */
		char c=table[i++]; /* character to print */
		for (int repeat=0;repeat<n;repeat++) /* print it n times */
			std::cout<<c<<" ";
	}
	return 0;

}

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Note that the above is just a nine-entry table; the relationship between 3 and 'q' is purely conceptual. 

The big advantage of table-driven programming is "separation of concerns".   The table, and the code that walks through the table (the function "foo" above), are completely separate pieces.  They can be written by different people.  They can be updated independently.  They can be read from different sources.  For example:
Your HW0 has several examples of table-driven programming--for the robot problems, you're preparing a command table, stored as a string.