Table-Driven Programming

The C++ "Array Initializer"

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

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, /* cube 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:

In a game, the code used to make a character do stuff (like attack the player, or drool, or buy and sell stuff) is part of the executable, written by the game engine company. The particular stuff the character should do, and when they should do it, comes from some sort of table, usually part of that level's map, and is read from disk or downloaded from the network.
The table can contain user interface strings from the program, like "Please enter a number". To translate the program into another human language, like German or Greek, you just have to change the table to contain "Bitte geben Sie eine Zahl" or "Παρακαλώ εισάγετε έναν αριθμό". This "look up user interface text from a table" idea is a key part of internationalization (i18n), as implemented in, e.g., GNU gettext.
A short program can look up "the answer" in a table. The answer can be the result of a much longer (slower, more resource-intensive) program, or even human effort. This only works if there are only a small possible number of questions. I've used this method for problems like designing playoff tournaments; big tournaments are regular (a big tree) and hence the general case is easy, but small tournaments are tricky to write code for, so I just encoded all the good small tournaments in a table!

The C++ "Switch" statement

In C++, if you have several values to test, you can use a series of "if" statements:

int x=0; std::cin>>x; /* read what to do */
if (x==3) std::cout<<"Lucky three!\n";
else if (x==7) std::cout<<"Seven!  Yes!\n";
else if (x==13) std::cout<<"NooooOOOO!!!!\n";
else std::cout<<"meh\n";

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If all the "if" statements are testing the same integral value, a "switch" does the same thing:

int x=0; std::cin>>x; /* read what to do */
switch (x) {
	case 3: std::cout<<"Lucky three!\n"; break;
	case 7: std::cout<<"Seven!  Yes!\n"; break;
	case 13: std::cout<<"NooooOOOO!!!!\n"; break;
	default: std::cout<<"meh\n"; break;
}

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Often "switch" is faster than a nested block of "if" statements, especially if there are many possibilities. I'm using "switch" below, so I thought I'd explain this syntax first.

How this relates to CS 301: Machine Code

"Machine code" is a block of binary data that the CPU interprets as a long table of commands. You'll be writing machine code for HW2. There's nothing magical about machine code, and in fact you can easily write a little program that walks through and executes a table of our own newly-defined "machine code":

const unsigned char table[]={
	0, /*yo! */
	1, /*print x */
	1, /*print x */
	0, /*yo! */
	2 /* exit */
};

int foo(void) {
	int i=0; /* our location in the table */
	while (1) /* always keep looping through the table */
	switch (table[i++]) { /* look at the next thing in the table */
	case 0: cout<<"Yo!\n"; break; /* single-Yo instruction */
	case 1: cout<<"x\n"; break; /* single-X instruction */
	case 2: return 0; /* stop looping through the table */
	default:
		cout<<"Unrecognized table entry!\n";
		return -999;
	}
}

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Rather than having two identical "print x" commands, we can make the "x" command repeatable, by adding a repetition count.

const unsigned char table[]={
	0, /*yo! */
	1, /*print x... */
	   2, /*       ... two times */
	0, /*yo! */
	2 /* exit */
};

int foo(void) {
	int i=0; /* our location in the table */
	while (1) /* always keep looping through the table */
	switch (table[i++]) { /* look at the next thing in the table */
	case 0: cout<<"Yo!\n"; break; /* single-Yo instruction */
	case 1: { /* multi-x instruction */
		int count=table[i++]; /* next byte in table is the x repeat count */
		for (int repeat=0;repeat<count;repeat++)
			std::cout<<'x'<<endl;
		break;
	}
	case 2: return 0; /* stop looping through the table */
	default:
		cout<<"Unrecognized table entry!\n";
		return -999;
	}
}

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Note that 0, a "Yo!" instruction, stands alone in the table, while 1, a "multi-x" instruction, takes two bytes, because the second byte is an x count. The indented "2" is not an exit command, it's the repetition count for the 1 instruction!

x86 Machine Code

You can of course use any numbers you like for the table values. Here's the same exact idea, but with x86-compatible instruction numbers:

const unsigned char table[]={
	0xb0, /*set x = ... */
	7, /*         ... this byte */
	0xc3 /* exit */
};

int foo(void) {
	int x=0; /* our "register" (temporary storage, and return value) */
	int i=0; /* our location in the table */
	while (1) /* always keep looping through the table */
	switch (table[i++]) { /* look at the next thing in the table */
	case 0xb0: { /* set-x instruction */
		x=table[i++]; /* next byte is the new value for x */
		break;
	}
	case 0xc3: return x; /* stop looping through the table */
	default:
		cout<<"Illegal instruction!\n";
		return -999;
	}
}

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Our table just has (8-bit) bytes in it, but sometimes we want to be able to set an entire (32-bit) int. The standard x86 solution to this is to split the integer into four bytes: first the low byte (lowest value, last two hex digits), then the not-so-low byte, the not-so-high byte, and the highest byte, like so.

const unsigned char table[]={
	0xb8, /* set x =... */
	4, /* low byte is 4 (that is, 0x04) */
	1, /* next byte is 1 (that is, 0x01) */
	0, /* highest two bytes are both zero */
	0,
	0xc3 /* return that */
};

int foo(void) {
	int x=0; /* register */
	int i=0;
	while (1) switch (table[i++]) {
	case 0xb8:
		x=table[i]|(table[i+1]<<8)|(table[i+2]<<16)|(table[i+3]<<24); 
		i+=4;
		break; 
	case 0xc3: return x;
	default:
		cout<<"Illegal instruction!\n";
		return -999;
	}
}

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This returns "0x104". The "0x04" is the low byte. "0x01" is the next higher byte, and all higher bytes are zero.

What's amazing is that I can tell the CPU to execute the bytes above, and it acts like a function that returns 0x104--the CPU is just table-driven hardware!