Jump Table Switch Case question

Question

I am trying to understand some things about jump tables and its relationship between a switch case statement.

I was told that a jump table is a O(1) structure that t

Answer 1

Suppose you had an array of procedures:

void fa() { 
 printf("a\n");
}

...

void fz() { 
 printf("it's z!\n");
}



typedef void (*F)();
F table[26]={fa,fb,...,fz};

Suppose you accept a character (from a-z) of input from the user and run fc:

char c;
switch(c) {
   case 'a': fa();break;
   case 'b': fb();break;
   ...
   case 'z': fz();break;       
   default: exit(-1);
}

Ideally this would be replaced with something like:

if (c<'a' || c>'z') exit(-1);
else (*table[c-'a'])();

Naturally, you might make the table bigger so the range check wouldn't be necessary.

The compiler would do this for arbitrary code, not necessarily function calls only, and would do it by storing the address to jump to (essentially, a goto). C doesn't directly support any sort of computed goto (indexing into a table or otherwise), but the CPU instructions for it are pretty simple.