Sure thing.

  : shuffle

This starts a declaration for our word 'shuffle'. This implementation expects the address of the array on the stack with the length of the array on top. Some Forth programmers would leave a comment to this effect like so:

  ( addr len -- addr )

Anything in parentheses is a comment, and this shows the contents of the stack we want as input (read from right to left) before the '--' followed by the contents of the stack we'll get as output. (We leave the array address on the stack. If we want to be void like the C example, add a drop to the end of this definition to discard it.)

  1- for

Subtract one from the length given and begin a for...next loop. This will take the length value off the stack and repeat everything up to the matching next n+1 times. Equivalent to:

  for(int z=n; z >= 0; z--) {}

Our stack now looks like this:

  ( addr )

While we're in the loop, a copy of the loop index can be pushed onto the stack via the word i. This actually accesses a secondary stack, so if you nest loops you can obtain progressive indices by using i' and j (or j and k, depending on your Forth dialect.)

  i 1+ random

Add one to the loop index and generate a random number between 0 and this value, exclusive. Our stack now looks like:

  ( addr rand_int(i+1) )

  over +

Over pushes a copy of the second element of our stack. This sequence adds the address of the array to the random index, leaving the address of that cell of the array. Our stack now looks like this:

  ( addr (rand_int(i+1)+addr) )

  over i +

Make another copy of the array address and add the loop index to it. The stack now looks like this:

  ( addr (rand_int(i+1)+addr) (i+addr) )

  swap@

This is a helper word which swaps the contents of two memory addresses. It should be easy to see that this will swap array indices for us in exactly the same way the C code did.

  next ;

Close the loop and terminate the word definition, respectively.