Problem - Kevin Tafuro

char *p = buf;
while (--len)
*p++ = (char)spc_rand_range(33, 126);
*p = 0;
return buf;
}
11.15 Shuffling Fairly
Problem
You have an ordered list of items that you would like to shuffle randomly, then visit
one at a time. You would like to do so securely and without biasing any element.
Solution
For each index, swap the item at that index with the item at a random index that has
not been fully processed, including the current index.
Discussion
Performing a statistically fair shuffle is actually not easy to do right. Many developers
who implement a shuffle that seems right to them off the top of their heads get it
wrong.
We present code to shuffle an array of integers here. We perform a statistically fair
shuffle, using the spc_rand_range( ) function from Recipe 11.11.
#include
void spc_shuffle(int *items, size_t numitems) {
int tmp;
size_t swapwith;
while (--numitems) {
/* Remember, it must be possible for a value to swap with itself */
swapwith = spc_rand_range(0, numitems);
tmp = items[swapwith];
items[swapwith] = items[numitems];
items[numitems] = tmp;
}
}
If you need to shuffle an array of objects, you can use this function to first permute
an array of integers, then use that permutation to reorder the elements in your array.
That is, if you have three database records, and you shuffle the list [1, 2, 3], getting
[3, 1, 2], you would build a new array consisting of the records in the listed order.
612 | Chapter 11: Random Numbers
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