Question

I’ve implemented this search algorithm for an ordered array of integers. It works fine for the first data set I feed it (500 integers), but fails on longer searches. However, all of the sets work perfectly with the other four search algorithms I’ve implemented for the assignment.

This is the function that returns a seg fault on line 178 (due to an unexpected negative m value). Any help would be greatly appreciated.

CODE:

155 /* perform Algortihm 'InterPolationSearch' on the set
156  * and if 'key' is found in the set return it's index
157  * otherwise return -1 */
158 int
159 interpolation_search(int *set, int len, int key)
160 {
161   int l = 0;
162   int r = len - 1;
163   int m;
164
165   while (set[l] < key &&  set[r] >= key)
166   {
167
168     printf ("m = l + ((key - set[l]) * (r - l)) / (set[r] - set[l])\n");
169
170     printf ("m = %d + ((%d - %d) * (%d - %d)) / (%d - %d);\n", l, key, set[l], r, l, set[r], set[l]);
171     m = l + ((key - set[l]) * (r - l)) / (set[r] - set[l]);
172     printf ("m = %d\n", m);
173
174 #ifdef COUNT_COMPARES
175     g_compares++;
176 #endif
177
178     if (set[m] < key)
179       l = m + 1;
180     else if (set[m] > key)
181       r = m - 1;
182     else
183       return m;
184   }
185
186   if (set[l] == key)
187     return l;
188   else
189     return -1;
190 }

OUTPUT:

m = l + ((key - set[l]) * (r - l)) / (set[r] - set[l])
m = 0 + ((68816 - 0) * (100000 - 0)) / (114836 - 0);
m = -14876

Thankyou!

Rhys

Answer 1

That is because the intermediate value of the calcualtion (68816 - 0) * (100000 - 0) exceeds the value that could be held inside an int. You can cast the intermediate value to a datatype which can hold the bigger number (for example a long long) to solve the problem