It's because numbers.size() returns a size_t (unsigned int), so when the array is empty numbers.size() - 1 is a large unsigned number (0xFFFFFFFF). Then the for loop executes that number of times and an invalid array index is used.

still does not work. using exactly what you did and still having a segmentation fault. makes no sense.

Well if you got a SIGSEGV (signal: segmentation violation), then it's likely that your program tried to access a spot in the vector numbers that was out of bounds. This might be because:

a) you used ++i in the for loop control instead of i++

or

b) you didn't include the -1 in the for loop control (needed to stay in bounds when grabbing [i+1] inside the loop)

Hope this was helpful!