Question

question about c++ find function

int num_to_id(int num) { static const int table[] = { 10, 15, 30, 50 }: assert(std::find(&table[0], &table[4], num) != &table[4]); return std::find(&table[0], &table[4], num) - &table[0]: } why do you need &table[0] and [4] and have to extract &table[0] from them ??

Answer 1

int num_to_id(int num) {
static const int table[] = { 10, 15, 30, 50 };
assert(std::find(&table[0], &table[4], num) != &table[4]);
return std::find(&table[0], &table[4], num);
}

Explanation:- We need &table[0] and &table[4] because here in the num_to_id function
we have to find the index of the element in the array table[] which
is equal to num. To find the index we use find() function that take
3 parameter first two are the range in which the search will done with
first parameter inclusive and second parameter exclusive that means if we
write &table[0] to &table[4] Then the search will start from element with
index 0 to element with index 3 (index 4 exclude).
  
We extract &table[0] from them because the function num_to_id must return
the index of the element in the array table[] which is equal to num.
when find() function is called it return the iterator pointing the first
element that matches with num. And for getting the index that is pointed by
iterator the it must be substrated from the reference point, Here the
reference point is &table[0] because we started the searching from this points.