Re: Search only one column of a multi-dimensional array

Victor Bazarov <v.bazarov@comcast.invalid>
Mon, 07 Mar 2011 14:08:45 -0500
On 3/7/2011 1:16 PM, Angus wrote:

On Mar 7, 6:05 pm, Victor Bazarov<v.baza...@comcast.invalid> wrote:

On 3/7/2011 12:17 PM, Angus wrote:

using lower_bound I want to pass a 2-dimensional array eg like this:
     static const int tbl[5][3] = {
        1000, 800, 801,
        1001, 802, 803,
        1002, 804, 805,
        1003, 806, 807,
        1004, 808, 809

I want to search in only the first column - eg the numbers 1000-1004.
I am trying to find the position in this 2-dimensional array, and then
once located I can access elements[position][1] and elements[position]

I tried this:
     int arrsize = sizeof(tbl)/sizeof(tbl[0]);
     low=lower_bound(tbl[0], tbl[0]+arrsize, 7);

But get compile error C2440 cannot convert from 'const int *' to 'int

How can I do this?

You should probably provide your own iterator, which when incremented
will skip the rest of the "row".

I do not respond to top-posted replies, please don't ask

Are you suggesting it 'jump' over the other items in row. Eg if there
were five columns of ints it would jump 5 int's worth on each

Yes, that's exactly what I'm suggesting.

Can you provide an outline example? Or place to find something like

Something like this:

   template<class T, size_type N, size_type M>
   class arr_col_iter_t
      T (&arr)[N][M];
      size_type col;
      size_type ind;
      arr_col_iter_t(T (&a)[N][M], size_type c, size_type ind0)
       : arr(a), col(c), ind(ind0) {}

      T& operator *() { return arr[ind][col]; }
      arr_col_iter_t& operator++() { ++ind; return *this; }
      arr_col_iter_t operator++(int) {
         arr_col_iter_t temp(*this);
         ++ind; return temp;

      bool operator!=(arr_col_iter_t const& other) const
         assert(&arr == &other.arr
               && col == other.col); // same array, same col
         return ind != other.ind;

   template<class T, size_type N, size_type M>
   arr_col_iter_t<T,N,M> arr_iter(T (&arr)[N][M], size_type c,
                                  size_type ind0 = 0)
      return arr_col_iter_t<T,N,M>(arr, c, ind0);
      // to iterate over "column 0" from 0 to arrsize-1:
      lower_boundary(arr_iter(tbl, 0), arr_iter(tb, 0, arrsize), 7);

There are undoubtedly holes in that (not to mention that it's not a
fully fledged iterator according to Standard's requirements - no tag, no
value_type, etc.)

I do not respond to top-posted replies, please don't ask

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