how to select a range in multidimensional array in C++?

Question

Lets say i have:

int array[9][9]= {

        {1 , 2 , 3 , 4, 5, 6, 7, 8, 9},
        {10, 11, 12, 13, 14, 15, 16, 17, 18},
        {19, 20, 21, 22, 23, 24, 25, 26, 27},
        {28, 29, 30, 31, 32, 33, 34, 35, 36},
        {37, 38, 39, 40, 41, 42, 43, 44, 45},
        {46, 47, 48, 49, 50, 51, 52, 53, 54},
        {55, 56, 57, 58, 59, 60, 61, 62, 63},
        {64, 65, 66, 67, 68, 69, 70, 71, 72},
        {73, 74, 75, 76, 77, 78, 79, 80, 81}

    };

how can i only apply some function to the first row (value 1 to 9 ) or the first column only (like value 1 to 73). lets say i want to say index 0 to 9 shall all have value 0.

is it possible to save this range in a variable?

Possible duplicate of [Get the first column of a matrix represented by a vector of vectors](http://stackoverflow.com/questions/15778377/get-the-first-column-of-a-matrix-represented-by-a-vector-of-vectors) — Teivaz, May 18 '16 at 07:02
have a look at the documentation for `std::slice` it does exactly this. — Richard Hodges, May 18 '16 at 08:11

101010 · Answer 1 · 2016-05-18T13:30:28.483

To isolate the rows of the array, you could take a reference to a row of the array:

int (&row)[9] = array[2];

For example the above line takes a reference to the 3rd row of the array.

Live Demo

For the columns, is more complicated.

Alternatevely, you could do the following construct that returns a vector of reference wrappers to either a column or a row of a 2D array.

// if flg == true you get row at idx else if flg == false you get column at idx
template<typename T, int N, int M>
std::vector<std::reference_wrapper<T>>
getRange(T (&arr)[N][M], std::size_t const idx, bool const flg = true) {
  if(flg) {
    return typename std::vector<std::reference_wrapper<T>>(std::begin(arr[idx]), std::end(arr[idx]));
  } else {
    typename std::vector<std::reference_wrapper<T>> out;
    out.reserve(N);
    for(int i(0); i < N; ++i) out.push_back(arr[i][idx]);

    return out;
  }
}

Live Demo

thank you very much. i want to make sure that there is only one 5 in that row or column. how would you do that? — Ali RJ, May 18 '16 at 08:17

Sergiy Shvets · Accepted Answer · 2016-05-18T08:10:55.337

1

Try to do like this:

for (int i = 0; i<10; i++) 
        array[0][i] = 0;

edited May 18 '16 at 08:10

answered May 18 '16 at 06:56

Sergiy Shvets

180
2
14

1

very simple and solved my problem right. thank you. now one more question. now i want to test this range see if a number is repeated or not. for example i want to make sure that there is only one 5 in that row or column. how would you do that? – Ali RJ May 18 '16 at 08:16
Look at this post, similar question http://stackoverflow.com/questions/4003584/more-elegant-way-to-check-for-duplicates-in-c-array – Sergiy Shvets May 18 '16 at 08:36

score 1 · Answer 3 · answered May 18 '16 at 07:26

There are no true multidimensional arrays in C.

In a true multidimensional array, all dimensions are on equal standing. Whatever you can do with rows, you can also do with columns.

This is not the case with C++. The third row of your array is just

array[3]

It's an array on its own in every regard. A range of rows, like any other range, can be represented as a (start, end) pair, e.g. make_pair(array[3], array[7]).

Nothingl like that can be done with columns. The third column, unlike the third row, is not an array, it's just a virtual collection of elements not sitting under any standard data structure umbrella.

The closest thing to a multidimensional array slices are custom iterators, such that ++i moves to either the next element to the right or to the next element below. While you're at it, consider moving away from C style arrays to STL style containers.

Vadim Key · Answer 4 · 2016-05-18T07:09:17.157

For rows it's easy, as you can pass them like:

void foo(int * row, int cols) {
    for (int col = 0; col < cols; ++col) {
        int * x = row + col;
    }
}
...
foo(array[3], 9);
...

For columns it's more difficult but you can thought about every column as something that have specific offset in the array:

void boo(int * col, int rows, int cols) {
    for (int row = 0; row < rows; ++row) {
        int * x = col + row * cols;
    }
}

....

// process fourth column:
boo(array[0]+4, 9, 9);

Of course using sizeof instead of '9' and C++ vectors/array instead of C-style int[][] will make life more easy and code more readable and supportable.

Another way is to use boost::matrix e.g.:

using namespace boost::numeric::ublas;
matrix<double> m(9, 9);
matrix_row<matrix <double> > row(m, 5);
matrix_column<matrix <double> > col(m, 4);

jblixr · Answer 5 · 2016-05-18T09:02:20.567

You can do it by specifying indices (start and end range) with your function and mention whether it should be applied on row or column. Since you are using plain C style array it's trickier to deal with pointers. I recommend you to use vectors and pairs (for ranges).

An example for C style array

void some_function(int array[][9], bool row_or_column, size_t major, size_t start, size_t end){
    if (row_or_column = true) {
        for (int i = start; i < end; i++) {
            cout << array[major][i]; //perform your operation on row
        }
    }
    else {
        for (int i = start; i < end; i++) {
            cout << array[i][major]; //perform your operation on column
        }
    }
}

Set row_or_column as either true for row or false for column, major should specify the column number or row number and the ranges in start and end. Note: end is exclusive

For processing second row with range start = 0 and end = 5 i.e 10 to 14 some_function(array, true, 1, 0, 5)

For processing second column with range start = 0 and end = 5 i.e 2 to 38 some_function(array, false, 1, 0, 5)

You can't convert int[9][9] to int** – Shangtong Zhang May 18 '16 at 08:11 — Shangtong Zhang, May 18 '16 at 08:11

how to select a range in multidimensional array in C++?

5 Answers5