Traverse Matrix in Diagonal strips

Question

I thought this problem had a trivial solution, couple of for loops and some fancy counters, but apparently it is rather more complicated.

So my question is, how woul

Answer 1

I thought this problem had a trivial solution, couple of for loops and some fancy counters

Precisely.

The important thing to notice is that if you give each item an index (i, j) then items on the same diagonal have the same value j+n–i, where n is the width of your matrix. So if you iterate over the matrix in the usual way (i.e. nested loops over i and j) then you can keep track of the diagonals in an array that is addressed in the above mentioned way.

Answer 2

I would probably do something like this (apologies in advance for any index errors, haven't debugged this):

// Operation to be performed on each slice:
void doSomething(const int lengthOfSlice,
                 elementType *slice,
                 const int stride) {
    for (int i=0; i<lengthOfSlice; ++i) {
        elementType element = slice[i*stride];
        // Operate on element ...
    }
}

void operateOnSlices(const int n, elementType *A) {
    // distance between consecutive elements of a slice in memory:
    const int stride = n - 1;

    // Operate on slices that begin with entries in the top row of the matrix
    for (int column = 0; column < n; ++column)
        doSomething(column + 1, &A[column], stride);

    // Operate on slices that begin with entries in the right column of the matrix
    for (int row = 1; row < n; ++row)
        doSomething(n - row, &A[n*row + (n-1)], stride);
}

Answer 3

A much easier implementation:

//Assuming arr as ur array and numRows and numCols as what they say.
int arr[numRows][numCols];
for(int i=0;i<numCols;i++) {
    printf("Slice %d:",i);
    for(int j=0,k=i; j<numRows && k>=0; j++,k--)
    printf("%d\t",arr[j][k]);
}

Answer 4

Pseudo code:

N = 2 // or whatever the size of the [square] matrix
for x = 0 to N
  strip = []
  y = 0
  repeat
     strip.add(Matrix(x,y))
     x -= 1
     y -= 1
  until x < 0
  // here to print the strip or do some' with it

// And yes, Oops, I had missed it... 
// the 2nd half of the matrix...
for y = 1 to N    // Yes, start at 1 not 0, since main diagonal is done.
   strip = []
   x = N
   repeat
      strip.add(Matrix(x,y))
      x -= 1
      y += 1
   until x < 0
  // here to print the strip or do some' with it

(Assumes x indexes rows, y indexes columns, reverse these two if matrix is indexed the other way around)