C print 2D array elements in ascending or descending order - c

Question

I have a 2D array in which many elements are zero. For example:
int array[10][10];
memset(array, 0, 100 * sizeof(int));
array[0][0] = 3;
array[2][8] = 5;
array[4][0] = 4;
How could I print the non-zero (only positive) elements in descending (or ascending) order. I was thinking about something like this:
int xpos, ypos;
while (getMaximumInArray(array, &xpos, &ypos))
{
printf("x=%i y=%i val=%i\n", xpos, ypos, array[xpos][ypos]);
array[xpos][ypos] = 0; // set to 0 to make sure we find the next biggest value.
}
However this makes me loop through the array many times (as much as there are non-zero elements). Would there be a better (ie less operations) way to do this?
EDIT : I forgot to specify that I would like to use the positions in my output; so I need not only the sorted values; but also their indices...

Answer 1

A simple way to make this more efficient is to simply sort the values. Sorting - or copying while sorting - has runtime complexity of O(n log n) which is an improvement over your O(n*n). An easy and efficient way to do that is to copy the non-zero values to a std::multiset and then iterate through it.
Update regarding your edit:
No problem. Simply store an object that contains the value and the indices in the set and supply a comparison functor that uses only the value.

Answer 2

You could create new array and then sort it.
For example:
int array[10][10];
array[0][0] = 3;
array[2][8] = 5;
array[4][0] = 4;
int positive[100];
int k = 0;
for(int i = 0; i < 10; i++){
for(int j = 0; j < 10; j++){
if(array[i][j] > 0){
positive[k++] = array[i][j];
}
}
}
sort(positive, positive + k);
for(int i = 0; i < k; i++){
printf("%d", positive[i]);
}

Answer 3

There are multiple approaches to this. Denis Gavrus proposed using an array, which is fine, but you can use a vector if you want to be more memory efficient:
Also, if you need to keep your values and their position, you'll need to store them in a struct/class (the ELEMENT struct), which will need a separate comparison class/function.
Here is the container and the comparison function:
struct ELEMENT{ ///Containing the value and the coordinates
int val, i, j;
};
bool cmp(ELEMENT a, ELEMENT b){
return a.val<b.val;
}
And here is the code that processes your data:
const int L=10; ///In case you want to modify the matrix dimensions
int array[L][L]; ///A really bad name for anything
vector<ELEMENT> v; ///The vector containing the sorted values
for(int i=0; i<L; i++) ///Initialize the matrix with 0
for(int j=0; j<L; j++)
array[i][j]=0;
array[0][0] = 3; ///Your values...
array[2][8] = 5;
array[4][0] = 4;
for(int i=0; i<L; i++)
for(int j=0; j<L; j++)
if(array[i][j]!=0)
v.push_back({array[i][j], i, j}); ///Insert into v the non-zero values and the coordinates
sort(v.begin(), v.end(), cmp);
for(ELEMENT i:v)///Your output
printf("%d at position (%d, %d)\n",i.val, i.i, i.j);
Also, you could use a priority queue:
The comparison class for the priority queue:
class cmp{ ///Containing the functor for the priority queue
public:
bool operator()(ELEMENT a, ELEMENT b){
return a.val<b.val;
}
};
The code that does stuff:
const int L=10; ///In case you want to modify the matrix dimensions
int array[L][L]; ///A really bad name for anything
priority_queue<ELEMENT, vector<ELEMENT>, cmp> v; ///The priority queue containing the sorted values
for(int i=0; i<L; i++) ///Initialize the matrix with 0
for(int j=0; j<L; j++)
array[i][j]=0;
array[0][0] = 3; ///Your values...
array[2][8] = 5;
array[4][0] = 4;
for(int i=0; i<L; i++)
for(int j=0; j<L; j++)
if(array[i][j]!=0)
v.push({array[i][j], i, j}); ///Insert into v the non-zero values
for(; !v.empty(); v.pop())
printf("%d at position (%d, %d)\n",v.top().val, v.top().i, v.top().j);