How to remove any unfilled slots from a 2d array without the STL?

Question

This is for the Traveling Sales program assigned to me in my Computing Foundations Class. One of the rules in the class is that are NOT allowed to use the STL in any way other than what our professor tells us for each individual assignment. In this case we are allowed to use string.

My current problem is creating a dynamic amount of cities/routes. My plan was to create a 13 by 13 array and then remove any elements that aren't used. In Java this is no big deal, but in C++ I am finding it hard to solve.(without using vector)

Is there a way (without using vector/anything in the STL) to remove any unfilled slots from an array? In other words decrease the size of the array until it reaches the first slot that has been taken up?

Another solution to this would be to create an array that is of a dynamic size, but google and cplusplus.com are telling me that you can't do that without using vector.

Example:

int matrix [13][13] = {};

for (int i = 0; i < 12; i++){
  matrix [i][i] = 4;
}

That would leave me with slots [12][13] and [13][13] ect.. empty how would I delete any slots that are empty?

Answer 1

Can you?

Yes, by using realloc().

Should you?

No, except if you really know what you are doing.

---

There are plenty of reasons for not doing what you want to do. You are using c++, thus an std::vector should be used instead. Moreover, realloc() may need to copy all the new array you are creating (but that's just for increasing the array's size).

---

how to have a dynamic amount of cities/routes?

Mark the empty slots of your array (or simple leave them empty), so that on the processing step, you simply ignore them.

Example:

Assume you want to find the sum of all the elements of the matrix, but ignore all the elements who are equal to 4. Then you could do this:

#include <iostream>

int main() {
        int matrix[13][13] = {0};
        for (int i = 0; i < 12; i++) {
                matrix[i][i] = 4;
        }
        // we want to sum all the elements
        // except the "emptyslots" (= 4)
        int sum = 0;
        for(int i = 0; i < 13; ++i)
                for(int j = 0; j < 13; ++j)
                        if(matrix[i][j] != 4)  // ignore empty slots!!
                                sum += matrix[i][j];
        std::cout << sum << std::endl;
        return 0;
}

Output:

gsamaras@gsamaras:~$ g++ -Wall px.cpp 
gsamaras@gsamaras:~$ ./a.out 
0        <-- as expected, since we initialized the matrix to 0

---

An alternative solution would be to use a simple linked list and delete the empty slots.