vector<string> mat(r,string(c,0));

This line of code creates a 2D matrix of size r x c, where each element is initialized to the null character ('\0').

The vector<string> data type indicates that each row in the matrix is represented by a string. The string constructor is used to create r strings, each of length c, initialized with the null character.

For example, if r = 3 and c = 4, the resulting matrix would be:

{
{'\0', '\0', '\0', '\0'},
{'\0', '\0', '\0', '\0'},
{'\0', '\0', '\0', '\0'}
}

This type of matrix initialization can be useful when working with string data, as it ensures that each element is initialized with a known value.