Comp 1672
Class Notes for Jan 17, 2002

Using 2D arrays in a program
Example of declaring:
     int arr[5][3];  // creates space in memory for 5 rows, 3 columns of ints
Examples of declaring and initializing
     // The next line declares and initializes a 2-row, 4 column array of floats
     float arr[][] = { {1.1 , 2.2, 0, 0 }, {0, 0, 0, 0} };
     // Initialers omitted are set to 0.  In fact, the next line has the
     // Identical outcome to the previous line 
     float arr[4][2] = { {1, 1} };
         
See 2Darray program from Wednesday for examples of how to manipulate
individual rows, individual columns, and the entire array

Remember that even though our pictures of arrays look two-dimensional,
the computer stores memory in a long linear (1-dimensional) list.
So, the computer has to do some calculations to make it look to us like it's
really 2-D.  It stores the data row-by-row, exactly in the order that we
initialize 2-D arrays.  So, when we access arrays with an expression like
testscores[1][2], it must calculate how many elements to skip.  To access
the element testscores[i][j], look at element number i*num_cols+j.
You can use this technique in your code as well, to store a 2-D array using a
single dimensional array.

The previous paragraph shows us that when we pass a 2-D array to a function,
we will have to also pass the number of columns.  This is done as
part of the array parameter, as in the following prototype
    void print_array(int arr[][num_cols]);
Note that you do not need to fill in the number of rows, although if you
want to you may fill that in as well, for documentation purposes.