Julia High Performance - Second Edition

Column-wise storage

When an array has only one dimension, its elements can be stored one after the other in a contiguous block of memory. As we observed in the previous section, operating on this array sequentially from its starting index to its end can be very fast, making it amenable to many compiler and CPU optimizations.

Two-dimensional (or greater) arrays can, however, be stored in two different ways. We can store them row-wise or column wise. In other words, we can store from the beginning of the array the elements of the first row, followed by the elements of the second row, and so on. Alternatively, we can store the elements of the first column, then the elements of the second column, and so on.

Consider, in the following diagram, ...