Row- and column-major order
The difference between the orders lies in which elements of an array are contiguous in memory. In a row-major order, the consecutive elements of a row reside next to each other, whereas the same holds true for consecutive elements of a column in a column-major order. While the terms allude to the rows and columns of a two-dimensional array, i.e. a matrix, the orders can be generalized to arrays of any dimension by noting that the terms row-major and column-major are equivalent to lexicographic and colexicographic orders, respectively.
Data layout is critical for correctly passing arrays between programs written in different programming languages. It is also important for performance when traversing an array because modern CPUs process sequential data more efficiently than nonsequential data. This is primarily due to CPU caching. Contiguous access makes it also possible to use SIMD instructions that operate on vectors of data. In some media such as tape or NAND flash memory, accessing sequentially is orders of magnitude faster than nonsequential access.
Explanation and example
The terms row-major and column-major stem from the terminology related to grouping objects. A general way to order objects with many attributes is to first group and order them by one attribute, and then, within each such group, group and order them by another attribute, etc. If more than one attribute participate in ordering, the first would be called major and the last minor. If two attributes participate in ordering, it is sufficient to name only the major attribute.
In the case of arrays, the attributes are the indices along each dimension. For matrices, the first index indicates the row, and the second indicates the column, e.g., given a matrix A , a1,2 is in its first row and second column. This convention is carried over to the syntax in programming languages, although often with indexes starting at 0 instead of 1.
Even though the row is indicated by the first index and the column by the second index, no grouping order between the dimensions is implied yet. We can then choose to group and order the indices either row-major or column-major. The terminology can be applied to even higher dimensional arrays. Row-major grouping starts from the leftmost index and column-major from the rightmost index, leading to lexicographic and colexicographics orders, respectively.
For example, for the array
The two possible ways are:
To select the correct element one must know whether an array A[a][b] in a particular language is stored in a row-major (a subscript indexes rows, b subscript indexes columns) or in a column-major mode (a subscript indexes columns, b subscript indexes rows), eg. in C A would result in , while in Fortran it would result in .
Programming languages and libraries
Programming languages or their standard libraries that support multi-dimensional arrays typically have a native row-major or column-major storage order for these arrays.
Support for multi-dimensional arrays may also be provided by external libraries, which may even support arbitrary orderings, where each dimension has a stride value, and row-major or column-major are just two possible resulting interpretations.
As exchanging the indices of an array is the essence of array transposition, an array stored as row-major but read as column-major (or vice versa) will appear transposed. As actually performing this rearrangement in memory is typically an expensive operation, some systems provide options to specify individual matrices as being stored transposed. The programmer must then decide whether or not to rearrange the elements in memory, based on the actual usage (including the number of times that the array is reused in a computation).
Address calculation in general
The concept generalizes to arrays with more than two dimensions.
For a d-dimensional array with dimensions Nk (k=1...d), a given element of this array is specified by a tuple of d (zero-based) indices .
In row-major order, the last dimension is contiguous, so that the memory-offset of this element is given by:
In column-major order, the first dimension is contiguous, so that the memory-offset of this element is given by:
For a given order, the stride in dimension k is given by the multiplication value in parentheses before index nk in the right hand-side summations above.
More generally, there are d! possible orders for a given array, one for each permutation of dimensions (with row-major and column-order just 2 special cases), although the lists of stride values are not necessarily permutations of each other, e.g., in the 2-by-3 example above, the strides are (3,1) for row-major and (1,2) for column-major.
- Matrix representation
- Vectorization (mathematics), the equivalent of turning a matrix into the corresponding column-major vector
- CSR format, a technique for storing sparse matrices in memory
- Morton order, another way of mapping multidimensional data to a one-dimensional index, useful in tree data structures
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