Why Julia arrays are stored in column-major order? Column-major vs row-major are not entirely symmetric regarding code style, because when we access the i,j’th element of an array, we write A[i,j] with the row index first, which can break the symmetry. For example, I find it surprising that in: […

As I understand it, it was mainly historical precedent, driven by the fact that most linear-algebra libraries (LAPACK etc.) are column-major (because Fortran is column-major, and hence Matlab is column-major), though it would have been possible to work around this with judicious use of transposition…

[image] DNF: like ‘first index is fastest, period. but in a simple table like this: John 54kg 1.80m Paul 80kg 1.70m Denis 70kg 1.60m isn’t it natural that running over columns (to compute average properties, for instance) should be the fastest?

[image] lmiq: isn’t it natural that What is “natural” in these discussions may be very application-dependent. Some applications benefit from either choice, or possible hybrids (eg block storage). The Julian answer to “row or column major” is that yes, the language provides a default for A…

Regarding your example: [image] lmiq: John 54kg 1.80m Paul 80kg 1.70m Denis 70kg 1.60m Its transpose is as simple or natural: John Paul Denis Weight 54kg 80kg 70kg Height 1.80m 1.70m 1.60m

Think about the weight and height of a population and put that in that table (or the grades of an exam). The point is that the number of values is much greater than the number of categories. But of course this is case dependent an application dependent. My note was only because sometimes seeing one…

[image] lmiq: but in a simple table like this: Sure, I was thinking of plain numeric arrays, tables would be different.

A = rand(2,2)' is a row major matrix in natural Julia syntax.

Let me add my my 2¢ too. I confess the issue and the arguments initially made my brain fry, it is indeed quite interesting. But this is how I view it now. With the notational convention A[i,j] we tend to call i the row index and j the column index. Taking into consideration that Julia is column-maj…

Okay, I’m confused now. What is the fast way to access/create A[i,j,k] in Julia … with nested for loops on different lines with nested for loops on one line with an array comprehension … and how do you reason through their order? I understand what column major storage means, but not necessarily …

this may help dispel the confusion [image] How much can hard-coded indexing be avoided in Julia? Internals & Design I’ve noticed a trend toward Julia’s new keywords trending towards using keywords and functions instead of integer indexing in Julia. For example, first(arra…

Each one remembers this as he/she wants. But one way it to remember that: run first over the first index: julia> function update_fast!(A) for k in axes(A,3) for j in axes(A,2) for i in axes(A,1) # <--- running first A[i,j,k] = A[i,j,k] -…

Why column major?

General Usage

Topic		Replies	Views
Row and column major order for arrays of different shape Performance column-major , row-major	23	2880	April 12, 2022
The columns and the rows - puzzled about performance Performance question	8	1003	October 14, 2021
Performance difference when accessing square matrix rows-first or cols-first Performance	14	1885	April 13, 2021
Array comprehension is inconsistent, given the column-major property of julia Internals & Design question , array	28	1371	January 18, 2026
What are the pros and cons of row/column major ordering? Internals & Design arrays , column-major , row-major	10	4027	February 13, 2024

Why column major?

Related topics