I found that (slightly) different results coming from matrix-matrix vs. matrix-vector multiplication! julia> m1 = randn(200, 300); julia> m2 = randn(300, 400); julia> m12 = m1 * m2; # matrix-matrix multiplication julia> m12col400 = m1 * m2[:, 400]; # matrix-vector multiplication j…

[image] PSA: floating-point arithmetic Offtopic Sometimes people are surprised by the results of floating-point calculations such as julia> 5/6 0.8333333333333334 # shouldn't the last digit be 3? julia> 2.6 - 0.7 - 1.9 2.2204460492503…

It’s not the problem of floating point resolution. It’s a matter of different implementations in matrix-matrix vs matrix-vector multiplication!

Yes, this is covered in that PSA a bit further down: [image] PSA: floating-point arithmetic Note there are still some ways you might get non-deterministic results: BLAS or LAPACK operations @fastmath: which lets the compiler do a lot of manipulations @simd: which allows re-association of arith…

[image] tomtom: It’s a matter of different implementations in matrix-matrix vs matrix-vector multiplication! I don’t see any surprise. One is a BLAS level-2 operation (gemv), the other one is a BLAS level-3 operation (gemm), they normally have different implementations in BLAS libraries.

Matrix multiplication and vector multiplication give different results!

Specific Domains Numerics

Mason June 24, 2025, 4:55pm 2

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Matrix multiplication and vector multiplication give different results!

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