Hi, I have been following tutorials to make the switch from Python&Matlab to Julia. And one reminder that pops up all the time is to use loops in Julia since they are really fast. I was wondering why exactly this is the case? Vectorizing the code feels intuitively faster, so I am really curious why …

Understanding why loops are fast in Julia first requires you to understand why loops are slow in languages like Python and Matlab. In addition to @mbauman ’s great post about this, I gave a course lecture about performance in dynamic languages based off of these notes . Vectorizing the code feels i…

To quote Chris, It’s not that vectorization is fast It’s that loops in Matlab/Python are slow. I think the reason has something to do w type checks, & the Jit compiler… Btw, Julia’s jit compiler uses the same architecture as Pythons Numba, so they achieve loops @ same speed. Key difference is n…

Vectorized code is expanded into loops anyway, in any language. That feeling comes from the fact that the loops are executed in lower level languages in R, Matlab, and Python. In addition, one may be confused with loop vectorization, which is an optimization that takes advantage of the fact that pr…

On the other hand @tullio a[i] = b[i,j]*c[j] is incredibly fast and easy to understand for one line of code :wink: ( see the thread ) Mostly don’t write loops, let macros do that for you!

[image] dlakelan: Mostly don’t write loops, let macros do that for you! Tullio is great, but I do not agree that this is a good advice in general. I would suggest better learning how to write performant loops. That gives us a flexibility that no macro can provide.

No doubt you should learn to write loops, but every opportunity you can turn 10 lines of code into 1 line of code with better or equal performance you should take it. Not everything is a tensor contraction or whatever, so loops are still a thing to learn.

Previously: [image] Why Are Languages Like Python and MATLAB So Much Slower than Julia? Internals & Design The simplest answer is that, naively, higher level dynamic languages need to ask meta-questions about everything in order to figure out what to do. That is, given a…

For a more ELI5 version of the excellent explanations above, I submit this simplistic, horrific generalization that also happens to be surprisingly accurate: When something computationally intensive runs fast in Python, it’s actually running in C (or Fortran or …). Vectorization is basically a tri…

Thanks for all the reactions everyone! This really helped me out.

Why are loops not slow in Julia?

New to Julia

mbauman June 20, 2021, 2:26am 7

Previously:

Topic		Replies	Views
Why Are Languages Like Python and MATLAB So Much Slower than Julia? Internals & Design question , first-steps , performance	8	2721	December 7, 2020
Comparison between Loops and Vectorization in Julia General Usage	3	686	May 3, 2024
Matlab versus Julia General Usage	33	5188	July 15, 2021
Julia significantly slower than Matlab for simple sin(x) loop – am I doing something wrong? New to Julia question , performance	8	835	October 28, 2025
Loop vs vectorization Performance	4	1749	October 10, 2023

Why are loops not slow in Julia?

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