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Is there a way to undo using in Julia?




Anonymous Functions


  1. Tutorial for plots

关于 mean()

  1. using Statistics 后才能用 mean(),而 using Distributions 后也能用 mean()。前者表示 generic function with 5 methods,后者称 generic function with 78 methods.

Normal 中标准差为 0 的问题

可知,最低可以支持 1e-323,所以似乎也支持 sqrt(1e-646),但并没有,而且当 sqrt(1e-324) 时精度就不够了,似乎 sqrt(x) 的精度与 x 的精度相当。


如果采用 a=b=c=ones(10) 形式赋值的话,则如果后面改变 a 的值,bc 的值也将随之改变。

但如果 a=b=c=1 为常值的话,则三个变量的值还是独立的。


如果配合 sharedarrays 使用时,需要加上 @sync, 参考@fetch

ERROR: expected Type{T}

参考 ERROR: LoadError: TypeError: Type{…} expression: expected Type{T}, got Module


module Foo end

会爆出这样的错误。但是一开始竟然没有仔细类比,最后在 REPL 中逐行试验才发现是,using SharedArrays 后直接用 SharedArrays{Float64}(10),这与上面 Foo 的错误形式完全一样,竟然没有仔细类比。哎,看来以后多思考一下错误可能的原因,不要一味蛮力试验。

type, instance, and object


  1. A type union is a special abstract type which includes as objects all instances of any of its argument types
  2. Nothing is the singleton type whose only instance is the object nothing.

从中分析知道,instance 相对于 types,而 object 相对 instance。一个 type 可能有多个 instance,每个 instance 称之为 object。

  1. instance of some types
  2. object of some instances

Couldn’t find libpython error


to install its own private Miniconda distribution for you, in a way that won’t interfere with your other Python installations.

Refer to Couldn’t find libpython error #199



  1. Julia parallel computing over multiple nodes in cluster
  2. Using julia -L startupfile.jl, rather than machinefiles for starting workers.
  3. Help setting up Julia on a cluster

pbsdsh (unsolved)

Submit a pbsdsh job and specify multiply nodes with multiply cores, say nodes=2:ppn=4

error log file (see full file in the cluster: dshmcmc.e21907):

fatal: error thrown and no exception handler available.
InitError(mod=:Base, error=ArgumentError(msg="Package Sockets not found in current path:
- Run `Pkg.add("Sockets")` to install the Sockets package.
rec_backtrace at /buildworker/worker/package_linux64/build/src/stackwalk.c:94
record_backtrace at /buildworker/worker/package_linux64/build/src/task.c:246
jl_throw at /buildworker/worker/package_linux64/build/src/task.c:577
require at ./loading.jl:817
init_stdio at ./stream.jl:237
jfptr_init_stdio_4446.clone_1 at /opt/share/julia-1.0.0/lib/julia/ (unknown line)
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
reinit_stdio at ./libuv.jl:121
__init__ at ./sysimg.jl:470
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2182
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1536 [inlined]
jl_module_run_initializer at /buildworker/worker/package_linux64/build/src/toplevel.c:90
_julia_init at /buildworker/worker/package_linux64/build/src/init.c:811
julia_init__threading at /buildworker/worker/package_linux64/build/src/task.c:302
main at /buildworker/worker/package_linux64/build/ui/repl.c:227
__libc_start_main at /lib64/ (unknown line)
_start at /opt/share/julia-1.0.0/bin/julia (unknown line)

But when I just use single node, and arbitrary cores, say nodes=1:ppn=2, it works well.


  1. PBSDSH - High Performance Computing at NYU - NYU Wikis
  2. pbsdsh usage

julia local package 失败折腾记录

  1. no error after add ~/GitHub/adm.jl, but using adm cannot work. refer to Adding a local package
  2. set startup.jl but still not work. refer to How does Julia find a module?
  3. one possible way: Finalizing Your Julia Package: Documentation, Testing, Coverage, and Publishing

julia for scope

The following code

i = 0
for j = 1:10
    i = i + 1

will report

ERROR: UndefVarError: i not defined

A (possible) reasonable explanation is, i is a global variable, we cannot modify a global variable in a local block without global keyword, but we can read i in the for loop.

Alternatively, we can use let block,

i = 0
for j = 1:10
    i = i + 1

then i isn’t really a global variable anymore.


  1. REPL and for loops (scope behavior change)
  2. Scope of variables in Julia
  3. Manual: Scope of Variables

convert a matrix into an array of array

mapslices(x->[x], randn(5,5), dims=2)[:]

refer to

Converting a matrix into an array of arrays

Conversely, we can converting the array of arrays to a matrix, refer to How to convert an array of array into a matrix?

index from 0

using OffsetArrays package, refer to

Github: OffsetArrays

plot kernel density

refer to Kernel density estimation status

自定义 == and hash()

对于自定义的 mutable struct, 直接用 == 返回 false,因此需要自己定义等号,比如

Base.:(==)(x::Reaction, y::Reaction) = all([
    x.substrate == y.substrate, 
    x.product == y.product, 
    x.reversible == y.reversible, 
    x.species == y.species])

而如果想用 unique 函数的话,其基于 isequal 函数,而 isequal 是通过判断 hash 值来判定的,所以仅仅定义了 == 仍不够,还需要定义 hash 函数,这很简单,比如

function Base.hash(obj::Reaction, h::UInt)
    return hash((obj.substrate, obj.product, obj.reversible, obj.species), h)


  1. Hash function for custom type
  2. 到底什么是hash?
  3. Surprising struct equality test