Is there a way to undo using in Julia?


  1. https://stackoverflow.com/questions/33927523/can-i-make-julia-forget-a-method-from-the-repl
  2. https://stackoverflow.com/questions/36249313/is-there-a-way-to-undo-using-in-julia

Although we cannot remove some defined function, with the powerful Revise.jl, we can update the functions without reloading the scripts.

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

ENV["PYTHON"]=""; Pkg.build("PyCall")

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

Actually, using PyPlot also encounters the similar question, cannot find matplotlib, then I changed to the conda environment contained such package, and then

# change to package command line
build PyCall

it works. And it seems that it does NOT depend on conda environment any more, i.e., PyPlot still works for different conda environment, even without matplotlib package in such environment.

In the words of the official documentation,

If you use Python virtualenvs, then be aware that PyCall uses the virtualenv it was built with by default, even if you switch virtualenvs. If you want to switch PyCall to use a different virtualenv, then you should switch virtualenvs and run rm(Pkg.dir(“PyCall”,”deps”,”PYTHON”)); Pkg.build(“PyCall”).

which supports my above guess.

Record for install PyCall

Switch to py37 conda environment, then run julia1.2.0

(v1.2) pkg> add PyCall

Then test to import module in the same REPL,

julia> using PyCall
[ Info: Recompiling stale cache file /home/weiya/.julia/compiled/v1.2/PyCall/GkzkC.ji for PyCall [438e738f-606a-5dbb-bf0a-cddfbfd45ab0]

julia> math = pyimport("math")
PyObject <module 'math' from '/home/weiya/anaconda3/lib/python3.7/lib-dynload/math.cpython-37m-x86_64-linux-gnu.so'>

Now open a new julia session under different conda environment (or no any conda environment), re-run the above code

julia> using PyCall

julia> math = pyimport("math")
PyObject <module 'math' from '/home/weiya/anaconda3/lib/python3.7/lib-dynload/math.cpython-37m-x86_64-linux-gnu.so'>

As you can see, no any compiling info when using PyCall, and return the same math module. However such module does not corresponds to the py37 conda environment. The official documentation says

On GNU/Linux systems, PyCall will default to using the python3 program (if any, otherwise python) in your PATH.

Then to specify the version of python such as

julia> ENV["PYTHON"] = Sys.which("python")

and rebuild, note that need to restart the julia session to recompiling,

julia> using PyCall
[ Info: Recompiling stale cache file /home/weiya/.julia/compiled/v1.2/PyCall/GkzkC.ji for PyCall [438e738f-606a-5dbb-bf0a-cddfbfd45ab0]

julia> pyimport("math")
PyObject <module 'math' from '/home/weiya/anaconda3/envs/py37/lib/python3.7/lib-dynload/math.cpython-37m-x86_64-linux-gnu.so'>

then the module corresponds to the specified conda environment, and then like the above test, it also can be used in other conda environment without recompiling.



  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/sys.so (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/libc.so.6 (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)



HASH函数是这么一种函数,他接受一段数据作为输入,然后生成一串数据作为输出,从理论上说,设计良好的HASH函数,对于任何不同的输入数据,都应该以极高的概率生成不同的输出数据,因此可以作为“指纹”使用,来判断两个文件是否相同。 数据 ---->输入 HASH 函数 ---->输出指纹数据


  1. Hash function for custom type
  2. 到底什么是hash?
  3. Surprising struct equality test
  4. What is the difference between “using” and “import”?

& 优先级


(x[1] <= width) & (x[1] >= 0) & (x[2] <= height) & (x[2] >=0)


x[1] <= width & x[1] >= 0 & x[2] <= height & x[2] >=0

MethodError: objects of type Module are not callable

check if the function is mistaken by the module name, such as AxisArray vs. AxisArrays.

/lib/x86_64-linux-gnu/libz.so.1: versionZLIB_1.2.9’ not found`

ref to https://stackoverflow.com/questions/48306849/lib-x86-64-linux-gnu-libz-so-1-version-zlib-1-2-9-not-found

Re-build after install cudnn

refer to GPU on cluster: conversion to pointer not defined for CuArray{Float32,2}

load ImageView on Server

when using ImageView, it throws an error,

ERROR: LoadError: InitError: Cannot open display:

fixed by ssh -X. If further ssh on the node, it still works.

-i vs -L

if need to REPL and also arguments, then -i would be suitable.

And note that -- should be separated the switches and program files. (but seems not necessary)

Julia docs: Getting Started

Unable to display plot using the REPL. GKS errors

The reason would be relate to the GR package,

cd .julia/packages/GR/ZI5OE/deps/gr/bin/
ldd gksqt

and then get

linux-vdso.so.1 =>  (0x00007ffdc1e58000)
libQt5Widgets.so.5 => not found
libQt5Gui.so.5 => not found
libQt5Network.so.5 => not found
libQt5Core.so.5 => not found
libGL.so.1 => /lib64/libGL.so.1 (0x00002ae608a73000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00002ae608ce5000)
libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00002ae608f01000)
libm.so.6 => /lib64/libm.so.6 (0x00002ae609209000)
libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00002ae60950b000)
libc.so.6 => /lib64/libc.so.6 (0x00002ae609721000)
libexpat.so.1 => /lib64/libexpat.so.1 (0x00002ae609aef000)
libxcb-dri3.so.0 => /lib64/libxcb-dri3.so.0 (0x00002ae609d19000)
libxcb-xfixes.so.0 => /lib64/libxcb-xfixes.so.0 (0x00002ae609f1c000)
libxcb-present.so.0 => /lib64/libxcb-present.so.0 (0x00002ae60a125000)
libxcb-sync.so.1 => /lib64/libxcb-sync.so.1 (0x00002ae60a328000)
libxshmfence.so.1 => /lib64/libxshmfence.so.1 (0x00002ae60a52f000)
libglapi.so.0 => /lib64/libglapi.so.0 (0x00002ae60a733000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x00002ae60a963000)
libXext.so.6 => /lib64/libXext.so.6 (0x00002ae60ab8a000)
libXdamage.so.1 => /lib64/libXdamage.so.1 (0x00002ae60ad9d000)
libXfixes.so.3 => /lib64/libXfixes.so.3 (0x00002ae60afa0000)
libX11-xcb.so.1 => /lib64/libX11-xcb.so.1 (0x00002ae60b1a6000)
libX11.so.6 => /lib64/libX11.so.6 (0x00002ae60b3a9000)
libxcb.so.1 => /lib64/libxcb.so.1 (0x00002ae60b6e7000)
libxcb-glx.so.0 => /lib64/libxcb-glx.so.0 (0x00002ae60b90f000)
libxcb-dri2.so.0 => /lib64/libxcb-dri2.so.0 (0x00002ae60bb2b000)
libXxf86vm.so.1 => /lib64/libXxf86vm.so.1 (0x00002ae60bd30000)
libdrm.so.2 => /lib64/libdrm.so.2 (0x00002ae60bf36000)
libdl.so.2 => /lib64/libdl.so.2 (0x00002ae60c148000)
/lib64/ld-linux-x86-64.so.2 (0x0000557abc358000)
libXau.so.6 => /lib64/libXau.so.6 (0x00002ae60c34c000)
libpcre.so.1 => /lib64/libpcre.so.1 (0x00002ae60c551000)

(refer to one comment in Error: GKS: can’t connect to GKS socket application)

So it seems that it is due to the missing of libqt5. If I have the sudo privilege, maybe just type

sudo apt-get install qt5-default

but I cannot. But I noticed that I have installed miniconda3, and the qt is installed,

qmake -v
# QMake version 3.1
# Using Qt version 5.9.7 in /users/xxx/miniconda3/lib

So a natural way is to append the library path of qt to LD_LIBRARY_PATH, that is,

# in .bashrc
export LD_LIBRARY_PATH=/users/xxx/miniconda3/lib:$LD_LIBRARY_PATH

then rebuild GR package in the julia REPL.

This solution should work, but actually it failed at the first time. Do not be too frustrated, I found the reason is that I type miniconda3 as minconda3 in the path.

It works now, although it still throw an error,

julia> libGL error: unable to load driver: swrast_dri.so
libGL error: failed to load driver: swrast


总是记不太清 sum, mean 的时候 dims=1 是按行求和还是按列求和,经常先玩个 toy example 才能分辨出来。比如,

julia> a = rand(4, 3)
4×3 Array{Float64,2}:
 0.279181  0.0903167  0.148329 
 0.486691  0.869156   0.0538834
 0.110781  0.836284   0.486467 
 0.810343  0.208659   0.759561 

julia> sum(a, dims=1)
1×3 Array{Float64,2}:
 1.687  2.00442  1.44824

即把 dims 所代表的维度元素加起来,或者说沿着 dims 进行运算。这一点与 R 的函数 applymargin 参数作用刚好相反,

> a = matrix(rnorm(12), 4, 3)
> a
           [,1]        [,2]       [,3]
[1,]  1.1535199  0.03198768  0.2857126
[2,] -1.3616720  0.32325598  1.9242805
[3,] -0.6519595 -1.14800119 -0.3635221
[4,]  1.2713182  0.98515312  0.2269218
> apply(a, 1, sum)
[1]  1.4712202  0.8858644 -2.1634828  2.4833932


reset kw... value

In the following situation, I want to reset a particular argument in kw....

function f(; kw...)
    g(; kw...)

Note that the type of kw is Base.Iterators.Pairs, firstly I want to directly reset the value via

h(;kw...) = kw
h(a=1).data.a = 2

but it throws

ERROR: setfield! immutable struct of type NamedTuple cannot be changed
 [1] setproperty!(::NamedTuple{(:a,),Tuple{Int64}}, ::Symbol, ::Int64) at ./Base.jl:21
 [2] top-level scope at none:0

So I need to find new method, or give up such idea. Fortunately, the official documentation of Julia says

The nature of keyword arguments makes it possible to specify the same argument more than once. For example, in the call plot(x, y; options..., width=2) it is possible that the options structure also contains a value for width.

Thus, I can write

function f(; kw...)
    g(; kw..., b = 6)
to solve my problem, where b is the argument I want to reset.

some interesting behavior

d = Dict{Int, Int}()
d[1] = 1

if we access

# d.keys

it will return a 16-element Array{Int64,1}, but if we use


it correctly returns the exactly one elements, but with type Base.ValueIterator for a Dict{Int64,Int64} with 1 entry.

function in function

julia> function f2()
           a = 10
           function g()
               a = a+10
f2 (generic function with 1 method)

julia> f2()


julia> function f2()
           a = 10
           g() = a + 10
f2 (generic function with 1 method)

julia> f2()

since the first one does not have return, but g() has return and the addition is not on a.

pass array into function

julia> a = [1,2]
2-element Array{Int64,1}:

julia> function f(a)
           a = a .+ 1
f (generic function with 1 method)

julia> f(a)
2-element Array{Int64,1}:

julia> a
2-element Array{Int64,1}:

julia> function g(a)
           a .= a .+ 1
g (generic function with 1 method)

julia> g(a)
2-element Array{Int64,1}:

julia> a
2-element Array{Int64,1}:

check whether array entry is undef

b = Array{Array{Int, 1}}(undef, 3)
isdefined(b, 1) 
# or
isassigned(b, 1)
# not
# isdefined(b[1])

refer to Julia: check whether array entry is undef

memory allocation of undef

julia> @time Array{Array{Int, 1}, 2}(undef, 100, 100);
  0.000024 seconds (6 allocations: 78.359 KiB)
julia> @time zeros(100,100, 1);
  0.000014 seconds (6 allocations: 78.359 KiB)
julia> @time zeros(100,100, 10);
  0.000077 seconds (6 allocations: 781.484 KiB)


julia> a = 'www'
ERROR: syntax: invalid character literal

julia> a = 'w'
'w': ASCII/Unicode U+0077 (category Ll: Letter, lowercase)

julia> a = "ww"

refer to #105.

Juno 中 Cltr + Enter 的 bug

当我使用 Ctrl + Enter 运行下列语句时,

using Images, ImageView
img = load(download("https://juliaimages.org/latest/assets/segmentation/horse.jpg"));

REPL 冻住了,一直停在

Dict{String,Any} with 4 entries:

而我如果直接将上述语句复制到 REPL 中运行时,一切 OK,正常显示为

Dict{String,Any} with 4 entries:
  "gui"         => Dict{String,Any}("window"=>GtkWindowLeaf(name="", parent, wi…
  "roi"         => Dict{String,Any}("redraw"=>50: "map(clim-mapped image, input…
  "annotations" => 3: "input-2" = Dict{UInt64,Any}() Dict{UInt64,Any} 
  "clim"        => 2: "CLim" = CLim{RGB{Float64}}(RGB{Float64}(0.0,0.0,0.0), RG…

找到类似的 Issue: [BUG] Evaluation in Editor freezes Atom occasionally

虽然其解决方案不太清楚,是有关 notification-daemon,但是维护者 @pfitzseb 的回复

OS level notifications are disabled completly in the latest Juno release because they were causing crashes on Mac and didn’t really seem to work on other platforms.

提醒我或许是因为 Juno 的版本问题,当前版本为 v0.7.2, 最新版本为 v0.8.1

仅仅更新 Juno.jl 的版本似乎还不行,另外将 Atom.jl 从 v0.12.8 更新到 v0.12.9


Juno 使用系统代理

当在 .bashrc 添加 http_proxyhttps_proxy 中并 source 之后,直接在 shell 里面开的 julia session 中,验证当前 ip

run(`curl ifconfig.me`)



发现可以使用系统代理。但是在 Atom 中开的 julia,则不行。找到 Atom 一条相关的 issue: [Atom 1.32.0] Does not inherit environment variables when launched not from the command line,但是我已经是最新版了,不应该存在这个问题。但是当我通过 Ctrl-Shift-i 打开 devtools,验证 process.env["http_proxy"],显示 undefined;而且直接在 source 之后的 terminal 中打开,也显示 undefined。猜想,只对 PATH 有效?或者因为 Juno.jl?

跳过这个问题,最后直接在 startup.jl 文件中添加

ENV["http_proxy"] = 

进行设置,这参考了 Install packages behind the proxy

wrong arrangement of multiple figures in a grid

Hi, I am confused by the arrangement of multiple figures in a grid. I begin with the example code in the README,

gui = imshow_gui((300, 300), (2, 1))  # 2 columns, 1 row of images (each initially 300×300)
canvases = gui["canvas"]
imshow(canvases[1,1], testimage("lighthouse"))
imshow(canvases[1,2], testimage("mandrill"))

it throws an error when accessing canvases[1,2]

julia> imshow(canvases[1,2], testimage("mandrill"))
ERROR: BoundsError: attempt to access 2×1 Array{Any,2} at index [1, 2]

and I check the size of canvases is

julia> size(canvases)
(2, 1)

it seems that the canvases will arrage by column while the gui declares it should be by rows.

On the other hand, if I run

gui = imshow_gui((300, 300), (2, 1))  # 2 columns, 1 row of images (each initially 300×300)
canvases = gui["canvas"]
imshow(canvases[1,1], testimage("lighthouse"))
imshow(canvases[2,1], testimage("mandrill"))
gui = imshow_gui((300, 300), (1, 2))  # 2 columns, 1 row of images (each initially 300×300)
canvases = gui["canvas"]
imshow(canvases[1,1], testimage("lighthouse"))
imshow(canvases[1,2], testimage("mandrill"))

Then I guess it might due to the version of some packages…

Manage Different Versions of Julia

  1. download the “Generic Linux Binaries for x86 (64-bit)” of a particular version from Download Julia
  2. put it into customed folder, such as src under home directory,
  3. link it to the system path, such as
cd /usr/local/bin
sudo ln -s /home/weiya/src/julia-1.3.1/bin/julia julia1.3.1

Currently, I have installed the following different versions.

$ ll | grep julia
lrwxrwxrwx  1 root root      37 8月  31  2018 julia -> /home/weiya/src/julia-1.0.0/bin/julia*
lrwxrwxrwx  1 root root      37 7月  30  2019 julia1.1.1 -> /home/weiya/src/julia-1.1.1/bin/julia*
lrwxrwxrwx  1 root root      37 9月  18 15:28 julia1.2.0 -> /home/weiya/src/julia-1.2.0/bin/julia*
lrwxrwxrwx  1 root root      37 3月  18 10:25 julia1.3.1 -> /home/weiya/src/julia-1.3.1/bin/julia*

array in functions

julia> function g(x)
           x += [1, 2]
g (generic function with 1 method)
julia> function g!(x)
           x .+= [1, 2]
g! (generic function with 1 method)
julia> x = [1,2];

julia> g(x)'
1×2 Adjoint{Int64,Array{Int64,1}}:
 2  4

julia> x'
1×2 Adjoint{Int64,Array{Int64,1}}:
 1  2

julia> g!(x)'
1×2 Adjoint{Int64,Array{Int64,1}}:
 2  4

julia> x'
1×2 Adjoint{Int64,Array{Int64,1}}:
 2  4

Note that sometimes .+= may make the program much slower, such as https://github.com/szcf-weiya/en/blob/3c8daeb4e0f477f5ea40dc2bb44d832faa4bbbb6/code/2019-06-14-ML/GD2.jl#L10

同时安装多个 package


add Distributions Combinatorics MATLAB PyPlot LightGraphsFlows LightGraphs Clp Plots


安装 PkgMirrors,则可以用中科大或者浙大的镜像源了

初次设置后,以后直接 using PkgMirrors 便切换到设置好的镜像,以后直接通过镜像源安装。

select text in the output pdf

Several weeks ago, the text, such as the label, title or legend, in the output pdf via savefig("xxx.pdf") can be selected, but recently I found that I cannot select the text in the pdf. The reason would be the version of the packages.

I had tried GRUtils, and GRUtils.savefig can output pdf to select the texts, so I think the reason is some changes in Plots package.

And I found that with Plots@0.27.0 in Julia1.0, I can select the text, but currently the newer Plots@1.2.0 cannot produce such pdf. To determine the change, maybe I need more effort to try different versions. But now I can downgrade the version to satisfy my requirement.

multiple labels

In Julia 1.4.0 with Plots.jl v1.0.14,

using Plots
x = rand(10, 2)
plot(1:10, x, label = ["a", "b"])

will produce

where these two lines share the same label instead of one label for one line. But if replacing the column vector with row vector,

plot(1:10, x, label = ["a" "b"])

will return the correct result,

Refer to Plots (plotly) multiple series or line labels in legend, which also works GR backend.

suptitle for subplots

currently, no a option to set a suptitle for subplots, but we can use @layout to plot the title in a grid, such as szcf-weiya/TB

refer to Super title in Plots and subplots

colorviews for images

check the help document by ?colorviews, and there are some examples to illustrate its usage, but here I add some examples that I used in my projects.

julia> colorview(RGB, rand(3,10,10))
10×10 reshape(reinterpret(RGB{Float64}, ::Array{Float64,3}), 10, 10) with eltype RGB{Float64}:

and we need to permutate the dims such that the first dim matches with RGB.

julia> colorview(RGB, permutedims(rand(10, 10, 3), (3, 1, 2)))
10×10 reshape(reinterpret(RGB{Float64}, ::Array{Float64,3}), 10, 10) with eltype RGB{Float64}:

we also can append the alpha channel to an image, such as

julia> colorview(RGBA, colorview(RGB, rand(3, 10, 10)), rand(10, 10))
10×10 mappedarray(RGBA{Float64}, ImageCore.extractchannels, reshape(reinterpret(RGB{Float64}, ::Array{Float64,3}), 10, 10), ::Array{Float64,2}) with eltype RGBA{Float64}:

but directly append the array does not work,

julia> colorview(RGBA, rand(3, 10, 10), rand(10, 10))
ERROR: DimensionMismatch("arrays do not all have the same axes (got (Base.OneTo(3), Base.OneTo(10), Base.OneTo(10)) and (Base.OneTo(10), Base.OneTo(10)))")

which should be replaced with

julia> colorview(RGBA, rand(4, 10, 10))
10×10 reshape(reinterpret(RGBA{Float64}, ::Array{Float64,3}), 10, 10) with eltype RGBA{Float64}:

To use RGB{N0f8}, the input array should be UInt8 not Int,

julia> colorview(RGB{N0f8}, Array{UInt8}(fill(1,3,10,10)))
10×10 reshape(reinterpret(RGB{N0f8}, view(::Array{UInt8,3}, [1, 2, 3], :, :)), 10, 10) with eltype RGB{Normed{UInt8,8}}:

return values of function

julia> versioninfo()
Julia Version 1.4.0
Commit b8e9a9ecc6 (2020-03-21 16:36 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i5-6300HQ CPU @ 2.30GHz
  LIBM: libopenlibm
  LLVM: libLLVM-8.0.1 (ORCJIT, skylake)

julia> function f(a, b, c, d)
           return a, b, c, d
julia> a = f(1,2,3,4);

julia> a
(1, 2, 3, 4)

julia> a, b = f(1,2,3,4);

julia> a

julia> b

julia> a, b, c = f(1,2,3,4);

julia> a

julia> b

julia> c

padding zero on the left

For example, convert “1” to “001”,

julia> lpad(1, 3, '0')