# General Usage

**URL:** https://discourse.julialang.org/c/usage/6.md?page=798

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**Page:** 799

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## [How to extract edge list/contour from image using Images.jl](https://discourse.julialang.org/t/how-to-extract-edge-list-contour-from-image-using-images-jl/25927)

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**Author:** [@ruszkipista](https://discourse.julialang.org/u/ruszkipista)\
**Replies:** 2\
**Last updated:** [January 9, 2020, 3:25pm UTC](https://discourse.julialang.org/t/how-to-extract-edge-list-contour-from-image-using-images-jl/25927 "2020-01-09T15:25:59Z")

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I am new to Julia and image processing. I need to implement geometrical shape (e.g. triangle, rectangle) detection for a course assignment. I want to solve it using Julia only (not e.g. opencv) I found an article doing …

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## [Nested environments in package](https://discourse.julialang.org/t/nested-environments-in-package/33157)

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**Author:** [@racinmat](https://discourse.julialang.org/u/racinmat)\
**Replies:** 5\
**Last updated:** [January 10, 2020, 9:52am UTC](https://discourse.julialang.org/t/nested-environments-in-package/33157 "2020-01-10T09:52:52Z")

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Hi, I have a package with the usual structure, but I also have examples here, because I believe runnable code is more useful than snippets in md. So the structure is something like - examples/ - src/ - test/ - Proje…

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## [Feedback on benchmark](https://discourse.julialang.org/t/feedback-on-benchmark/33186)

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**Author:** [@jw3126](https://discourse.julialang.org/u/jw3126)\
**Replies:** 0\
**Last updated:** [January 10, 2020, 9:23am UTC](https://discourse.julialang.org/t/feedback-on-benchmark/33186 "2020-01-10T09:23:06Z")

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It is my understanding that hardware operations do not always take the same amount of cycles on a given type. For instance it seems that fma is slower on subnormal floats then on normal floats. I wanted to observe this …

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## [Strategy for using multiple threads in an in-place matrix operation](https://discourse.julialang.org/t/strategy-for-using-multiple-threads-in-an-in-place-matrix-operation/33039)

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**Author:** [@dmbates](https://discourse.julialang.org/u/dmbates)\
**Replies:** 3\
**Last updated:** [January 9, 2020, 8:33pm UTC](https://discourse.julialang.org/t/strategy-for-using-multiple-threads-in-an-in-place-matrix-operation/33039 "2020-01-09T20:33:21Z")

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When fitting a linear mixed effects model using the MixedModels package most of the time is spent updating a Cholesky factorization of a blocked array. Within that updating operation a large part of the time is spend on…

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## [Are the entries in a vector of immutable structs stored in place, or is there always a pointer to the heap?](https://discourse.julialang.org/t/are-the-entries-in-a-vector-of-immutable-structs-stored-in-place-or-is-there-always-a-pointer-to-the-heap/33141)

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**Author:** [@Sean\_Matthews](https://discourse.julialang.org/u/Sean_Matthews)\
**Replies:** 10\
**Last updated:** [January 9, 2020, 6:06pm UTC](https://discourse.julialang.org/t/are-the-entries-in-a-vector-of-immutable-structs-stored-in-place-or-is-there-always-a-pointer-to-the-heap/33141 "2020-01-09T18:06:04Z")

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Are the entries in a vector of immutable structs stored in place, or is there always a pointer to the heap? (Or how can I get Julia to do something equivalent). I have a question about the interplay between vectors a…

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## [Correct argument type for ::Array{Array{\<:Real,1},1})](https://discourse.julialang.org/t/correct-argument-type-for-array-array-real-1-1/33164)

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**Author:** [@HenriDeh](https://discourse.julialang.org/u/HenriDeh)\
**Replies:** 6\
**Last updated:** [January 9, 2020, 6:03pm UTC](https://discourse.julialang.org/t/correct-argument-type-for-array-array-real-1-1/33164 "2020-01-09T18:03:45Z")

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I would like to understand why function foo(A::Array{Array{\<:Real,1},1}) end does not accept foo(fill(Float64\[\],10)) I know that function foo(A::Array{Array{T,1},1}) where T \<: Real end works but I need to understa…

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## [CSV.write("\*.txt",DataFrame) ReadOnlyMemoryError()](https://discourse.julialang.org/t/csv-write-txt-dataframe-readonlymemoryerror/33085)

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**Author:** [@BMval](https://discourse.julialang.org/u/BMval)\
**Replies:** 14\
**Last updated:** [January 9, 2020, 5:42pm UTC](https://discourse.julialang.org/t/csv-write-txt-dataframe-readonlymemoryerror/33085 "2020-01-09T17:42:46Z")

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Does somebody have the same error? How to deal with it. Dataframe is quite big: 339000\*18 (String type). Is the problem connected to size? Thank you in advance.

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## [How to update the initial condition in DifferentialEquations?](https://discourse.julialang.org/t/how-to-update-the-initial-condition-in-differentialequations/32696)

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**Author:** [@fedoroff](https://discourse.julialang.org/u/fedoroff)\
**Replies:** 8\
**Last updated:** [January 9, 2020, 2:19pm UTC](https://discourse.julialang.org/t/how-to-update-the-initial-condition-in-differentialequations/32696 "2020-01-09T14:19:11Z")

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Hello! I try to solve a nonlinear differential equation using a split-step method. The right hand side of my equation has two terms: \\frac{du}{dt} = f(u,t) + g(u,t) According to the split-step method, I can split ev…

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## [Update existing imshow with new image data](https://discourse.julialang.org/t/update-existing-imshow-with-new-image-data/8296)

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**Author:** [@yakir12](https://discourse.julialang.org/u/yakir12)\
**Replies:** 11\
**Last updated:** [January 9, 2020, 2:18pm UTC](https://discourse.julialang.org/t/update-existing-imshow-with-new-image-data/8296 "2020-01-09T14:18:59Z")

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I want to set up a rapid loop of updating a screen with some image data and then fetching the result with a webcam that is pointed at the screen (the idea is to sample hundreds of cycles, change some external filter betw…

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## [HTTP.jl connection refused](https://discourse.julialang.org/t/http-jl-connection-refused/33142)

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**Author:** [@Steven\_Sagaert](https://discourse.julialang.org/u/Steven_Sagaert)\
**Replies:** 1\
**Last updated:** [January 9, 2020, 10:48am UTC](https://discourse.julialang.org/t/http-jl-connection-refused/33142 "2020-01-09T10:48:17Z")

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I used to have a webservice running on HttpServer and it just worked. Now I migrated that same server to HTTP.jl and when I try to connect to it via its dns name I get “connection refused” (either from a remote machine o…

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## [Need help with Chrome OS](https://discourse.julialang.org/t/need-help-with-chrome-os/33116)

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**Author:** [@kimran](https://discourse.julialang.org/u/kimran)\
**Replies:** 6\
**Last updated:** [January 9, 2020, 3:39am UTC](https://discourse.julialang.org/t/need-help-with-chrome-os/33116 "2020-01-09T03:39:58Z")

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Good day everyone With the advent of some amazing chromebooks, more and more developers are looking into working on Chrome OS I mean, who does not want to write code on this baby: https://store.google.com/us/product/pi…

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## [DifferentialEquations with Turing](https://discourse.julialang.org/t/differentialequations-with-turing/33100)

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**Author:** [@virgile-baudrot](https://discourse.julialang.org/u/virgile-baudrot)\
**Replies:** 3\
**Last updated:** [January 9, 2020, 4:22am UTC](https://discourse.julialang.org/t/differentialequations-with-turing/33100 "2020-01-09T04:22:28Z")

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Dear all, I’d like to use DifferentialEquations.jl with Turing.jl, I read that this was possible, but I’ve trouble to run it. Here is a simple try with Lotka-Voltera equation, how to fix it to make it running? using D…

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## [Occursin strange behavior](https://discourse.julialang.org/t/occursin-strange-behavior/33127)

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**Author:** [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Replies:** 2\
**Last updated:** [January 8, 2020, 11:24pm UTC](https://discourse.julialang.org/t/occursin-strange-behavior/33127 "2020-01-08T23:24:07Z")

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function f(arg) # Wrap a string with spaces between quotes if (isa(arg, AbstractString) || isa(arg, Symbol)) out = string(arg) if (occursin(" ", out)) out = "\\"" \* out \* "\\"" end else error("Bla") end end …

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## [Interpolate all variables in expression without using $ for each variable](https://discourse.julialang.org/t/interpolate-all-variables-in-expression-without-using-for-each-variable/33101)

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**Author:** [@rvignolo](https://discourse.julialang.org/u/rvignolo)\
**Replies:** 6\
**Last updated:** [January 8, 2020, 5:39pm UTC](https://discourse.julialang.org/t/interpolate-all-variables-in-expression-without-using-for-each-variable/33101 "2020-01-08T17:39:16Z")

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Hi, is there any alternative way to do this: x = 1 y = 2 z = 3 i = 1 j = 2 k = 3 expr = :($x + 2 \* $y + $z + $i + $j + $k) For example, something like: expr = @$ :(x + 2 \* y + z + i + j + k) Thanks!

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## [DataDeps.jl - replacing a dependency](https://discourse.julialang.org/t/datadeps-jl-replacing-a-dependency/33064)

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**Author:** [@mthelm85](https://discourse.julialang.org/u/mthelm85)\
**Replies:** 2\
**Last updated:** [January 8, 2020, 1:05pm UTC](https://discourse.julialang.org/t/datadeps-jl-replacing-a-dependency/33064 "2020-01-08T13:05:50Z")

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The docs say: …you can’t force users to download new copies of your data using DataDeps. There are work arounds, such as using DataDeps.jl + deps/build.jl to rm(datadep"MyData", recursive=true, force=true every pac…

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## [Julia 1.2 on CentOS: package manager says ERROR: no active project](https://discourse.julialang.org/t/julia-1-2-on-centos-package-manager-says-error-no-active-project/33074)

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**Author:** [@Steven\_Sagaert](https://discourse.julialang.org/u/Steven_Sagaert)\
**Replies:** 15\
**Last updated:** [January 8, 2020, 12:41pm UTC](https://discourse.julialang.org/t/julia-1-2-on-centos-package-manager-says-error-no-active-project/33074 "2020-01-08T12:41:22Z")

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After installing julia 1.2 by extracting the tar.gz file I can run the julia console but when I enter the package manager console and do add I get ERROR: no active project. Tere’s no prefix (v1.2) in the pm prompt. Bef…

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## [\`nfields\` vs \`fieldcount\`](https://discourse.julialang.org/t/nfields-vs-fieldcount/33084)

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**Author:** [@singularitti](https://discourse.julialang.org/u/singularitti)\
**Replies:** 4\
**Last updated:** [January 8, 2020, 8:36am UTC](https://discourse.julialang.org/t/nfields-vs-fieldcount/33084 "2020-01-08T08:36:40Z")

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What are the differences between nfields and fieldcount? Is nfields(x) == fieldcount(typeof(x)) ?

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## [Accidental overwrite base function name](https://discourse.julialang.org/t/accidental-overwrite-base-function-name/33080)

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**Author:** [@Egwene\_al\_Vere](https://discourse.julialang.org/u/Egwene_al_Vere)\
**Replies:** 2\
**Last updated:** [January 8, 2020, 2:04am UTC](https://discourse.julialang.org/t/accidental-overwrite-base-function-name/33080 "2020-01-08T02:04:16Z")

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Not sure if it’s a bug or expected behavior: people may not know all the names of the built-in functions, and on a fresh start, the built-in function may be overwritten: on a fresh start, things like diff = 0 would run…

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## [Error building "pycall"](https://discourse.julialang.org/t/error-building-pycall/16270)

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**Author:** [@lannisite110](https://discourse.julialang.org/u/lannisite110)\
**Replies:** 10\
**Last updated:** [January 8, 2020, 12:56am UTC](https://discourse.julialang.org/t/error-building-pycall/16270 "2020-01-08T00:56:51Z")

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Error: Error building PyCall: │ ERROR: LoadError: UndefVarError: v not defined │ Stacktrace: │ \[1\] find\_libpython(::String) at /home/users/xdlan/.julia/packages/PyCall/0jMpb/deps/build.jl:101 │ \[2\] top-level scope …

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## [GMT.jl GPS points plotting](https://discourse.julialang.org/t/gmt-jl-gps-points-plotting/33047)

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**Author:** [@Dervis](https://discourse.julialang.org/u/Dervis)\
**Replies:** 1\
**Last updated:** [January 7, 2020, 8:38pm UTC](https://discourse.julialang.org/t/gmt-jl-gps-points-plotting/33047 "2020-01-07T20:38:25Z")

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Dear all, I would like to plot the GPS points (latitude and longitude) on satellite map similar to this: If anyone knows any other packages rather than GMT.jl, that would be appreciated. Thank you.

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## [Testing modifications to Base](https://discourse.julialang.org/t/testing-modifications-to-base/33063)

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**Author:** [@tro3](https://discourse.julialang.org/u/tro3)\
**Replies:** 5\
**Last updated:** [January 7, 2020, 4:45pm UTC](https://discourse.julialang.org/t/testing-modifications-to-base/33063 "2020-01-07T16:45:55Z")

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Do I recall on one of these threads that there is a way to easily test Base modifications without rebuilding Julia? Can’t find it through search, so maybe my memory is off…

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## [Five-argument mul! C](https://discourse.julialang.org/t/five-argument-mul-c/33068)

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**Author:** [@Paul\_Soderlind](https://discourse.julialang.org/u/Paul_Soderlind)\
**Replies:** 2\
**Last updated:** [January 7, 2020, 3:55pm UTC](https://discourse.julialang.org/t/five-argument-mul-c/33068 "2020-01-07T15:55:00Z")

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mul! on 1.3.1 does not update x\[:,:,1\] in x = zeros(2,2,2) mul!(x\[:,:,1\],\[1,1\],\[1 2\],1.0,1.0) In contrast, it does update x\[1\] in x = \[zeros(2,2) for i=1:2\] mul!(x\[1\],\[1,1\],\[1 2\],1.0,1.0) Is this difference obvious?

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## [Converting String to Datatype with Meta.parse](https://discourse.julialang.org/t/converting-string-to-datatype-with-meta-parse/33024)

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**Author:** [@YongHee-Kim](https://discourse.julialang.org/u/YongHee-Kim)\
**Replies:** 8\
**Last updated:** [January 7, 2020, 1:30pm UTC](https://discourse.julialang.org/t/converting-string-to-datatype-with-meta-parse/33024 "2020-01-07T13:30:48Z")

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I’ve made a package to create JSONPointer from a string, then construct the Dictionary from JSONPointer data. But I’ve added extra functionality to define static type for JSONPointer. here is example pkg\>add XLSXasJSO…

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## [Subtyping Conventions for Dicts are Unexpected](https://discourse.julialang.org/t/subtyping-conventions-for-dicts-are-unexpected/33040)

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**Author:** [@chelseas](https://discourse.julialang.org/u/chelseas)\
**Replies:** 7\
**Last updated:** [January 6, 2020, 9:30pm UTC](https://discourse.julialang.org/t/subtyping-conventions-for-dicts-are-unexpected/33040 "2020-01-06T21:30:08Z")

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If Int64 \<: Real is true, and I presume Julia follows normal function typing rules (for dicts) where the “return” type can be more specific than indicated by the type annotation, why isn’t the following true? Dict(:x…

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## [DelimitedFiles:writedlm usage](https://discourse.julialang.org/t/delimitedfiles-writedlm-usage/33009)

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**Author:** [@tanya](https://discourse.julialang.org/u/tanya)\
**Replies:** 17\
**Last updated:** [January 6, 2020, 9:51pm UTC](https://discourse.julialang.org/t/delimitedfiles-writedlm-usage/33009 "2020-01-06T21:51:28Z")

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with using DelimitedFiles What would be the equivalent of matlab command dlmwrite(‘filename.csv’,arrayname,‘delimiter’,‘,’,‘-append’) ? Basically, I need to write to the same csv file and append the existing data. wri…

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## [Example of Async/Distributed loading into a single shared Channel](https://discourse.julialang.org/t/example-of-async-distributed-loading-into-a-single-shared-channel/11815)

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**Author:** [@oxinabox](https://discourse.julialang.org/u/oxinabox)\
**Replies:** 2\
**Last updated:** [January 6, 2020, 11:45am UTC](https://discourse.julialang.org/t/example-of-async-distributed-loading-into-a-single-shared-channel/11815 "2020-01-06T11:45:39Z")

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I think this is a fairly common senario: You have a collection of files, and each file contains a set of records (e.g. one JSON entry per line). And you which to kinda lazily load up those records, without caring abou…

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## [Ranges with zero step](https://discourse.julialang.org/t/ranges-with-zero-step/32970)

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**Author:** [@Xnartharax](https://discourse.julialang.org/u/Xnartharax)\
**Replies:** 3\
**Last updated:** [January 6, 2020, 2:23am UTC](https://discourse.julialang.org/t/ranges-with-zero-step/32970 "2020-01-06T02:23:44Z")

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This is my first post here and English is not my native language so please excuse some (hopefully minor) mistakes. I ran across a problem when trying to iterate over some cubic space. I want to give a function two corne…

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## [Change symbol size inside legend with Plots, pyplot()](https://discourse.julialang.org/t/change-symbol-size-inside-legend-with-plots-pyplot/30632)

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**Author:** [@juliauser](https://discourse.julialang.org/u/juliauser)\
**Replies:** 0\
**Last updated:** [November 2, 2019, 5:10pm UTC](https://discourse.julialang.org/t/change-symbol-size-inside-legend-with-plots-pyplot/30632 "2019-11-02T17:10:49Z")

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No matter how large is markesize in a scatter plot, the symbol in the legend is always tiny. Any idea on how to fix it? Thank you

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## [ImageMajick.jl, Gtk.jl fail to load in Julia 1.3.1](https://discourse.julialang.org/t/imagemajick-jl-gtk-jl-fail-to-load-in-julia-1-3-1/32989)

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**Author:** [@magister-ludi](https://discourse.julialang.org/u/magister-ludi)\
**Replies:** 3\
**Last updated:** [January 5, 2020, 10:32pm UTC](https://discourse.julialang.org/t/imagemajick-jl-gtk-jl-fail-to-load-in-julia-1-3-1/32989 "2020-01-05T22:32:39Z")

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I have just updated ImageMagick.jl to v1.1.1 and Gtk.jl to v1.1.2. They installed without any apparent problem, but when I try, say, using ImageMagick precompilation fails: \[ Info: Precompiling ImageMagick \[6218d12a-5…

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## [Infix macros and tuple valued function broadcasting](https://discourse.julialang.org/t/infix-macros-and-tuple-valued-function-broadcasting/33002)

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**Author:** [@Jakub\_Wronowski](https://discourse.julialang.org/u/Jakub_Wronowski)\
**Replies:** 5\
**Last updated:** [January 5, 2020, 9:17pm UTC](https://discourse.julialang.org/t/infix-macros-and-tuple-valued-function-broadcasting/33002 "2020-01-05T21:17:09Z")

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Hello :slight\_smile: I am using modular arithmetic extensively and there are two things that I am missing: Possibility to define infix macro. I’d like to use syntax like this: x = H .\* Y @mod 17 and @mod macro wou…

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