# General Usage

**URL:** https://discourse.julialang.org/c/usage/6.md?page=994

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**Page:** 995

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## [Unable to download the nightly for windows](https://discourse.julialang.org/t/unable-to-download-the-nightly-for-windows/9249)

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**Author:** [@colintbowers](https://discourse.julialang.org/u/colintbowers)\
**Replies:** 5\
**Last updated:** [February 22, 2018, 10:21pm UTC](https://discourse.julialang.org/t/unable-to-download-the-nightly-for-windows/9249 "2018-02-22T22:21:56Z")

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Hi all, I’m getting the following error when trying to download the latest 64 bit nightly for windows (the .exe at https://julialang.org/downloads/nightlies.html) This XML file does not appear to have any style informa…

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## [Mapslices failing on n x 0 matrices](https://discourse.julialang.org/t/mapslices-failing-on-n-x-0-matrices/9256)

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**Author:** [@vvjn](https://discourse.julialang.org/u/vvjn)\
**Replies:** 2\
**Last updated:** [February 22, 2018, 7:14pm UTC](https://discourse.julialang.org/t/mapslices-failing-on-n-x-0-matrices/9256 "2018-02-22T19:14:19Z")

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Is this supposed to happen? I tried mapslices(x-\>x\*2, rand(3,0), 1) and it errors. mapslices(x-\>x\*2, rand(3,1), 1) works as expected. map(x-\>x\*2, rand(0)) works, so I would assume mapslices should work on empty matrice…

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## [Revise.jl does not trigger recompilation](https://discourse.julialang.org/t/revise-jl-does-not-trigger-recompilation/9220)

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**Author:** [@tk3369](https://discourse.julialang.org/u/tk3369)\
**Replies:** 3\
**Last updated:** [February 22, 2018, 5:17pm UTC](https://discourse.julialang.org/t/revise-jl-does-not-trigger-recompilation/9220 "2018-02-22T17:17:04Z")

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For some reasons, Revise.jl does not trigger after I made changes to the source file in a package. The funny thing is that it works on a different package, and I’ve been scratching my head for past 2 hours. Any idea wh…

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## [Using PDMats](https://discourse.julialang.org/t/using-pdmats/9236)

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**Author:** [@orosen1](https://discourse.julialang.org/u/orosen1)\
**Replies:** 4\
**Last updated:** [February 22, 2018, 5:07pm UTC](https://discourse.julialang.org/t/using-pdmats/9236 "2018-02-22T17:07:01Z")

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Are there any examples of using the package PDMats? For example, how can I find the inverse of a symmetric positive definite matrix?

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## [Ifelse function in @transform](https://discourse.julialang.org/t/ifelse-function-in-transform/8799)

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**Author:** [@Yifan\_Liu](https://discourse.julialang.org/u/Yifan_Liu)\
**Replies:** 13\
**Last updated:** [February 22, 2018, 3:56pm UTC](https://discourse.julialang.org/t/ifelse-function-in-transform/8799 "2018-02-22T15:56:38Z")

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I came across an issue with using ifelse function in @transform, below is a simplified example, I am not sure where the issue comes from. df1 is correct but df2 is wrong. using DataFrames, DataFramesMeta, Lazy, Distribu…

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## [Open a dialog asynchronously](https://discourse.julialang.org/t/open-a-dialog-asynchronously/9251)

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**Author:** [@yakir12](https://discourse.julialang.org/u/yakir12)\
**Replies:** 0\
**Last updated:** [February 22, 2018, 1:30pm UTC](https://discourse.julialang.org/t/open-a-dialog-asynchronously/9251 "2018-02-22T13:30:45Z")

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I’m building a simple GUI where the user is asked to choose a file for further processing. Because the subsequent steps involved in the process can take some time (loading packages etc), I figured it would be cool if I c…

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## [How to lazily \`collect(zip(fn(a), b))\` for some function \`fn\`?](https://discourse.julialang.org/t/how-to-lazily-collect-zip-fn-a-b-for-some-function-fn/9244)

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**Author:** [@xiaodai](https://discourse.julialang.org/u/xiaodai)\
**Replies:** 2\
**Last updated:** [February 22, 2018, 10:51am UTC](https://discourse.julialang.org/t/how-to-lazily-collect-zip-fn-a-b-for-some-function-fn/9244 "2018-02-22T10:51:36Z")

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I want to lazily collect a zip iterator. See MWE below where I applied pointer. to a before using a zip and collect, but pointer.(a) allocated a new vector, so I don’t want that. Is there a way to lazily do pointer.(a) s…

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## [Converting Float64 to string with run-time generated format string](https://discourse.julialang.org/t/converting-float64-to-string-with-run-time-generated-format-string/9228)

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**Author:** [@colintbowers](https://discourse.julialang.org/u/colintbowers)\
**Replies:** 4\
**Last updated:** [February 22, 2018, 10:44am UTC](https://discourse.julialang.org/t/converting-float64-to-string-with-run-time-generated-format-string/9228 "2018-02-22T10:44:35Z")

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Hi all, I’m fairly sure this topic has come up before, either on discourse, or on the old google group, but I can’t seem to find the link. Regardless, I thought I might see if the answer has changed given all the langua…

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## [Spooky Missing action at a distance](https://discourse.julialang.org/t/spooky-missing-action-at-a-distance/9089)

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**Author:** [@jamii](https://discourse.julialang.org/u/jamii)\
**Replies:** 6\
**Last updated:** [February 22, 2018, 10:40am UTC](https://discourse.julialang.org/t/spooky-missing-action-at-a-distance/9089 "2018-02-22T10:40:26Z")

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I understand why this happens, but it surprised me. Some complex code that was previously type-stable became unstable after upgrading unrelated dependencies. I’m not really sure what I think about this - I’m just posting…

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## [PyCall: install a wheel package](https://discourse.julialang.org/t/pycall-install-a-wheel-package/9246)

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**Author:** [@phlavenk](https://discourse.julialang.org/u/phlavenk)\
**Replies:** 1\
**Last updated:** [February 22, 2018, 9:43am UTC](https://discourse.julialang.org/t/pycall-install-a-wheel-package/9246 "2018-02-22T09:43:47Z")

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I need to install a private python package during my (private) package build process. I have it in a form of a wheel file and due to licensing I cannot upload it to public internet but need to keep it in local directory …

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## [SubArray is not AbstractArray](https://discourse.julialang.org/t/subarray-is-not-abstractarray/9245)

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**Author:** [@tk3369](https://discourse.julialang.org/u/tk3369)\
**Replies:** 1\
**Last updated:** [February 22, 2018, 5:50am UTC](https://discourse.julialang.org/t/subarray-is-not-abstractarray/9245 "2018-02-22T05:50:40Z")

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I was trying to fit a SubArray into a field of AbstractArray type and was surprised by this. I thought most, if not all, array types should be AbstractArray’s… Any reason why not? julia\> SubArray isa AbstractArray fal…

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## [Extract single element from container](https://discourse.julialang.org/t/extract-single-element-from-container/9200)

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**Author:** [@jw3126](https://discourse.julialang.org/u/jw3126)\
**Replies:** 7\
**Last updated:** [February 22, 2018, 3:09am UTC](https://discourse.julialang.org/t/extract-single-element-from-container/9200 "2018-02-22T03:09:56Z")

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Every now and then I have a container like \[1\] or Set(\["foo"\]), which contains only a single element and I want to extract it. Is there a function that does this somewhere? It should Throw an error, if the container is…

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## [Why isn't this true: \`\[mean, mean, mean\] isa Vector{Function}\`? but this is: \`\[mean, mean, sum\] isa Vector{Function}\`?](https://discourse.julialang.org/t/why-isnt-this-true-mean-mean-mean-isa-vector-function-but-this-is-mean-mean-sum-isa-vector-function/9238)

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**Author:** [@xiaodai](https://discourse.julialang.org/u/xiaodai)\
**Replies:** 7\
**Last updated:** [February 22, 2018, 12:40am UTC](https://discourse.julialang.org/t/why-isnt-this-true-mean-mean-mean-isa-vector-function-but-this-is-mean-mean-sum-isa-vector-function/9238 "2018-02-22T00:40:09Z")

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Why isn’t this true: \[mean, mean, mean\] isa Vector{Function}? but this is: \[mean, mean, sum\] isa Vector{Function}? Basically I am writing a function like this, but it’s not dispatching on applyfns2tuples(\[mean, mean,mea…

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## [Array/Struct hybrids](https://discourse.julialang.org/t/array-struct-hybrids/9195)

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**Author:** [@Raf](https://discourse.julialang.org/u/Raf)\
**Replies:** 5\
**Last updated:** [February 21, 2018, 11:44pm UTC](https://discourse.julialang.org/t/array-struct-hybrids/9195 "2018-02-21T23:44:16Z")

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A package I’m building needs a datastructure that has properties of both structs and arrays. I’m not totally across all of the options and their relative speed or type stability. I’m currently using axis arrays and vario…

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## [Broadcasting zip](https://discourse.julialang.org/t/broadcasting-zip/9229)

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**Author:** [@yakir12](https://discourse.julialang.org/u/yakir12)\
**Replies:** 7\
**Last updated:** [February 21, 2018, 10:26pm UTC](https://discourse.julialang.org/t/broadcasting-zip/9229 "2018-02-21T22:26:26Z")

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Say I have one column vector x with n elements in it, and one row vector y with m elements in it. If I want to zip the broadcast of these two vectors, zip(x .+ 0\*y, 0\*x .+ y), can I do it without generating these tempor…

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## [Packaging mixed Julia and non-Julia code](https://discourse.julialang.org/t/packaging-mixed-julia-and-non-julia-code/9091)

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**Author:** [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Replies:** 3\
**Last updated:** [February 21, 2018, 7:56pm UTC](https://discourse.julialang.org/t/packaging-mixed-julia-and-non-julia-code/9091 "2018-02-21T19:56:58Z")

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Let’s say I have a repository foo\_app consisting of an application (written in another language) and also the Julia bindings for that application. In my mind, this application is not, itself, a Julia package, so it makes…

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## [Prepend! usage / what methods to implement?](https://discourse.julialang.org/t/prepend-usage-what-methods-to-implement/9232)

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**Author:** [@Qiyamah](https://discourse.julialang.org/u/Qiyamah)\
**Replies:** 3\
**Last updated:** [February 21, 2018, 5:46pm UTC](https://discourse.julialang.org/t/prepend-usage-what-methods-to-implement/9232 "2018-02-21T17:46:25Z")

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Hi, What methods should i be implementing if i am to use prepend! on a user defined datatype? Can you show me an example? Thanks julia\> struct R x::Int end julia\> function f(r::R) lineariz…

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## [Dispatch function based on value of parametric type](https://discourse.julialang.org/t/dispatch-function-based-on-value-of-parametric-type/9177)

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**Author:** [@RandomString123](https://discourse.julialang.org/u/RandomString123)\
**Replies:** 11\
**Last updated:** [February 21, 2018, 4:43pm UTC](https://discourse.julialang.org/t/dispatch-function-based-on-value-of-parametric-type/9177 "2018-02-21T16:43:27Z")

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I have been thinking about parametric types lately and how to use them so I came up with a thought experiment that I can’t seem to figure out, but I am pretty sure is possible. For this experiment lets assume there is …

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## [Installation error: Can't initialize plug-ins directory](https://discourse.julialang.org/t/installation-error-cant-initialize-plug-ins-directory/9235)

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**Author:** [@yha](https://discourse.julialang.org/u/yha)\
**Replies:** 0\
**Last updated:** [February 21, 2018, 4:25pm UTC](https://discourse.julialang.org/t/installation-error-cant-initialize-plug-ins-directory/9235 "2018-02-21T16:25:51Z")

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I’m trying to install Julia 0.6.2 from Download Julia on Windows 7, and get the following error: I have verified the installer MD5 checksum and tried installing after a reboot. My machine also has JuliaPro 0.6.2.1 ins…

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## [Power function numerical mystery](https://discourse.julialang.org/t/power-function-numerical-mystery/9203)

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**Author:** [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)\
**Replies:** 6\
**Last updated:** [February 21, 2018, 2:46pm UTC](https://discourse.julialang.org/t/power-function-numerical-mystery/9203 "2018-02-21T14:46:37Z")

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I’m trying to hunt down a numerical platform deviation. Much simplification leads to this power computation, where I don’t understand the variation in the results (in the last bit of the mantissa). julia\> 15.632459f0 ^ …

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## [Udp timeout?](https://discourse.julialang.org/t/udp-timeout/9215)

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**Author:** [@purplishrock](https://discourse.julialang.org/u/purplishrock)\
**Replies:** 1\
**Last updated:** [February 21, 2018, 2:45pm UTC](https://discourse.julialang.org/t/udp-timeout/9215 "2018-02-21T14:45:00Z")

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I want to timeout if I don’t get any messages. The equivalent of that would be to simply do some sort of wait condition on the channel, and if there was no data ready then timeout. I thought I had found something to do…

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## [Row major arrays?](https://discourse.julialang.org/t/row-major-arrays/9225)

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**Author:** [@tk3369](https://discourse.julialang.org/u/tk3369)\
**Replies:** 2\
**Last updated:** [February 21, 2018, 2:20pm UTC](https://discourse.julialang.org/t/row-major-arrays/9225 "2018-02-21T14:20:04Z")

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I have to integrate with a C library that expects row-major 2D data. I could transpose/reshape an array before calling it but it seems awkward and not efficient. What’s the best way to handle this? Maybe I could creat…

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## [FactorAnalysis, KernelPCA in MultivariateStats.jl broken?](https://discourse.julialang.org/t/factoranalysis-kernelpca-in-multivariatestats-jl-broken/9064)

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**Author:** [@burfel](https://discourse.julialang.org/u/burfel)\
**Replies:** 8\
**Last updated:** [February 19, 2018, 7:39pm UTC](https://discourse.julialang.org/t/factoranalysis-kernelpca-in-multivariatestats-jl-broken/9064 "2018-02-19T19:39:22Z")

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It seems like some dimensionality reduction methods in MultivariateStats.jl are broken. I tried the sample code given in the documentation: # train a FactorAnalysis model M22 = fit(FactorAnalysis, Xtr; maxoutdim=100) …

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## [Is it possible to search the Interface of a certain type/abstract type?](https://discourse.julialang.org/t/is-it-possible-to-search-the-interface-of-a-certain-type-abstract-type/9122)

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**Author:** [@Roger-luo](https://discourse.julialang.org/u/Roger-luo)\
**Replies:** 2\
**Last updated:** [February 21, 2018, 8:10am UTC](https://discourse.julialang.org/t/is-it-possible-to-search-the-interface-of-a-certain-type-abstract-type/9122 "2018-02-21T08:10:30Z")

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Many abstract type or type have a list of required functions, (like AbstractArray) will it be possible to search them rather than using methodswith which gives you all the methods that use this type.

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## [Covert BitArray to Int64](https://discourse.julialang.org/t/covert-bitarray-to-int64/9193)

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**Author:** [@Snir\_Gazit](https://discourse.julialang.org/u/Snir_Gazit)\
**Replies:** 6\
**Last updated:** [February 21, 2018, 12:52am UTC](https://discourse.julialang.org/t/covert-bitarray-to-int64/9193 "2018-02-21T00:52:56Z")

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Hi, Is there a simple (and efficient) method to convert a BitArray to an Int64, treating the BitArray as the binary representation an Int64? Thanks!

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## [Converting bytes to floats when byte order is reversed](https://discourse.julialang.org/t/converting-bytes-to-floats-when-byte-order-is-reversed/7585)

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**Author:** [@purplishrock](https://discourse.julialang.org/u/purplishrock)\
**Replies:** 2\
**Last updated:** [February 21, 2018, 12:35am UTC](https://discourse.julialang.org/t/converting-bytes-to-floats-when-byte-order-is-reversed/7585 "2018-02-21T00:35:00Z")

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I’ve got a stream of data that i’m reading which are float values, but each of the 4-bytes in the value are in byte-reversed order. That means that 1 reinterpret(Float32, x\[1:4\]) gives me an invalid value but 2 reint…

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## [Atom Juno stopped providing backtraces with in-function line numbers](https://discourse.julialang.org/t/atom-juno-stopped-providing-backtraces-with-in-function-line-numbers/9218)

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**Author:** [@clinton](https://discourse.julialang.org/u/clinton)\
**Replies:** 0\
**Last updated:** [February 20, 2018, 11:29pm UTC](https://discourse.julialang.org/t/atom-juno-stopped-providing-backtraces-with-in-function-line-numbers/9218 "2018-02-20T23:29:09Z")

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Since the latest updates of Juno, the line numbers no longer reference the specific location of any errors, and instead refer to the initial function call. This is true with both the legacy console and the integrated rep…

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## [Julia 0.7 nightlies for ARM](https://discourse.julialang.org/t/julia-0-7-nightlies-for-arm/9209)

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**Author:** [@cdsousa](https://discourse.julialang.org/u/cdsousa)\
**Replies:** 1\
**Last updated:** [February 20, 2018, 8:12pm UTC](https://discourse.julialang.org/t/julia-0-7-nightlies-for-arm/9209 "2018-02-20T20:12:48Z")

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Nightly builds download links for ARM at https://julialang.org/downloads/nightlies.html are not working. Is that expected? Thanks.

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## [Is mem of compiled/eval'ed functions garbage collected?](https://discourse.julialang.org/t/is-mem-of-compiled-evaled-functions-garbage-collected/2231)

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**Author:** [@robertfeldt](https://discourse.julialang.org/u/robertfeldt)\
**Replies:** 6\
**Last updated:** [February 20, 2018, 7:32pm UTC](https://discourse.julialang.org/t/is-mem-of-compiled-evaled-functions-garbage-collected/2231 "2018-02-20T19:32:14Z")

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If I create a large number of functions and compile them (say by evaluating expressions that define functions) will they then be garabage collected if I let go of the references to these functions? I need to know this fo…

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## [How to quickly create array of array?](https://discourse.julialang.org/t/how-to-quickly-create-array-of-array/9152)

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**Author:** [@jebej](https://discourse.julialang.org/u/jebej)\
**Replies:** 12\
**Last updated:** [February 20, 2018, 2:43am UTC](https://discourse.julialang.org/t/how-to-quickly-create-array-of-array/9152 "2018-02-20T02:43:01Z")

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I find that creating uninitialized arrays of arrays, e.g. with a comprehension: f1(N,n) = \[Matrix{Complex128}(N,N) for i=1:n\] is relatively slow, especially compared to creating a single large array: f2(N,n) = Matrix{…

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