legend-julia-cheat-sheet.md

Legend Julia Cheat Sheet

This is a cheat sheet to help you with your coding projects in Julia.

In the Julia REPL (read-eval-print loop —> command window) one can enter different modes:

• ? → help mode
• ] → package manager
• ; → terminal/shell mode

VSCode (Editor) shortcuts:

• ⌘+shift+P (Mac) open command palette → from there open Julia REPL

Useful Julia commands:

• typeof(x) returns variable type of x
• size(array) , length(array) similar to size and numel in matlab
• readdir() similar to "ls" in shell
• replace(string,"pattern"=>"new_pattern")
• isequal(x,y) returns bool
• ifelse(condition, value_if_true, value_if_false) function that chooses between two values based on a condition

Anonymous functions:

• define function without giving it a name
• example: x->x*2
• can be used in other functions, example: map(x->x*2,[1,2,3])

Map: map()

• Apply function to each element of array/vector/...
• example see anonymous function.

Pipe operator: |>

• passes result of the expression on its left-hand side of |> and passes it as the first argument to the function on its right-hand side
• example: [3,1,2] |> x-> sort(x) returns 3-element Vector{Int64}: 1, 2, 3

Macros: @

• symbol is used to denote macros, e.g. build-in macros like @time begin a = 5 end to measure the execution time
• macros are different from functions (compile time vs. runtime)?

PropertyFunctions @pf

• Macro (Doc) to broadcast (element-wise) operation to properties of an object
• Faster (reads only relevant entries, less data traffic) than other methods

Using StructArrays
xs = StructArrays.StructArray((
        a = [1,2,3,4,5],
        b = [6,7,8,9,10],
        c = [11,12,13,14,15] ))
out    = xs.b + xs.a.^2 # standard wat to do some elementwise operation 
out_pf = @pf($b + $a.^2)(xs)  # same result as out, but faster

Base.Fix1

• Function fun1 with mutltiple argument arg1, arg2
• Create a new function fun2 that is identical to fun1 except that one of the arguments is fixed to a certain value

fun1 = (arg1, arg2) -> print("$arg1 $arg2")
fun2 = Base.Fix1(fun1,"Hallo") #fix first arg1 in fun1
fun2("Duda") # returns: "Hallo Duda"

• also possible to fix 2nd, 3rd,..argument. Using the same example as above:

fun3 = Base.Fix2(fun1,"Hallo") #fix first arg2 in fun1
fun3("Duda") # returns: "Duda Hallo"

Regular expressions (regex):

• standard julia package
• define a regex via r"hallo"
• match("pattern","string", startindex (opt)) search for pattern in string, pattern has to be regex
• pattern examples:
  • r"\d+" one or more digits, r"\D" any non-digit
  • r"l.*" the character "l" and everything that comes after
  • r"(?=)"

Let block:

• let blocks create a new scope and (optionally) introduce new local variables

let var1 = value1, var2, var3 = value3
    code
end

• in this examble var1, var2, var3 are new variables that are only accessible within let block
• the values value1 and value3 are either actual values or variables defined before the let block

Julia package manager:

• import Pkg load package manager (alternatively enter package manager in REPL with ])
• Pkg.activate("Environmentpath") create a new or activate existing environment ---> can also be defined in settings.json (see "Environments:")
• Pkg.add("LegendDataManagement") add to package to project environment. clones package from git/main brain. Only for registered package
• Pkg.update("Packagename") update from git
• Pkg.instantiate() download/precompile correct version of packages that are listed in Manifest.toml
• Pkg.resolve() check that the Manifest.toml is consistent with Project.tomls. then does instantiate()

Package development:

• pathof(PackageName) → location of package source code (package has to be used in environment)
• Development of a package:
  1. Clone (fork of) package, e.g. LegendSpecFits in path/LegendSpecFits.jl
  2. In package manager: dev path/LegendSpecFits.jl , using LegendSpecFits note that packages can be developed "live", that means changed in functions are seen immediately
  3. To go back to the "official" package version: add LegendSpecFits.jl , using LegendSpecFits
• Test packet changes with julia benchmark tools --> Google

Environments (package versions):

• In each folder with Julia code, there is a hidden directory called .vscode
• Inside .vscode there is a file called settings.json that configures settings for this directory
• example:

{
    "julia.environmentPath": "/home/iwsatlas1/schluete/.julia/environments/legend-dev",
    "git.ignoreLimitWarning": true  // Ignores the warning when there are too many changes in a repository
}

• The path in julia.environmentPath points to a Project.toml and Manifest.toml
• → they define the package environment for all subfolder inside the main folder.