legend-julia-cheat-sheet coding help for software dev in julia

legend-julia-cheat-sheet.md

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+# 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.