Programming Tricks

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Julia

parentmodule: determine the package a function in Julia originates from

names: Get a vector of the public names of a Module, excluding deprecated names

undef: undef is a special marker used when constructing arrays (including vectors) to indicate that the elements of the array should not be initialized to any specific value.

collect: return an Array of all items in a collection or iterator, collect(2:5), collect('B':'D'), collect("HELLO")

!: a function naming convention to indicate that a function mutates its arguments in place, meaning the changes will be visible outside the function. When a function is designed to modify its arguments, it is good practice to append a ! (exclamation mark) to its name

eltype: To find the type of the elements that are iterated over in a collection

typeof: To determine the specific type of any given value

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pizza_tuple = ("hawaiian", 'S', 10.5)
typeof(pizza_tuple) # Tuple{String, Char, Float64}
eltype(pizza_tuple)	# Any

arr = [1,2,3]
typeof(arr)		# Vector{Int64} (alias for Array{Int64, 1})
eltype(arr)		# Int64

Any: It is used to construct a heterogeneous array that can hold elements of any type, like Any[1, "hello", 3.14]

^: Repeat a regex n times (s^n is same with repeat(s, n)); Exponentiation operator

parse: convert a text string to anything else. parse(Int, "42"), parse(Float64, "42")

String * : concatenate strings, "The " * engine *

String $: string interpolation, use $(variable) instead of $variable when there is no whitespace that can clearly distinguish the variable name from the surrounding text

vec: Reshape the array as a one-dimensional column vector

pkg> dev|develop: add a local package, which not initialized by git

using|import .MyModule: relative import followinginclude("path/to/MyModule.jl") custom module

Instantiate the project

instantiate command to download and install all packages and their dependencies listed in Project.toml (and Manifest.toml if present)

create a new project

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$ julia

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(@v1.12) pkg> generate MyNewProj

;

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shell> cd MyNewProj/

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(@v1.12) pkg> activate .

dictionary

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pizza = Dict("name" => "hawaiian", "size" => 'S', "price" => 10.5)

A problem with using a dictionary is that it requires every value to be of the same type

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typeof(pizza)	# Dict{String, Any}

symbols

It is denoted by : (colon), followed by the name of the symbol, built-in Julia type to represent identifiers

named tuples

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pizza = (name = "hawaiian", size = 'S', price = 10.5)

A named tuple only allows you to use symbols as keys

All types of tuples are immutable, meaning you cannot change them

implicit naming from identifiers

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x = 0
t = (; x) # t is (x = 0,)
u = (; t.x) # u is (x = 0,)

composite type (struct) & type annotation

:: is used to annotate variables and expressions with their type. x::T means variable x should have type T. It helps Julia figure out how many bytes are needed to hold all fields in a struct

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struct Archer
    name::String
    health::Int
    arrows::Int
end

closure

varargs

"varargs" (variable arguments) refers to the ability of a function to accept an arbitrary number of arguments. This is achieved using the splat operator (...) in the function definition.

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function my_sum(a, b, rest...)
    total = a + b
    for x in rest
        total += x
    end
    return total
end
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my_sum(1, 2, 3, 4, 5)	# 15

keyword arguments

a semicolon (;) separates positional arguments from keyword arguments in the function signature. All arguments to the right of the semicolon are treated as keyword arguments. They can optionally have default values

views

A view is essentially a pointer to a sub-section of another vector, but not a standalone vector itself

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one2ten = collect(1:10);
println(one2ten)

one2five = one2ten[1:5]
one2five_view = view(one2ten, 1:5)

one2ten[1] = 10
println(one2five)
println(one2five_view)

# [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# [1, 2, 3, 4, 5]
# [10, 2, 3, 4, 5]

@views is a macro that converts sliced arrays into views (pointers are much cheaper than creating copies of arrays). For more information on how to use the view syntax correctly

semicolon after steprange

placing a semicolon ; after a step range expression inside square brackets, e.g., [1:10;], changes the resulting object from a UnitRange to a Vector

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b = [1:2:10]	# Vector{StepRange{Int64, Int64}}
a = [1:2:10;]	# Vector{Int64}

size(b)	# (1)
size(a)	# (5)

element-wise operations

Dot syntax for operators: For binary operators like +, -, *, /, ^

Dot syntax for functions: For functions, the dot is placed after the function name.

Property destructuring

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

julia> a
1

julia> b
2

One-line functions

"one-line function" also known as the compact "assignment form"

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function_name(parameters) = expression

Anonymous Functions (Lambda Functions)

functions without a name (parameters) -> expression, often defined inline for use with higher-order functions like map, filter, or reduce

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numbers = [1, 2, 3]
doubled_numbers = map(x -> 2 * x, numbers) # doubled_numbers will be [2, 4, 6]

using vs import

that’s the difference between using and import - the former brings all exported names into scope, while the latter only brings NiceStuff (the module identifier) into scope.

[https://discourse.julialang.org/t/difference-between-include-use-and-import/65918/5]

Conditional Compilation In C++

Using g++ only

conditional.cpp

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#include <iostream>

#define na 4

int main() {
  int a[na];

  a[0] = 2;
  for (int n = 1; n < na; n++) a[n] = a[n-1] + 1;

#ifdef DEBUG
  // Only kept by preprocessor if DEBUG defined
  for (int n = 0; n < na; n++) {
    std::cout << "a[" << n << "] = " << a[n] << std::endl;
  }
#endif 
  
  return 0;
}

-DDEBUG args

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$ g++ -Wall -Wextra -Wconversion conditional.cpp -o conditional
$ ./conditional
$ g++ -Wall -Wextra -Wconversion conditional.cpp -o conditional -DDEBUG
$ ./conditional
a[0] = 2
a[1] = 3
a[2] = 4
a[3] = 5

Using CMakeLists.txt add_definitions

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cmake_minimum_required(VERSION 3.2)

option(DEBUG "Option description" OFF)

if(DEBUG)
    add_definitions(-DDEBUG)
endif(DEBUG)

add_executable(cond conditional.cpp)

without debug

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$ cmake ..
$ make
$ ./cond

with debug

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$ cmake -DDEBUG=ON ..
$ make
$ ./cond
a[0] = 2
a[1] = 3
a[2] = 4
a[3] = 5