Custom searchsortedfirst method

Question

I'm kinda new in Julia lang, so I'm still struggling with reading Julia documentation. Here is a piece of it and I am looking for explanation specifically the bolded part.

Base.Sort.searchsortedfirst — Function.

searchsortedfirst(a, x, [by=,] [lt=,] [rev=false])

Returns the index of the first value in a greater than or equal to x, according to the specified order. Returns length(a)+1 if x is greater than all values in a. a is assumed to be sorted.

Website

My array looks like this:

A = Vector{Record}()

where

type Record
     y::Int64
     value::Float64
end

Now here is my problem. I would like to call above-mentioned method on my array and obtain Record where given x equals y in this Record (Record.y == x). Guess I have to write 'by' transfrom or 'lt' comparator? or both?

Any help would be appraciated :)

Answer 1

@crstnbr has provided a perfectly good answer for the case of one-off uses of searchsortedfirst. I thought it worth adding that there is also a more permanent solution. If your type Record exhibits a natural ordering, then just extend Base.isless and Base.isequal to your new type. The following example code shows how this works for some new type you might define:

struct MyType ; x::Float64 ; end  #Define some type of my own
yvec = MyType.(sort!(randn(10)))  #Build a random vector of my type
yval = MyType(0.0)                #Build a value of my type
searchsortedfirst(yvec, yval)     #ERROR: this use of searchsortedfirst will throw a MethodError since julia doesn't know how to order MyType
Base.isless(y1::MyType, y2::MyType)::Bool = y1.x < y2.x   #Extend (aka overload) isless so it is defined for the new type
Base.isequal(y1::MyType, y2::MyType)::Bool = y1.x == y2.x #Ditto for isequal
searchsortedfirst(yvec, yval)     #Now this line works

Some points worth noting:

1) In the step where I overload isless and isequal, I preface the method definition with Base.. This is because the isless and isequal functions are originally defined in Base, where Base refers to the core julia package that is automatically loaded every time you start julia. By prefacing with Base., I ensure that my new methods are added to the current set of methods for these two functions, rather than replacing them. Note, I could also achieve this by omitting Base. but including a line beforehand of import Base: isless, isequal. Personally, I prefer the way I've done it above (for the overly pedantic, you can also do both).

2) I can define isless and isequal however I want. It is my type and my method extensions. So you can choose whatever you think is the natural ordering for your new type.

3) The operators <. <=, ==, >=, >, all actually just call isless and isequal under the hood, so all of these operators will now work with your new type, eg MyType(1.0) > MyType(2.0) returns false.

4) Any julia function that uses the comparative operators above will now work with your new type, as long as the function is defined parametrically (which almost everything in Base is).