CODEWITHSUNDEEP
functionmethodsjuliadispatch
Dominique Makowski
Dominique Makowski

Reputation: 1673

Julia: Same function for multiple types?

I have this big function that I defined on a vector, but I'd like it to work also with a single value. I'd like the type of the first argument to be either a vector or a number.

I triend the following:

function bigfunction(x::Vector, y::Float64=0.5)

  # lots of stuff
  z = x .+ y
  return z
end


bigfunction(x::Number) = bigfunction()

The function works on a vector, but not on the number. 

bigfunction([0, 1, 3])
bigfunction(2)

Should I do something with Union{} as I've seen sometimes? Or redefining the method in a different way?

Upvotes: 5

Views: 2510

Answers (2)

Julia Learner
Julia Learner

Reputation: 3032

This question and the responses help illustrate for me the points made at the great blog post by Chris Rackauckas on type dispatch in Julia.

I have collated the responses into the following code:

# I ran this only in Julia 1.0.0.

## ========== Original function ==========
## function bigfunction(x::Vector, y::Float64=0.5)
##     # lots of stuff
##     z = x .+ y
##     return z
## end
## bigfunction(x::Number) = bigfunction()
## println(bigfunction([0, 1, 3]))
## println(bigfunction(2))
## ---------- Output has ERROR ----------
## [0.5, 1.5, 3.5]
## ERROR: LoadError: MethodError: no method matching bigfunction()


# ========== Answer Suggested by Picaud Vincent in comments ==========
# Note use of Union in function signature.
function bigfunction(x::Union{Vector, Number}, y::Float64=0.5)
    # lots of stuff
    z = x .+ y
    return z
end
println(bigfunction([0, 1, 3]))
println(bigfunction(2))
## ---------- Output Okay ----------
## [0.5, 1.5, 3.5]
## 2.5


# ========== Answer Suggested by Robert Hönig in comments ==========
# Note change in line right after function definition.
function bigfunction(x::Vector, y::Float64=0.5)
    # lots of stuff
    z = x .+ y
    return z
end
bigfunction(x::Number) = bigfunction([x])
println(bigfunction([0, 1, 3]))
println(bigfunction(2))
## ---------- Output Okay ----------
## [0.5, 1.5, 3.5]
## 2.5


# ========== Answer Suggested by Chris Rackauckas ==========
# Note change in function signature using duct typing--no type for x.
function bigfunction(x, y=0.5)
    # lots of stuff
    z = x .+ y
    return z
end
println(bigfunction([0, 1, 3]))
println(bigfunction(2))
## ---------- Output Okay ----------
## [0.5, 1.5, 3.5]
## 2.5

Upvotes: 4

Chris Rackauckas
Chris Rackauckas

Reputation: 19162

Or duck type. Remember that functions always auto-specialize so choosing restrained dispatches does not impact the performance at all.

function bigfunction(x, y=0.5)

  # lots of stuff
  z = x .+ y
  return z
end

This will be just as performant yet will work on many more types. See this blog post on type-dispatch designs for more information.

Upvotes: 3

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