Why no mutating filter function for strings?

New to Julia
1

Is there a reason why

julia> str = " a"
" a"

julia> filter(isspace, str)
" "

works but

julia> filter!(isspace, str)
ERROR: MethodError: no method matching filter!(::typeof(isspace), ::String)
Closest candidates are:
  filter!(::Any, ::AbstractArray{T,1} where T) at array.jl:2503
  filter!(::Any, ::IdDict) at iddict.jl:162
  filter!(::Any, ::Dict{K,V}) where {K, V} at dict.jl:686
  ...
Stacktrace:
 [1] top-level scope at REPL[3]:1

doesn’t? Why there isn’t a mutating form of filter for strings?

2

This is probably because strings are immutable. Methods with ! work by modifying the container passed to them, but you can’t modify a string in Julia (for performance reasons).

1 Like
3

Putting 2 and 2 together, I realise that other functions return SubStrings.

Though I just end up using str = filter(..., str) anyway, changing the original variable.

Though I’m surprised that there’s no equivalent of push! and deleteat!. I’ll read up more on this.

Thanks!

1 Like
4

You could always make an issue asking for this feature. That said, one critical difference between a function like filter and a function like findfirst is that the latter always returns a result which can be described by a single start and end index, while the former can require extensive auxiliary space to store the result (e.g. filter(x->x=='0', "0110010101010") )

1 Like
5

What would they do on an immutable object?

6

Sorry, I’m coming across wrong. I’m new to Julia and am learning why the design of the language is in certain ways. My asking was not an expectation of conformation to a structure, but an inquiry for better understanding, challenging my intuition. I couldn’t find anything in the documentation explaining, so I thought I’d ask.

Throw an error I suppose.

Again, not expecting a change from my question. Just looking for understanding, and strings being immutable for performance makes sense. Thanks!

7

But they already do, eg

julia> push!("I am immutable", 'q')
ERROR: MethodError: no method matching push!(::String, ::Char)
1 Like
8

And likewise for filter! as per my original post, yes.

I was thinking an error that mentions an inappropriate attempt to mutate an immutable object, rather than no method defined for that combination of input variable types.