# Char vs. String for Dict key

**URL:** <https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332>\
**Category:** New to Julia\
**Created:** [August 11, 2017, 2:17pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332 "2017-08-11T14:17:35Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![RemyX](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/remyx/32/1599_2.png) [@RemyX](https://discourse.julialang.org/u/RemyX)\
**Post date:** [August 11, 2017, 2:17pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332/1 "2017-08-11T14:17:35Z")

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I’m getting to know Julia and found some quick little exercises at [exercism.io](http://exercism.io) ([http://exercism.io/languages/julia/exercises](http://exercism.io/languages/julia/exercises)). In the Nucleotide Count exercise, based on the runtests.jl file, the preferred Dict structure is:  
Dict(‘A’ =\> 0, ‘C’ =\> 0, ‘G’ =\> 0, ‘T’ =\> 0)

Is this preferred to String based keys? i.e. Dict(“A”=\>0).

My first solution did this for String keys (trimmed slightly). `count` is the Dict, `myStr` the input string.  
`
for i in myStr
count[“$i”] += 1
end
`

This failed the test because of the data type of the key. So I changed to using indexes on the input string, which worked as well, but felt less elegant somehow. I realize it’s quite subjective.  
`
for i in 1:length(myStr)
count[myStr[i]] += 1
end
`

Is there a reason to choose one over the other, aside from passing the supplied runtest.jl? Style, idiom, performance?

Thanks.

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**Author:** ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)\
**Post date:** [August 11, 2017, 2:29pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332/2 "2017-08-11T14:29:19Z")

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Most idiomatic, I think, is to use symbols:

```julia
Dict(:A => 0, :C => 0, :G => 0, :T => 0)

```

although for this particular exercise that might not be an option.

I think chars are immutable and strings are not, so they should be more performant. I would think symbols are also performant. Why don’t you benchmark your variants using Benchmarktools.jl?

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**Author:** ![RemyX](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/remyx/32/1599_2.png) [@RemyX](https://discourse.julialang.org/u/RemyX)\
**Post date:** [August 11, 2017, 2:49pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332/3 "2017-08-11T14:49:28Z")

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Thanks for the reply. I played around with converting a String to a Symbol and realized that iterating over a string creates a Char. I’m not sure how I missed that before. The following works just fine for the purposes of the provided exercise.  
`
for i in str1
if !haskey(count,i)
throw(ErrorException(“Key not found”))
end
count[i] += 1
end
`  
On a side note, figuring out how to get from String to Symbol to Char and back is entertaining.

Thanks for your recommendation of Benchmarktools, I’ll take a look at it.

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**Author:** ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)\
**Post date:** [August 11, 2017, 3:06pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332/4 "2017-08-11T15:06:07Z")

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(Note you can use triple backticks to typeset whole blocks of code at once)

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [August 11, 2017, 3:32pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332/5 "2017-08-11T15:32:10Z")

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Note that `String` is generally a more heavyweight object than a `Char`. A `String` combines a length and an an array of (UTF8-encoded) characters (though technically it no longer uses an `Array`), whereas a `Char` is just one number (that can fit in a single CPU register). Operations on strings will typically be more expensive than operations on chars, e.g.

```julia
julia> k = "A"; @btime hash($k);
  10.671 ns (0 allocations: 0 bytes)

julia> k = 'A'; @btime hash($k);
  3.790 ns (0 allocations: 0 bytes)

julia> k = "A"; @btime isequal($k,$k);
  5.041 ns (0 allocations: 0 bytes)

julia> k = 'A'; @btime isequal($k,$k);
  1.893 ns (0 allocations: 0 bytes)

```

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<div class="post-metadata">

**Author:** ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)\
**Post date:** [August 11, 2017, 3:46pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332/6 "2017-08-11T15:46:46Z")

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And symbols are as fast as chars:

```julia
julia> @btime isequal(:a,:a);                                                                                                                         
  1.689 ns (0 allocations: 0 bytes)                                                                                                                   
                                                                                                                                                      
julia> @btime isequal('k', 'c');                                                                                                                      
  1.688 ns (0 allocations: 0 bytes)                                                                                                                   
                                                                                                                                                      
julia> @btime isequal("k", "v");                                                                                                                      
  5.033 ns (0 allocations: 0 bytes)                                                                                                                   

```

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<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [August 11, 2017, 4:18pm UTC](https://discourse.julialang.org/t/char-vs-string-for-dict-key/5332/7 "2017-08-11T16:18:21Z")

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> [@mauro3](#):
>
> And symbols are as fast as chars:

Not for all operations. e.g. `hash` and `isless` are both slower for symbols.