# Find the position of a single non-matching character between two strings
**URL:** https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560
**Category:** General Usage
**Tags:** strings, unicode
**Created:** [August 30, 2022, 5:07pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560 "2022-08-30T17:07:38Z")
**Posts on this page:** 8
**Page:** 2
### Author:  [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)
#### Post date: [August 30, 2022, 7:24pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/21 "2022-08-30T19:24:46Z")
> [@stevengj](#):
>
> `const UTF8String = Union{String,SubString{String}}`
Would you mind a footnote on the above? š
---
### Author:  [@digital\_carver](https://discourse.julialang.org/u/digital_carver)
#### Post date: [August 30, 2022, 7:39pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/22 "2022-08-30T19:39:41Z")
> [@stevengj](#):
>
> What would the user do with a grapheme index?
Graphemes are what (end-)users care about in most cases. If the goal was to show āthe filenames differ at position Nā to an average, non-developer user, then N being in terms of graphemes would make the most intuitive sense. (For eg. I originally tested the code segments here with strings in my native language, and was myself confused for a moment at the result, until I counted how many codepoints were there in each of the graphemes upto the difference.)
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### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [August 30, 2022, 7:52pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/23 "2022-08-30T19:52:49Z")
> [@digital\_carver](#):
>
> If the goal was to show āthe filenames differ at position Nā to an average, non-developer user, then N being in terms of graphemes would make the most intuitive sense.
I agree, if itās a non-programming interface (as opposed to, say, an exception string) where the user will never take the position and use it to index into a Julia string. (Though Julia 1.9 will include [grapheme slicing](https://github.com/JuliaLang/julia/pull/44266).)
(It might be reasonable to implement a `graphemeind(s, i)` function that returns the index of the grapheme containing `s[i]` in the `Unicode` stdlib. I havenāt seen that functionality in other languages, though, e.g. it doesnāt seem to be in the Python [`grapheme`](https://pypi.org/project/grapheme/) library.)
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### Author:  [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)
#### Post date: [August 30, 2022, 8:00pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/24 "2022-08-30T20:00:02Z")
the intention was to provide a different proposal from the many already arrived.
If I had tried to find a better performing solution it would have taken 50 times the time it took me to find this one.
Regarding correctness, I point out that a āsimplerā solution was required to the problem of finding the position of a SINGLE non-matching character, not the first of many non-matching characters.
I used findfirst because a āsimpleā find did not come to mind (and I donāt know if it exists).
But the same function works with findlast (or even findall), for the given problem.
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### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [August 30, 2022, 8:02pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/25 "2022-08-30T20:02:38Z")
> [@rocco\_sprmnt21](#):
>
> Regarding correctness, I point out that a āsimplerā solution was required to the problem of finding the position of a SINGLE non-matching character, not the first of many non-matching characters.
It also fails this test:
```julia
julia> s1, s2 = "zzzz", "zzzx"
("zzzz", "zzzx")
julia> findfirst(==(only(setdiff(s1,s2))), s1)
ERROR: ArgumentError: Collection is empty, must contain exactly 1 element
```
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### Author:  [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)
#### Post date: [August 30, 2022, 8:03pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/26 "2022-08-30T20:03:48Z")
@rocco_sprmnt21, your proposal seems very simple and elegant. I was looking for a function to `subtract` strings, which doesnāt seem to exist, probably for a good reasonā¦ But your `setdiff` (nearly) does the trick, although slowly compared to the rest.
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### Author:  [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)
#### Post date: [August 30, 2022, 8:06pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/27 "2022-08-30T20:06:40Z")
it can be useful, but you have to be careful, as @stevengj shows, for corner cases.
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### Author:  [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)
#### Post date: [August 30, 2022, 9:47pm UTC](https://discourse.julialang.org/t/find-the-position-of-a-single-non-matching-character-between-two-strings/86560/28 "2022-08-30T21:47:40Z")
if you like setdiff, here are some amended versions
```julia
# for ASCII strings
setdiff(enumerate(s1),enumerate(s2))
# for Unicode strings
first(setdiff(pairs(s1),pairs(s2)))
```
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