# Compare patterns in byte array
**URL:** https://discourse.julialang.org/t/compare-patterns-in-byte-array/55463
**Category:** New to Julia
**Created:** [February 17, 2021, 2:29pm UTC](https://discourse.julialang.org/t/compare-patterns-in-byte-array/55463 "2021-02-17T14:29:31Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Michal](https://discourse.julialang.org/u/Michal)
#### Post date: [February 17, 2021, 2:29pm UTC](https://discourse.julialang.org/t/compare-patterns-in-byte-array/55463/1 "2021-02-17T14:29:31Z")
How can I find the position in array “a”, where is pattern match from array “b”? Does standard library have implemented this functionality?
```julia
a = [0x54, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x61]
b = [0x73, 0x20, 0x69]
```
Expected result is 4, or 4:6 or similar.
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [February 17, 2021, 2:41pm UTC](https://discourse.julialang.org/t/compare-patterns-in-byte-array/55463/2 "2021-02-17T14:41:18Z")
I don’t know of a built-in tool to do exactly this, but it can be built out of existing tools:
```julia
julia> function find_sub_array(needle, haystack)
findfirst(firstindex(haystack):(lastindex(haystack) - length(needle))) do i
@view(haystack[i:(i + length(needle) - 1)]) == needle
end
end
find_sub_array (generic function with 1 method)
julia> find_sub_array(b, a)
4
```
This just uses `findfirst` to find the first index `i` such that `a[i:(i + 2)] == b`. If `b` is not present, then `findfirst` will return `nothing`.
---
### Author:  [@Michal](https://discourse.julialang.org/u/Michal)
#### Post date: [February 19, 2021, 12:34pm UTC](https://discourse.julialang.org/t/compare-patterns-in-byte-array/55463/3 "2021-02-19T12:34:50Z")
Thank you. It is working. I modified your solution to get first indexes of all patterns in array:
```julia
a = [0x54, 0x68, 0x69, 0x73, 0x20, 0x69, 0x70, 0x20, 0x61, 0x70, 0x20, 0x61, 0x73, 0x20, 0x69, 0x20]
b = [0x73, 0x20, 0x69]
function find_sub_array_index(pattern, arr)
findfirst(firstindex(arr):(lastindex(arr) - length(pattern))) do i
@view(arr[i:(i + length(pattern) - 1)]) == pattern
end
end
function find_sub_array_indexes(pattern, arr)
arrResult = []
result = 0
resultLast = 1-size(pattern,1)
patternSize = size(pattern,1)
while result !== nothing
nextIndex = (resultLast + patternSize)
if nextIndex > size(arr, 1)
return arrResult
end
result = resultLast + patternSize + find_sub_array_index(pattern, arr[nextIndex:end])
if result === nothing
return arrResult
end
push!(arrResult, result-1)
resultLast = result
end
end
println(find_sub_array_indexes(b, a))
```
This is not optimized solution, just working example.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [February 19, 2021, 1:07pm UTC](https://discourse.julialang.org/t/compare-patterns-in-byte-array/55463/4 "2021-02-19T13:07:18Z")
> [@Michal](#):
>
> How can I find the position in array “a”, where is pattern match from array “b”? Does standard library have implemented this functionality?
In Julia 1.6 you can use `findfirst` and `findnext` for this:
```julia
julia> findfirst(b, a)
4:6
```
See also this discussion: [Finding position of a sequence in a UInt8 array?](https://discourse.julialang.org/t/finding-position-of-a-sequence-in-a-uint8-array/45757) which resulted in this PR: [https://github.com/JuliaLang/julia/pull/37283](https://github.com/JuliaLang/julia/pull/37283)
---
### Author:  [@Michal](https://discourse.julialang.org/u/Michal)
#### Post date: [February 19, 2021, 2:25pm UTC](https://discourse.julialang.org/t/compare-patterns-in-byte-array/55463/5 "2021-02-19T14:25:43Z")
Ok, I’ll check it. Thank you for advice.