Hi there. I want to find a file by its name within a given directory (and possible subdirectories). ...

# getting-started

[tomas-mrkvička]

Hi there.
I want to find a file by its name within a given directory (and possible subdirectories).
I found that FileWalkDirection.BOTTOM_UP https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.io/-file-walk-direction/ should do the thing because I want to check files first and then go deep. But BOTTOM_UP goes to directory first regardless its documentation: Depth-first search, directory is visited AFTER its files. What am I missing here?
Thank you.

[ephemient]

do you have a reproducible example? File.walk works as expected for me,

File(".").walk(FileWalkDirection.TOP_DOWN).first() == File(".")
File(".").walk(FileWalkDirection.BOTTOM_UP).last() == File(".")

[tomas-mrkvička]

I have just tried some examples and it looks like that filter is the one to blame.

File("/C:*/foo*").walkBottomUp().filter { file -> file.name == "gradle.properties" }.forEach { file -> println(file.absolutePath) }

prints:

C:\foo\xxx\gradle.properties
C:\foo\gradle.properties
C:\foo\asd\gradle.properties
C:\foo\zzz\gradle.properties
C:\foo\yyy\gradle.properties

So it looks like that filtering does something fishy with ordering.

In our real code we use .firstOrNull() after filter {}. So forEach {} in my example is used to just print occurrences.

[ephemient]

what's wrong with that order? there's no guarantee on relative ordering between children at the same level, such as C:\foo\gradle.properties and C:\foo\xxx

[tomas-mrkvička]

I am expecting that if C:\foo\gradle.properties is a file, it should go to the filter method before C:\foo\xxx\gradle.properties (because BOTTOM_UP observers files first) and therefore should be first in the terminal operation.

[tomas-mrkvička]

File("/C:/foo").walkBottomUp().find { file -> file.name == "gradle.properties" }?.absolutePath) also prints C:\foo\xxx\gradle.properties as a winner even though there is a C:\foo\gradle.properties in the root of C:/foo directory.
Am I missing any fundamental Kotlin knowledge here?

[ephemient]

it only means that a file's parent directory is visited before or after the file, not all directories before or after all files

[ephemient]

think of a tree traversal:

A
 /|\
B C D
|
E

in preorder (top-down), A is visited before BCD, but BCD have no order among themselves, and thus E may be visited either before or after CD (but always after B). in postorder (bottom-up), E is visited before B, and BCD is visited before A, but similarly there is no order among BCD so E may be visited before or after CD.

[tomas-mrkvička]

Ouch. Now I make sense to me. Thank you very much for the explanation!!