ResetDirectory causes a fail on stored file names

Question

Consider some directory dir with sub dirs and files. Our aim is to count the no of files in dir (not in sub-dirs, just the dir.)
dir="C:/folder";
SetDirectory[dir];
names = FileNames[];
Length@Select[names, FileType@# == File&]
(*2*)

There were in fact 2 files and 6 sub-dirs.
FileType /@ names // Tally
(*{{Directory, 6}, {File, 2}}*)

Now consider the following code
dir = "C:/folder";
SetDirectory[dir];
names = FileNames[];
ResetDirectory[];
Length@Select[names, FileType@# == File &]
(*0*)

Here even though we saved the file names in names, the count is 0. Why is this? Why is a directory reset affecting stored file names?

Jason B. · Accepted Answer

Herein lies the issue:
In[6]:= SetDirectory[$UserDocumentsDirectory];
files = FileNames[];
SetDirectory[$UserBaseDirectory];
CountsBy[files, FileExistsQ]

Out[9]= <|False -> 54, True -> 1|>

The filenames returned by FileNames are given relative to the current path, and so when the path changes they aren't valid filenames.
To get around this limitation, use ExpandFileName to make sure you always have an absolute file path.  In the above, use
files = ExpandFileName /@ FileNames[];

In the snippet above the only file that returns True from FileExistsQ is easily guessed by any OSX user:
In[11]:= Select[files, FileExistsQ]

Out[11]= {".DS_Store"}
```

ResetDirectory causes a fail on stored file names

One Answer

Add your own answers!

Ask a Question