Skip to main content

files and directories - What kind of check does DirectoryQ exactly perform?


I'm using Mathematica 9.0.1 on Windows XP and my drive letters are assigned as follows: C: and D: are hard local disks, while E: is assigned to some removable media. Now, the situation is as follows


DirectoryQ[# <> ":\\"] & /@ {"c", "d", "e"}


returns {True,True,True}, even when an optical disk isn't in the drive.


To outflank the problem I use a directoryQ function, defined in the code below.


However, it obviously fails against a full hard disk or when I don't have write access. Therefore, I' m wondering about the kind of check DirectoryQ basically performs. Is there a better solution to fix the problem?


   directoryQ[unit_]:=Module[
       {root,file,ret},
       ret=False;
       root=unit<>":\\";
       file = FileNameJoin[{root, "testFile.txt"}];
       If[
         DirectoryQ[root],

         Quiet[Export[file,"test","Text"]];
         If[
           FileExistsQ[file],
           DeleteFile[file];
           ret=True
         ];
       ];
       Return[ret];
];


directoryQ["c"]
directoryQ["e"]

Answer



Since Mathematica is written in the C programming language, it is likely that any file-system status operation is doing nothing more than calling C's built-in functions. In this case, that would be stat.


So let's test what stat does on Windows. According to the C language documentation, stat is given the name of something in the file system, and returns a C structure whose fields describe the properties of that thing. In particular, the st_mode field seems relevant:



st_mode: Bit mask for file-mode information. The _S_IFDIR bit is set if path specifies a directory; the _S_IFREG bit is set if path specifies an ordinary file or a device. User read/write bits are set according to the file's permission mode; user execute bits are set according to the filename extension.



I wrote a simple test program in Microsoft Visual C that tests the stat function. Drive C: is my Windows system drive; drive E: is my DVD drive, which currently doesn't have anything in it. Here's what I found:


stat("C:\\WINDOWS") returns success and sets _S_IFDIR

stat("C:\\WINDOWS\\SYSTEM32\\NOTEPAD.EXE") returns success and does not set _S_IFDIR
stat("C:\\") returns success and sets _S_IFDIR
stat("E:\\") returns success and sets _S_IFDIR
stat("W:\\") returns failure

So we can conclude from this that Windows, or at least the stat function on Windows, considers empty DVD drives to be "there." I also have a flash memory stick that I can plug into a USB slot, which I have mapped to drive W, but that "isn't there" when it's unplugged.


This is consistent with what Windows Explorer shows me when I click on "My Computer". I see drive C: with a name, and I see drive E:, even though I get an error message when I double-click on it. I don't see drive W: (my flash memory stick) after I pulled it out of the socket. So Windows does consider an empty DVD drive to be present, which is probably why stat says it's a valid directory.


I have no idea how to find out whether a DVD drive is full or empty in C, much less in Mathematica. But at least we can see the core of the problem.


Comments

Post a Comment

Popular posts from this blog

front end - keyboard shortcut to invoke Insert new matrix

I frequently need to type in some matrices, and the menu command Insert > Table/Matrix > New... allows matrices with lines drawn between columns and rows, which is very helpful. I would like to make a keyboard shortcut for it, but cannot find the relevant frontend token command (4209405) for it. Since the FullForm[] and InputForm[] of matrices with lines drawn between rows and columns is the same as those without lines, it's hard to do this via 3rd party system-wide text expanders (e.g. autohotkey or atext on mac). How does one assign a keyboard shortcut for the menu item Insert > Table/Matrix > New... , preferably using only mathematica? Thanks! Answer In the MenuSetup.tr (for linux located in the $InstallationDirectory/SystemFiles/FrontEnd/TextResources/X/ directory), I changed the line MenuItem["&New...", "CreateGridBoxDialog"] to read MenuItem["&New...", "CreateGridBoxDialog", MenuKey["m", Modifiers-...
Post a Comment
Read more

How to thread a list

I have data in format data = {{a1, a2}, {b1, b2}, {c1, c2}, {d1, d2}} Tableform: I want to thread it to : tdata = {{{a1, b1}, {a2, b2}}, {{a1, c1}, {a2, c2}}, {{a1, d1}, {a2, d2}}} Tableform: And I would like to do better then pseudofunction[n_] := Transpose[{data2[[1]], data2[[n]]}]; SetAttributes[pseudofunction, Listable]; Range[2, 4] // pseudofunction Here is my benchmark data, where data3 is normal sample of real data. data3 = Drop[ExcelWorkBook[[Column1 ;; Column4]], None, 1]; data2 = {a #, b #, c #, d #} & /@ Range[1, 10^5]; data = RandomReal[{0, 1}, {10^6, 4}]; Here is my benchmark code kptnw[list_] := Transpose[{Table[First@#, {Length@# - 1}], Rest@#}, {3, 1, 2}] &@list kptnw2[list_] := Transpose[{ConstantArray[First@#, Length@# - 1], Rest@#}, {3, 1, 2}] &@list OleksandrR[list_] := Flatten[Outer[List, List@First[list], Rest[list], 1], {{2}, {1, 4}}] paradox2[list_] := Partition[Riffle[list[[1]], #], 2] & /@ Drop[list, 1] RM[list_] := FoldList[Transpose[{First@li...
Post a Comment
Read more

functions - Get leading series expansion term?

Given a function f[x] , I would like to have a function leadingSeries that returns just the leading term in the series around x=0 . For example: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x)] x and leadingSeries[(1/x + 2 + (1 - 1/x^3)/4)/(4 + x)] -(1/(16 x^3)) Is there such a function in Mathematica? Or maybe one can implement it efficiently? EDIT I finally went with the following implementation, based on Carl Woll 's answer: lds[ex_,x_]:=( (ex/.x->(x+O[x]^2))/.SeriesData[U_,Z_,L_List,Mi_,Ma_,De_]:>SeriesData[U,Z,{L[[1]]},Mi,Mi+1,De]//Quiet//Normal) The advantage is, that this one also properly works with functions whose leading term is a constant: lds[Exp[x],x] 1 Answer Update 1 Updated to eliminate SeriesData and to not return additional terms Perhaps you could use: leadingSeries[expr_, x_] := Normal[expr /. x->(x+O[x]^2) /. a_List :> Take[a, 1]] Then for your examples: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x), x] leadingSeries[Exp[x], x] leadingSeries[(1/x + 2 + (1 - 1/x...
Post a Comment
Read more