Session 8
Files & Directories

File Lists

So far, our scripts were able to manage individual files. But scripts are particularly useful for batch processing of many files, e.g. all wav files in a given directory. Batch processing is applicable with and without user interaction. For instance, a script can incrementally read files from a target directory, open every file in an editor, wait for the user to do something (e.g. an annotation), finish off with some post-processing, and load the next file. Or a script may ask the user for a target directory and then process all files without further interaction (e.g. calculating pitch parameters), presenting results in Praat Info. Or a standard directory is declared in the script, so that the script processes files autonomously without any interaction as soon as it is launched.

The standard procedure for Praat batch processing consists of two steps: (1) Create a list of all target files (the dedicated container for file lists is a Strings object). (2) Loop through the file list, incrementally processing file after file.

Listing and filtering files

Step 1 is implemented using the GUI command Create Strings as file list... (located in the New menu of Praat Objects). As the name suggests, the command creates a Strings object containing a list of filenames, one filename per line. Create Strings as file list has two arguments: a name for the newly created Strings object and a file specification. The only idiosyncrasy concerning object naming is the replacement of spaces with underscores, other than that there's nothing to bear in mind. The file specification argument is more interesting:

• If only a directory path is given, all files in the specified directory will be part of the list.
• If a full file specification is given, only the one specified file is listed—which is useless, you don't need a list for one file…
• Instead of listing all files or only one file, the range of target files can be restricted with a wildcard: The asterisk (*) is a joker matching any character. It is used to define filename patterns. Only one wildcard is permitted. Some examples:

Lists created with Create Strings as file list contain only filenames, the path information is not part of the list!

To practice file list creation and become familiar with filename patterns using a wildcard, select New > Create Strings as file list..., try different patterns in the File path input field, click apply, and review the resulting list with View & Edit.

Having unraveled the mechanism of file list creation, implementation is straightforward:

Create Strings as file list: "Filelist", "*.wav"

This statement

• creates a Strings object called Filelist
• containing a list of all filenames ending with .wav
• of files located in the working directory.

Algorithmic thinking

Step 2 of batch processing involves looping through the file list which we created in step 1. We have to put off implementation of this step until we finish discussing loops in the next session. What we can do now, however, is pondering over an abstract algorithm, so that the structure of the script is ready for implementation as soon as we're able to use loops. What we have: A list of filenames, referencing files in a given directory. What we need: Incremental processing of all files referenced in the file list.

First the number of filenames in the list must be determined to find out the number of passes through the loop. Then the loop is set up. It provides an index, starting with 1, which is incremented by 1 until the number of files is reached, i.e. until all files in the list are processed. Within the loop, the filename with the current index is pulled from the list, then the referenced file is loaded, processed, and removed:

create file list
n = determine number of files to process
start loop with i = 1
    extract filename[i] from file list (first, second, third, ..., last)
    load file[i] as object
    statement 3
    statement 4
    statement 5
    ...
    remove object[i]
    increment i by 1
end loop if i = n
remove file list

This, of course, is not valid Praat scripting code, it's just an outline of what we want our script to do. It's formulated in 'code-aware', i.e. formalized natural language and is a good exercise in 'algorithmic thinking'.