# Sorting an array of strings based on number pattern

217 views (last 30 days)
Niels on 14 Jul 2015
Edited: Stephen Cobeldick on 22 Feb 2016
I'm looking for an efficient way to solve the following issue:
Typically I read out all files in a directory that match a certain pattern by means of the dir function:
dirFiles = dir('Common_*_common.ext');
fileNames= {dirFiles.name};
This does what it is supposed to do and gives me a nice cell array with files I need to load.
However, the pattern is of the format 0, 1, 2, ..., 8, 9, 10 and the order of my fileNames comes out as 0, 10, 1, ..., 7, 8, 9. Unfortunately, changing the filenames themselves to 00, 01, 02, ..., 10 is not an option. Is there an efficient way to sort such arrays to the correct format?
Of course this should also hold when the sequence increases to larger numbers (e.g. 0, 1, 2, ..., 29, 30, 31).

#### 1 Comment

Stephen Cobeldick on 22 Feb 2016
You could use my FEX submission natsortfiles. The function natsortfiles does not perform a naive natural-order sort, but also sorts the filenames and file extensions separately so that the file extension period character does not influence the sort output.
>> A = {'doc_10.txt'; 'doc_20.txt'; 'doc_2.txt'; 'doc_1.txt'; 'doc_11.txt'};
>> sort(A) % gives the wrong order!
ans =
'doc_1.txt'
'doc_10.txt'
'doc_11.txt'
'doc_2.txt'
'doc_20.txt'
>> natsortfiles(A) % the correct number order :)
ans =
'doc_1.txt'
'doc_2.txt'
'doc_10.txt'
'doc_11.txt'
'doc_20.txt'

Niels on 14 Jul 2015
Edited: Niels on 14 Jul 2015
Ok, I should've searched better...
Problem solved with a PotW tool by Stephen Cobeldick.

Jos (10584) on 14 Jul 2015
Here is a way:
% create some example names
filenames = {'xx_1_yy.txt','xx_8_yy.txt','xx_10_yy.txt','xx_2_yy.txt'}
sort(filenames) % wrong!
% extract the numbers
filenum = cellfun(@(x)sscanf(x,'xx_%d_yy.txt'), filenames)
% sort them, and get the sorting order
[~,Sidx] = sort(filenum)
% use to this sorting order to sort the filenames
SortedFilenames = filenames(Sidx)

Niels on 14 Jul 2015
Thanks. Also does the job nicely.
I actually prefer your solution over the tool I found, as I can add that in just 2 additional lines of code. Unfortunately I do not seem to be able to change my accepted answer anymore...
Stephen Cobeldick on 22 Feb 2016
@Niels: this is a good solution, but it works only for this filename format: any change in filename format will mean that you need to change your code. For multiple numbers in one name, or sorting by characters and numeric values it gets quite complicated. That is why I wrote my FEX submission natsortfiles, which naturally deals with all of these situations. You can change the filename format and natsortfiles will still work.