How to sort strings by lenght?

Hi, I want to do this:

>> A = ['adam  '
        'bertil'
        'cesar '
        'Ada   '
        'berit '];

>> Asort = charsort(A)  
Asort = 
 Ada
 adam
 cesar
 berit
 bertil

I use strtrim to remove the blanks but i have no idea how i could sort mi list by lenght. Any sugestions?

Answer 1

[Farook Sadarudeen]

If your list is as below:

A = {'adam'; 'bertil'; 'cesar'; 'Ada'; 'berit'};

You can sort it based on the length as below:

[~,stringLength] = sort(cellfun(@length,A),'descend');
OutputListSorted = A(stringLength);

[Jan]

"stringLength" is a confusing name for a sorting index.

Answer 2

[Geoff Hayes]

Since you asked for suggestions, here is an almost solution. Rather than using a matrix of strings, convert this to a cell array, as

A = {'adam ' 'bertil' 'cesar ' 'Ada ' 'berit '};

Then in your function, charsort, remove (as you already said) the whitespace (blanks). So you have a cell array of strings, each of which you want to find the length of. You can use cellfun to apply a function to each element of your cell array. In this case, we want to apply length as

 stringLengthsOfA = cellfun(@(x)length(x),A)';

which returns a column vector (I added the apostrophe so that the result of cellfun is a column (this assumes that A is a row cell array)) like

 stringLengthsOfA =

     4
     6
     5
     3
     5

We have the appropriate length of each string which we can now sort. But as soon as we sort this vector, we lose the original order. So just prepend a column that has the indices of each of these lengths (so the first element 4 is for row 1, the second element 6 is for row 2, etc.) as

     1     4
     2     6
     3     5
     4     3
     5     5

(The above is easy to do.) Now consider using sortrows which will allow you to specify which column to sort on and preserve the relationship between elements of each row. See what happens when you sort (the above matrix) on the second column. You will get the lengths (second column) sorted in ascending order with their corresponding indices (first column). You can then use the first column to grab the (now) sorted data from A.

Try putting the above together and see what happens!

Answer 3

[per isakson]

Without using a cell array

>> A = ['adam  '
        'bertil'
        'cesar '
        'Ada   '
        'berit '];

>> len = arrayfun( @(jj) length(strtrim(A(jj,:))), [1:size(A,1)] );
>> [ ~, ix ] = sort( len );
>> A(ix,:)
ans =
Ada   
adam  
cesar 
berit 
bertil

I guess, this is faster and uses less memory than solutions based on a cell array.

Answer 4

[Stephen23]

Simpler:

>> [~,idx] = sortrows(A~=32);
>> A(idx,:)
ans =
Ada
adam
cesar
berit
bertil

Answer 5

[Jan]

   A = ['adam  '
        'bertil'
        'cesar '
        'Ada   '
        'berit '];
   [~, Index] = sort(cellfun('length', cellstr(A)));
   Cs         = C(Index);

Similar answers have been given already, but it is worth to mention, that cellfun('length') is faster than cellfun(@length) or cellfun(@(x)length(x)).

This implies, that trailing spaces do not belong to the strings. Note that CHAR matrices are a really bad method to store multiple strings. Using a cell string or a modern STRING object is much better.

[per isakson]

"CHAR matrices are a really bad method to store multiple strings" That depends. "bad" from what point of view?

One case, in which CHAR is faster and uses less memory than CELL

• a list of a zillion names/words
• search, match, select, ...

>> cac = {'adam'; 'bertil'; 'cesar'; 'Ada'; 'berit'};
>> chr = ['adam  '
          'bertil'
          'cesar '
          'Ada   '
          'berit '];
>> whos cac chr
  Name      Size            Bytes  Class    Attributes
  cac       5x1               606  cell               
  chr       5x6                60  char

An order of magnitude in memory use

[Jan]

@per: Char matrices have the disadvantage, that the padding with spaces implies, that trailing spaces are not significant. This is a magic number problem (see https://www.mathworks.com/matlabcentral/answers/83075-magic-strings-and-numbers-in-matlab), and here even the non-exotic space character got a special job. Using char(0) would have been a smarter choice.

This problem is severe. Implementing general algorithms like sorting or string matching will always be impeded by the requirement to treat trailing spaces separately.

So you can compare the speed for sorting and the memory consumption:

CStr    = sprintfc('%6d', 0:1e6-1);
CStr    = CStr(randperm(1e6));
S       = string(CStr);
CharMat = char(CStr);
tic; CStrS    = sort(CStr); toc
tic; SS       = sort(S); toc
tic; CharMatS = sortrows(CharMat); toc

R2016b/Win64:

 Elapsed time is 1.261178 seconds.
 Elapsed time is 1.605999 seconds.
 Elapsed time is 1.202420 seconds.

Memory consumption:

 whos CStr S CharMat
  Name               Size                     Bytes  Class     
  CStr               1x1000000            124000000  cell                
  CharMat      1000000x6                   12000000  char                
  S                  1x1000000             62000072  string

But what happens, if you want to sort the strings '1', '1 ', '1 ' (none, one and two trailing spaces)? As soon as you try this with a CHAR matrix, you need much more complicated methods.

Imagine the same situation for storing numbers: Would you ever store a list of vectors with different lengths using a matrix padded with the value 32? Any other value could collide with the input data also, even Inf or NaN. Any algorithm based on such a data representation would suffer from this design. Even if the actual processing is very fast, it takes time to check the user inputs or results of computations for conflicts with the padding with magic numbers.

I use CHAR matrices for a tabular output to the command window. For all other applications and algorithms, I cannot exclude reliably, if a trailing space is significant, and therefore I consider this data representation as flawed.

Where do you see benefits for "search, match, select, ..." for char matrices? Are these benefits worth to restrict the usage of trailing spaces?

[per isakson]

• Magic strings and numbers in Matlab is a good read.
• Yes, the "padding with spaces" requires precaution.
• column/row-major also requires precaution. To take advantage of CHAR in the work with zillions of names the characters of single names must be stored in consecutive order in memory.
• ... but then there is no function, sortcolumns
• btw: I finally concatenated my zillions of short strings into a long string with char(31) as separator. Now, I keep my fingers crossed.