How to slice each string in a string array without using for loop

For a string array, for example,

celldata =

3×1 cell array

    {'2018-12-12'}
    {'2018-11-05'}
    {'2018-09-02'}

Is there array operation (i.e. without using for loop) to extract the months from each cell and convert them to a 3*1 numeric matrix, which should be [12;11;09]. I don't want to use for loop because it was too slow.

Answer 1

[Cedric]

If you favor performance over readability/maintainability, you can build an approach around the following:

 buffer = vertcat( dates{:} ) ;
 months = (buffer(:,6:7) - '00') * [10;1] ;

where dates is a cell array of date strings ( celldata in your example).

Here is the benchmark:

% Build "large" data set.
N = 1e4 ;
dates = repmat( {'2018-12-12'; '2018-11-05'; '2018-09-02'}, N, 1 ) ;

% Basic FOR loop, STR2DOUBLE.
tic ;
n = numel( dates ) ;
months_forStr2double = zeros( n, 1 ) ;
for k = 1 : n
    months_forStr2double(k) = str2double( dates{k}(6:7) ) ;
end
fprintf( 'Basic FOR, STR2DOUBLE : %.3fs\n', toc ) ;

% Basic FOR loop, STR2NUM.
tic ;
n = numel( dates ) ;
months_forStr2num = zeros( n, 1 ) ;
for k = 1 : n
    months_forStr2num(k) = str2num( dates{k}(6:7) ) ;
end
fprintf( 'Basic FOR, STR2NUM : %.3fs\n', toc ) ;

% Basic FOR loop, SSCANF.
tic ;
n = numel( dates ) ;
months_forScanf = zeros( n, 1 ) ;
for k = 1 : n
    months_forScanf(k) = sscanf( dates{k}(6:7), '%d' ) ;
end
fprintf( 'Basic FOR, SSCANF : %.3fs\n', toc ) ;

% CELLFUN (hidden FOR), SSCANF.
tic ;
months_cellfun = cellfun( @(date) sscanf( date(6:7), '%d' ), dates ) ;
fprintf( 'CELLFUN, SSCANF : %.3fs\n', toc ) ;

% REGEXP
tic ;
months_regexp = str2double( regexp( dates,'(?<=-)\d+(?=-)','match','once' )) ;
fprintf( 'REGEXP : %.3fs\n', toc ) ;

% CELL2MAT, STR2NUM
tic ;
chardata = cell2mat( dates ) ;
months_cell2matStr2num = str2num( chardata(:,6:7) ) ;
fprintf( 'CELL2MAT, STR2NUM : %.3fs\n', toc ) ;

% DATETIME
tic ;
dt = datetime( dates) ;
months_datetime = month( dt ) ;
fprintf( 'DATETIME : %.3fs\n', toc ) ;

% SSCANF
tic ;
months_sscanf = sscanf([dates{:}],'%*4d-%2d-%*2d') ;
fprintf( 'SSCANF : %.3fs\n', toc ) ;

% EXTRACTBETWEEN
tic ;
months_extractBetween = extractBetween( dates, '-', '-' ) ;
months_extractBetween = cellfun( @str2double, months_extractBetween ) ;
fprintf( 'EXTRACTBETWEEN : %.3fs\n', toc ) ;

% Trick.
tic ;
buffer = vertcat( dates{:} ) ;
months_trick = (buffer(:,6:7) - '00') * [10;1] ;
fprintf( 'Trick: %.3fs\n', toc ) ;

% Check
disp( [isequal( months_forStr2num, months_forStr2double ), ...
       isequal( months_forScanf, months_forStr2double ), ...
       isequal( months_cellfun, months_forStr2double ), ...
       isequal( months_regexp, months_forStr2double ), ...
       isequal( months_cell2matStr2num, months_forStr2double ), ...
       isequal( months_datetime, months_forStr2double ), ...
       isequal( months_sscanf, months_forStr2double ), ...
       isequal( months_extractBetween, months_forStr2double ), ...
       isequal( months_trick, months_forStr2double )] ) ;

Output:

Basic FOR, STR2DOUBLE : 0.489s
Basic FOR, STR2NUM : 0.975s
Basic FOR, SSCANF : 0.356s
CELLFUN, SSCANF : 0.550s
REGEXP : 0.673s
CELL2MAT, STR2NUM : 0.015s
DATETIME : 0.201s
SSCANF : 0.023s
EXTRACTBETWEEN : 0.624s
Trick: 0.008s
   1   1   1   1   1   1   1   1   1

[Stephen23]

sscanf does not require a loop, simply concatenate the char vectors and use an appropriate format string:

C =  {'2018-12-12','2018-11-05','2018-09-02'}
sscanf([C{:}],'%*4d-%2d-%*2d')

[Cedric]

Thanks Stephen, just added this to the benchmark!

Answer 2

[Christopher Wallace]

chardata = cell2mat(a);
numdata = str2num(chardata(:,6:7));

[Cedric]

No, I am CW ;-)

Answer 3

[Paolo]

str2double(regexp(celldata,'(?<=-)\d+(?=-)','match','once'))

ans =

      12
      11
       9

Answer 4

[Star Strider]

Using datetime and its functions:

celldata = [{'2018-12-12'}
    {'2018-11-05'}
    {'2018-09-02'}];
dt = datetime(celldata);
M = month(dt)

M = 
    12
    11
     9

Answer 5

[Akira Agata]

Another possible solution:

celldata = [{'2018-12-12'}
  {'2018-11-05'}
  {'2018-09-02'}];
M = extractBetween(celldata,'-','-');
M = cellfun(@str2double,M);