How can I create new variables based on groups?

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Hannah_Mad 2020-4-27

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https://ww2.mathworks.cn/matlabcentral/answers/521018-how-can-i-create-new-variables-based-on-groups

评论： Hannah_Mad 2020-4-29

Hello everyone,

I want to create new variables in order to perform a t-test based on the group membership of my subjects. I have this code here:

clearvars
close all
filepath = ['filepath'];
T =readtable('filename');
G = findgroups(T(:,1))
if G == 1 
    X = T(:,:)
else G == 2 
    Y = T(:,:)
end

I am encountering the following problem: It does not work. I will only get table T again for Y and not what I want, two entirely seperate tables based on whether a subject is in group 1 or 2. Any help or tips would be appreciated.

Thank you

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Hannah_Mad 2020-4-27

在 MATLAB Online 中打开

Thank you Rik,

Please see below an excerpt from my data.

'0,1188'	'0,1103'	'1,4'	'1,3'	'-13,00950292'	'-1,000894239'	'3,728322672'	'12,81289888'	'0,468820547'	'1,169608552'
'0,1103'	'0,2376'	'1,3'	'2,8'	'-11,8'	'-2'	'3,6'	'13,4'	'-0,9'	'2,9'
'0,1313'	'0,1717'	'1,3'	'1,7'	'-13,28540783'	'-3,043789654'	'1,401630356'	'13,32603837'	'-2,987182197'	'0,545827005'
'0,0971'	'0,0883'	'1,1'	'1'	'-15,71450602'	'-3,962745391'	'3,050642807'	'13,45261762'	'-1,497263892'	'3,083489585'
'0,295'	'0,295'	'2,8'	'2,8'	'-14,5881751'	'-2,603528618'	'3,518819139'	'14,33740562'	'-1,870682366'	'3,525744346'
'0,0883'	'0,0883'	'1'	'1'	'-12,86394769'	'-5,766465114'	'3,120227299'	'13,97601291'	'-4,209455419'	'3,276772679'
'0,2191'	'0,402'	'2'	'3,3'	''	''	''	''	''	''
'0,1424'	'0,1442'	'1,6'	'1,5'	'-17,17220026'	'2,691067249'	'6,865599728'	'14,59057189'	'4,206039042'	'5,34181054'
''	''	''	''	'-13,1'	'-4,9'	'1,5'	'12,7'	'-2,7'	'3,1'

If I try and use a loop:

clearvars
close all
filepath = ['C:\Hannah\ET\Statistik'];
T =readtable('ET2mat.csv');
G = findgroups(T(:,1))
for k = 1:44
    if T(:,1) == 1 
        x = T(:,:)
    else T(:,1) == 2
        y = T(:,:)
    end
end

I will get the following error message: Undefined operator '==' for input arguments of type 'table'.

So what do I need to do? Make it an array? I understand that maybe this will not work because the grouping variable is not a vector but part of the table.

Thank you

Stephen23 2020-4-27

在 MATLAB Online 中打开

test.txt

"I do in fact have negative values in my table. Is that the reason?"

The actual reason is your data file, which is imported as character, not as numeric. The reasons are:

The file is typical of regions which use a decimal comma, namely tab-separated values (and a misleading .CSV file extension). Whilst readtable can cope with the tab delimiter, it cannot parse decimal commas.
single quotes around all "numeric" values. I cannot image what badly written application does that.

Because of these, readtable imports that data (which you think is numeric) as character vectors in cell vectors, complete with single quotes. You can check this quite easily (because you did not upload a sample file I had to create it myself based on your earlier comment, attached, including column headers):

>> T = readtable('test.txt','delimiter','\t')
T = 
    AA        BB            CC          DD         EE              FF                  GG                 HH                 II                  JJ                 KK       
    __    __________    __________    _______    _______    ________________    ________________    _______________    _______________    ________________    _______________
    1     ''0,1188''    ''0,1103''    ''1,4''    ''1,3''    ''-13,00950292''    ''-1,000894239''    ''3,728322672''    ''12,81289888''    ''0,468820547''     ''1,169608552''
    1     ''0,1103''    ''0,2376''    ''1,3''    ''2,8''    ''-11,8''           ''-2''              ''3,6''            ''13,4''           ''-0,9''            ''2,9''        
    1     ''0,1313''    ''0,1717''    ''1,3''    ''1,7''    ''-13,28540783''    ''-3,043789654''    ''1,401630356''    ''13,32603837''    ''-2,987182197''    ''0,545827005''
    1     ''0,0971''    ''0,0883''    ''1,1''    ''1''      ''-15,71450602''    ''-3,962745391''    ''3,050642807''    ''13,45261762''    ''-1,497263892''    ''3,083489585''
    2     ''0,295''     ''0,295''     ''2,8''    ''2,8''    ''-14,5881751''     ''-2,603528618''    ''3,518819139''    ''14,33740562''    ''-1,870682366''    ''3,525744346''
    2     ''0,0883''    ''0,0883''    ''1''      ''1''      ''-12,86394769''    ''-5,766465114''    ''3,120227299''    ''13,97601291''    ''-4,209455419''    ''3,276772679''
    2     ''0,2191''    ''0,402''     ''2''      ''3,3''    ''''                ''''                ''''               ''''               ''''                ''''           
    2     ''0,1424''    ''0,1442''    ''1,6''    ''1,5''    ''-17,17220026''    ''2,691067249''     ''6,865599728''    ''14,59057189''    ''4,206039042''     ''5,34181054'' 
    2     ''''          ''''          ''''       ''''       ''-13,1''           ''-4,9''            ''1,5''            ''12,7''           ''-2,7''            ''3,1''        
>> cellfun(@class,T.BB,'uni',0)
ans = 
    'char'
    'char'
    'char'
    'char'
    'char'
    'char'
    'char'
    'char'
    'char'
>> +T.BB{1} % first and last characters are single-quotes.
ans =
    39    48    44    49    49    56    56    39

Essentially you have two choices:

write or edit the file so that all numeric data are written without single quotes and using decimal points, then efficiently import the whole file in one step using readtable, or
parse those character vectors inside of MATLAB, replacing the decimal commas with decimal points and then converting to numeric. Not particularly efficient, but it can work with your existing data files, e.g.:

T.KKnum = str2double(strrep(strrep(T.KK,'''',''),',','.'));

You can then apply numeric functions to that numeric data. I recommend that you use the variable names to refer to the data columns, rather than indexing.

Tommy 2020-4-28

编辑：Tommy 2020-4-28

在 MATLAB Online 中打开

Have you also gotten rid of the quotes in your text file?

This line:

[h, p ] = splitapply(@ttest,F,G)

would pass every column within F to ttest at once, as separate arguments. If you want to consider each column individually, you could use

for n = 2:col
    fprintf('This is column %d. \n' , n)
    [h, p ] = splitapply(@ttest,F{:,n},G)
end 

(although this does use indexing rather than variable names.)

Then, the last argument to splitapply must be G, so you cannot have

[h, p ] = splitapply(@ttest,F{:,n},G,'Alpha',0.05 )

because of the 'Alpha' and 0.05. splitapply thinks the 0.05 specifies the group numbers, which is not allowed because the group numbers need to be positive integers. If you want, you could use this syntax:

[h, p ] = splitapply(@(x,y) ttest(x,y,'Alpha',0.05),F{:,n},?,G)

or this syntax:

[h, p ] = splitapply(@(x,m) ttest(x,m,'Alpha',0.05),F{:,n},?,G)

both of which are explained in the documentation for ttest, but this would require you to pass a y or m to ttest, perhaps in place of the ?s above. However, the default alpha value is 0.05, so you shouldn't need to provide it anyway.

(edit) You can only choose and vote for answers, but so far everything here is a comment.