Existing table, populate a column with string (receiving errors)

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newbie9 2019-11-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/491607-existing-table-populate-a-column-with-string-receiving-errors

评论： Cris LaPierre 2022-9-19

I have a table, and I want to populate every row in a specific column with the same string. I keep getting errors. Using Matlab R2018a, no fancy packages.

mytable = nan(5,2); 
% I'm creating my table like this; it's actually much bigger, and most of the columns are numerical
% is there a better way to do this? the order of the column matters, and I don't want to have to add extra code to re-order
mytable = array2table(mytable);
mytable.Properties.VariableNames = {'colA', 'colB'};
mytable.colA(:) = 459; % just a random number, proof-of-concept that it works
mytable.colB(:) = {'words'}; % this throws errors

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Thiago de Aquino Costa Sousa 2022-9-18

移动：the cyclist 2022-9-18

在 MATLAB Online 中打开

I have something similar, I have a master table with more then 270k lines and 85 columns. I added to my master table one column with subject Id of my participants, wich I get from the name of my first level of subfolders, but I am not being able to populate every line with the subject Id, only the first line is being populated, can you help on that??? @Cris LaPierre or anyoner expert???

clear
clc
mytopfolder = '/Users/Participants data';
filepattern = fullfile(mytopfolder, '**/VIVE/Walking_ST/Trial_*.txt');  
thefiles = dir(filepattern);
nfiles = length(thefiles);
for k = 1:nfiles
    basefilename = thefiles(k).name;
    fullfilename = fullfile(thefiles(k).folder, basefilename);
    fprintf(1, 'Now reading %s\n', fullfilename);
    thisTable = readtable(fullfilename);
    thisTable.Subject_Id(k,:) = fullfilename(98:102);
    fprintf('    It has %d rows in it.\n', height(thisTable));
    if k == 1
        masterTable = thisTable;
    else
        masterTable = [masterTable; thisTable];
    end
    fprintf('    Now there are a total of %d rows in the master table.\n', height(masterTable));
end

Cris LaPierre 2022-9-19

This is probably best asked as a new question, as more people will see it. Quick observation, though, is that you are assigning the Subject_ID to the kth row, all columns. Perhaps you meant to assign to all rows?

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Answer 1

Cris LaPierre 2019-11-18

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在 MATLAB Online 中打开

You are close, but you are trying to assign a character vector to a double variable (NaN = not a number). You can see that if you run the command summary(mytable).

You also do not need to put the text into a cell if you use the string data type.

See if you can get this code to work for your needs.

myArray = nan(5,2)
mytable = array2table(myArray);
mytable.Properties.VariableNames = {'colA', 'colB'};
mytable2 = convertvars(mytable,"colB",'string');
mytable2.colA(:) = 459;
mytable2.colB(:) = "words"

You say you have a lot of data. If the variables already exist, you could do something like this:

colA = nan(5,1);
colB = strings(5,1);
mytable = table(colA,colB);
mytable.colA(:) = 459; % just a random number, proof-of-concept that it works
mytable.colB(:) = "words" % this throws errors