how to change a cell in a string to a string in a table

Hey everyone, I'm trying to use the 'unique' matlab function.

unique(table.row)

When I try this I get the error "Cell array input must be a cell array of character vectors.". I think this is because my table is in the form

table.row(1) = {'string'}

But it needs to be in the form

table.row(1) = 'string'.

I tried to fix this by using the for loop below

for n= 1:numel(table.row)
    table.row(n)=table.row{n};
end

but doing this I get the output error "Conversion to cell from char is not possible."

[Jon]

I understand that you are having a problem using the MATLAB unique function and you get the error message "Cell array input must be a cell array of character vectors" but I don't know what variable (argument) you are passing to the unique function. I see it has something to do with data that is in a table but it is not clear to me what is in your table or what you are passing to the unique function. Can you please be more specific. Best if you could please make a small, self contained example (very short script) that demonstrates the problem.

[tiwwexx]

Jon, I changed up the wording of the question so it has a more concrete example of the problem. Does this help clairify?

[Adam Danz]

% This reproduces your error
T = table(num2cell(string(['a':'z']')),'VariableNames',{'row'})

T = 26x1 table
      row  
    _______

    {["a"]}
    {["b"]}
    {["c"]}
    {["d"]}
    {["e"]}
    {["f"]}
    {["g"]}
    {["h"]}
    {["i"]}
    {["j"]}
    {["k"]}
    {["l"]}
    {["m"]}
    {["n"]}
    {["o"]}
    {["p"]}

unique(T.row) % ERROR!

Error using matlab.internal.math.uniqueCellstrHelper
Cell array input must be a cell array of character vectors.

Error in cell/unique (line 86)
[varargout{1:nlhs}] = matlab.internal.math.uniqueCellstrHelper(A,[false true false true false]);

I've updated my answer to include 3 ways to convert the data type of table variables.

Answer 1

[Adam Danz]

Three ways to convert data types within a table or timetable

1. Use convertvars (R2018b) to convert table or timetable variables to another datatype.

Example: Convert the first and last columns from a cellstring to string array.

T1 = readtable('outages.csv');
head(T1,3)

       Region           OutageTime        Loss     Customers     RestorationTime          Cause      
    _____________    ________________    ______    __________    ________________    ________________

    {'SouthWest'}    2002-02-01 12:18    458.98    1.8202e+06    2002-02-07 16:50    {'winter storm'}
    {'SouthEast'}    2003-01-23 00:49    530.14    2.1204e+05                 NaT    {'winter storm'}
    {'SouthEast'}    2003-02-07 21:15     289.4    1.4294e+05    2003-02-17 08:14    {'winter storm'}

T2 = convertvars(T1,["Region","Cause"],"string");
head(T2,3)

      Region          OutageTime        Loss     Customers     RestorationTime         Cause     
    ___________    ________________    ______    __________    ________________    ______________

    "SouthWest"    2002-02-01 12:18    458.98    1.8202e+06    2002-02-07 16:50    "winter storm"
    "SouthEast"    2003-01-23 00:49    530.14    2.1204e+05                 NaT    "winter storm"
    "SouthEast"    2003-02-07 21:15     289.4    1.4294e+05    2003-02-17 08:14    "winter storm"

2. Set the VariableTypes property of the table or timetable (R2024b)

Example: Convert the last names to strings and the Gender to categorical.

T1 = readtable("patients.xls");
head(T1,3)

      LastName        Gender      Age              Location               Height    Weight    Smoker    Systolic    Diastolic    SelfAssessedHealthStatus
    ____________    __________    ___    _____________________________    ______    ______    ______    ________    _________    ________________________

    {'Smith'   }    {'Male'  }    38     {'County General Hospital'  }      71       176      true        124          93             {'Excellent'}      
    {'Johnson' }    {'Male'  }    43     {'VA Hospital'              }      69       163      false       109          77             {'Fair'     }      
    {'Williams'}    {'Female'}    38     {'St. Mary's Medical Center'}      64       131      false       125          83             {'Good'     }      

T1.Properties.VariableTypes([1,2]) = ["string","categorical"];
head(T1,3)

     LastName     Gender    Age              Location               Height    Weight    Smoker    Systolic    Diastolic    SelfAssessedHealthStatus
    __________    ______    ___    _____________________________    ______    ______    ______    ________    _________    ________________________

    "Smith"       Male      38     {'County General Hospital'  }      71       176      true        124          93             {'Excellent'}      
    "Johnson"     Male      43     {'VA Hospital'              }      69       163      false       109          77             {'Fair'     }      
    "Williams"    Female    38     {'St. Mary's Medical Center'}      64       131      false       125          83             {'Good'     }      

Tip: To replace all "cell" types to "string", use:

T1.Properties.VariableTypes = strrep(T1.Properties.VariableTypes,'cell','string');

3. Reassign the table variable

Example: Convert the "Cause" variable from cellstring to categorical.

T1 = readtable('outages.csv');
head(T1,3)

       Region           OutageTime        Loss     Customers     RestorationTime          Cause      
    _____________    ________________    ______    __________    ________________    ________________

    {'SouthWest'}    2002-02-01 12:18    458.98    1.8202e+06    2002-02-07 16:50    {'winter storm'}
    {'SouthEast'}    2003-01-23 00:49    530.14    2.1204e+05                 NaT    {'winter storm'}
    {'SouthEast'}    2003-02-07 21:15     289.4    1.4294e+05    2003-02-17 08:14    {'winter storm'}

T1.Cause = categorical(T1.Cause); 
head(T1,3)

       Region           OutageTime        Loss     Customers     RestorationTime        Cause    
    _____________    ________________    ______    __________    ________________    ____________

    {'SouthWest'}    2002-02-01 12:18    458.98    1.8202e+06    2002-02-07 16:50    winter storm
    {'SouthEast'}    2003-01-23 00:49    530.14    2.1204e+05                 NaT    winter storm
    {'SouthEast'}    2003-02-07 21:15     289.4    1.4294e+05    2003-02-17 08:14    winter storm

Example, convert between character vectors or cellstrings and string arrays using convertStringsToChars or convertCharsToStrings (R2017b)

T1.Region = convertCharsToStrings(T1.Region); 
head(T1,3)

      Region          OutageTime        Loss     Customers     RestorationTime        Cause    
    ___________    ________________    ______    __________    ________________    ____________

    "SouthWest"    2002-02-01 12:18    458.98    1.8202e+06    2002-02-07 16:50    winter storm
    "SouthEast"    2003-01-23 00:49    530.14    2.1204e+05                 NaT    winter storm
    "SouthEast"    2003-02-07 21:15     289.4    1.4294e+05    2003-02-17 08:14    winter storm

[tiwwexx]

Perfect! Just needed the convertStringsToChars / convertCharstoStrings. Thanks for the help.

[Adam Danz]

Nice! Thanks for the feedback!