Change table variable data type

I used readtable() to read a text file into a table. One of the columns of the table was read (by default) as a double rather than a string. How does one change a columns datatype within a table?

Answer 1

[Sean de Wolski]

Change it before reading it in by modifying the options passed to readtable.

https://www.mathworks.com/help/releases/R2017a/matlab/ref/detectimportoptions.html

Or

t.Var1 = num2str(t.Var1)

[Nikhil Tawakley]

The approach to modifying the read options was what I tried first but unfortunately detectimportoptions doesnt seem to work with 2016a...

num2str worked though so the issue is resolved...thanks!

[Peter Perkins]

Without meaning to contradict Sean, num2str is almost certainly not what you want, at least not by itself. It is a very old function that creates a char matrix, which will likely be pretty awkward to work with. One possibility is to apply cellstr to that char matrix, another (if you're using a recent version of MATLAB) would be to convert using the string function (as in, string, the new class).

Answer 2

[Jeffrey Daniels]

fileName = table.xlsx;

opts = detectImportOptions(fileName);

opts.VariableTypes{15} = 'char'; % This will change column 15 from 'double' or whatever it is to 'char', which is what you want.

new_table = readtable(fileName,opts);

[yunyu Hu]

This is a simple and clean solution.

Answer 3

[Peter Perkins]

readtable also accepts a Format parameter that would allow you to override the automatic type detection.

[Nikhil Tawakley]

Thanks Peter. Is there a way to copy formats from one table to another? I want to take all the datatypes/formatting of one table and mimic it in another.

I tried

table2.Variable_1_Name.Formats=table1.Variable_1_Name.Formats

but that returned an error: "Field assignment to a non-structure array object."

Another approach could be to modify table2 when importing: (https://www.mathworks.com/help/matlab/import_export/import-mixed-text-and-numeric-data-from-a-text-file.html)

However, I don't know how to get the formatSpec argument out of table1 and don't want to type it out manually.

-Nikhil

[Peter Perkins]

I don't know what this means. Tables don't have formats.

[Nikhil Tawakley]

I'm referring to the data types of the variables in a table. For example, a Table with variables: Name and Age with datatypes String and Integer respectively.

Is it possible to "copy" the datatypes from one table and then use them while creating another table? Continuing from the example above, if there's another table with variables: Item Name & Quantity, is it possible to "copy-paste" the datatypes String and Integer from the previous table?

[Walter Roberson]

No, it is not possible to copy-paste datatypes. However, you could use

old_class = arrayfun(@class, OldTable.VariableName, 'Uniform', 0);
new_variable = arrayfun(@(value, newclass) cast(value, newclass), NewTable.VariableName, old_class, 'uniform', 0);
NewTable.VariableName = new_variable;

This assumes that each entry in the variable is a potentially different type -- which would not be the case for numeric entries for example. If you have numeric entries, then

NewTable.VariableName = cast( NewTable.VariableName, class(OldTable.VariableName) );