convert categorical to numeric

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Fischer Zheng
Fischer Zheng on 19 Jan 2016
Commented: the cyclist on 25 Jul 2022
I have a categorical array and I want to convert it back to the numerical matrix. What is the syntax?
Walter Roberson
Walter Roberson on 24 Jul 2022
double() returns category index
the cyclist
the cyclist on 25 Jul 2022
@AMEN BARGEES, if you look at other solutions, besides the accepted one, you will see that others suggested double(), followed by comments about how it did not actually solve the problem that was posed.

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Accepted Answer

the cyclist
the cyclist on 19 Jan 2016
Edited: the cyclist on 19 Jan 2016
If you have the Statistics and Machine Learning Toolbox, you could use the grp2idx command:
c = categorical({'Male','Female','Female','Male','Female'})
n = grp2idx(c)
That will simply encode the categories as numerical variables (which is handy for some other software packages). But that does not really change the fact that "1", "2" etc are still really just categories.
If you have categories that somehow embed numbers inside of them, that you want to convert to truly numerical (e.g. ordinal or interval) data, you'll need to be more specific about what your input is.
Ayachitula Radha Krishna
Ayachitula Radha Krishna on 23 Aug 2017
I have an array of 3500 strings(some are repeated values) in my work space. I'm not able to access them while building a machine learning model. Is there any way that I can convert them into numerical value and then use it in the model or else can you suggest me any other better approach.
the cyclist
the cyclist on 23 Aug 2017
I suggest you open a new question. You will get the attention of more people with a new, unanswered question rather than a comment on an answered question.
In that new question, I suggest that you include a small example of your data, or upload the entire array in a MAT file. You have not given enough information here to help you.

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More Answers (5)

Matthew Parkan
Matthew Parkan on 19 Mar 2018
Juste use the unique() function (which does not require any toolbox).
For example:
c = categorical({'Red','Blue','Red','Red','Blue','Blue','Green'});
[GN, ~, G] = unique(c)
Will return:
GN =
1×3 categorical array
Blue Green Red
G =
  1 Comment
the cyclist
the cyclist on 19 Mar 2018
My comment on Xingyu Li's answer applies here as well. It works well if arbitrary numeric values are OK as output, but will not convert categorical '12' to numeric 12.

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Peter Perkins
Peter Perkins on 23 Mar 2018

Calling categorical is a data conversion, so

   c = categorical([12 12 13])

completely throws away the numeric values. In general, there is no way to get them back unless you have saved them, any more than you can get back the original values from int8([1.1 2.2 3.3]). Calling categorical is a data conversion.

That being said, you can certainly save the unique numeric values, and then index into those using the categorical array:

   n = uniqueNumericValues(c)

You can also call double on a categorical, but what you will get back are the category numbers, not the original numeric values.

But here's the question: if you need to convert back to the original numbers, and you are not using meaningful category names when converting from those numbers, why use categorical to begin with? There may be things you haven't mentioned.

Ian Blake
Ian Blake on 10 Jun 2019
Edited: Ian Blake on 10 Jun 2019
I've come to the conclusion that would have been easiest, although I've developed an effective though crude workaround.
vfdbdata.DD01km is my categorical data array (from a table of data)
odocats = categories (vfdbdata.DD01km);
odoval = zeros (1, length (odocats) ); % preallocate space
for kk=1:length(odocats),
So this is run before the main processing, the numeric data can then be extracted as required by
odotemp = double ( vfdbdata.DD01km(vidx) ) ;
odotemp = odoval (max (1, odotemp) ) ;
The max ensures that 'undefined' values are processed without throwing an error (they give a NaN after translation to double, which causes a subscript error), I also have some code to process specific values that can occur (hence the use of a temporary variable).
Matthew Anderson
Matthew Anderson on 13 Apr 2020
a = categorical(["2" "3" "3"])
double(a) % returns [1 2 2] - maybe desired for some reason
double(string(a)) % returns [2 3 3] - maybe desired for some reason
categorical(double(string(a)) % returns the same thing as a

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Milan Andrejevic
Milan Andrejevic on 29 Apr 2018
It's an intuitive functionality that should exist. There are so many instances one needs to treat certain variables as categorical when using some modelling functions, and as continuous for other analyses, or simply be able to index the array comparing it to a number. This is so easy to do in other programming languages.

Xingyu Li
Xingyu Li on 15 Dec 2017
  1 Comment
the cyclist
the cyclist on 15 Dec 2017
Edited: the cyclist on 19 Mar 2018
This is a great solution for the use case of assigning arbitrary numeric values to general categorical variables, e.g.
c = categorical({'Male','Female','Female','Male','Female'})
But this will not solve this poster's particular use case of
c = categorical([12 12 13]);
and wanting numeric [12 12 13] as the output.

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nathan blanc
nathan blanc on 16 Jan 2021
I converted the categorical data into a char and then used str2num. worked for me :)
  1 Comment
Walter Roberson
Walter Roberson on 16 Jan 2021
In most cases it is better to use str2double() rather than str2num(). str2num() invokes the full power of eval(), which can lead to problems.

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