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convert categorical to numeric

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I have a categorical array and I want to convert it back to the numerical matrix. What is the syntax?
Thanks,

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.
  7 Comments
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)

Xingyu Li
Xingyu Li on 15 Dec 2017
double(categorical)
  1 Comment
the cyclist
the cyclist on 15 Dec 2017
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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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 =
3
1
3
3
1
1
2
  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.

  4 Comments
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.

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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