How do I convert a cell array of strings representing roman numbers to double?

Question

I am trying to convert a cell array of strings into double so that I can use it.

hourNumbers = {'I','II','III','IV','V','VI','VII','VIII','IX','X','XI','XII'}

finHourNumbs = cell(1,length(hourNumbers))
for i=1:length(hourNumbers)
finHourNumbs {i} = str2double(hourNumbers{i});
end

I get the below output which I am NOT expecting

finHourNumbs = 

Columns 1 through 9

[NaN]    [NaN]    [NaN]    [NaN]    [NaN]    [NaN]    [NaN]    [NaN]    [NaN]

Columns 10 through 12

[NaN]    [NaN]    [NaN]

I am expecting it to contain the strings so that I can display.

Thanks in Advance

Answer 1

You can use containers.Map, and create your own roman2double like this

hourNumbers = {'I','II','III','IV','V','VI','VII','VIII','IX','X','XI','XII'};

roman2double = containers.Map(hourNumbers, 1:12);

finHourNumbs = cell(1,length(hourNumbers));
for i=1:length(hourNumbers)
    finHourNumbs {i} = roman2double(hourNumbers{i});
end

or in a shorter form

hourNumbers = {'I','II','III','IV','V','VI','VII','VIII','IX','X','XI','XII'};
roman2double = containers.Map(hourNumbers, 1:12);
finHourNumbs = values(roman2double, hourNumbers);

Answer 2

Elaborating on Gunther's solution, you're dealing with a small set of roman numbers that you need to convert to a decimal representation, so you can use a simple conversion. Here's a fast native solution with ismember:

romans = {'I','II','III','IV','V','VI','VII','VIII','IX','X','XI','XII'};
hourNumbers = {'I', 'III', 'XII'}; %// Just a sample input data
[tf, finHourNumbs] = ismember(hourNumbers, romans);

The second output of ismember is what you need, so just assign it to finHourNumbs and presto!

Answer 3

I'd be very surprised if any srt2dbl-type conversion code recognised Roman numerals. I think you'll probably want to convert them into decimal numbers first.

The str2double doc page states the requirements:

The string may contain digits, a comma (thousands separator), a decimal point, a leading + or - sign, an e preceeding a power of 10 scale factor, and an i for a complex unit.

If str does not represent a valid scalar value, str2double returns NaN.

No mention whatsoever of Roman numerals in there.

Answer 4

A simple google search returns plenty of ways of converting a roman numeral to arabic, of which this one seems to be the simplest.

This is just for one string to convert, if you have an array of strings (in a cell array), just use a loop or cellfun:

finHourNumbs = cellfun(@roman2arabic, hourNumbers);

OR as @EitanT points out, since you're dealing with (a small set of) hour numbers, you can also use a simple conversion array:

numsRoman = {'I','II','III','IV','V','VI','VII','VIII','IX','X','XI','XII'};
nums=1:numel(numsRoman);

and then:

finHourNumbs = cellfun(@(str) nums(strcmpi(str,numsRoman)), hourNumbers);