why does str2num create a figure?

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Pepijn 2012-2-16

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编辑： Jan 2013-10-14

I am using the property that str2num(string) returns an empty when the string is not solely made of numbers. In my case, I want to ignore the lines of my file that are not numers only so if ~isempty(str2num(fgetl(fid))) do something. However, a window popped up. It turns out "lines 42" - an entry in my file - caused this. Upon looking into it, I found

str2num('lines 1') or variations pop up a figure window; str2num('line') actually draws a line from (0,0) to (1,1). Also, str2num('lines') returns a matrix of values.

Why? And moreover, is there a workaround or a function I can use instead?

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Jan 2012-2-16

Thanks for pointing to this old problem.

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Answer 1

Jan 2012-2-16

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STR2NUM uses eval to get the numerical contents of a string. If the string contains a Matlab command like line, it is evaluated with the obeserved side-effects. This is explained in the documentation, see help str2num. There you find the recommendation to use str2double to avoid this. In addition you can use sscanf also, but this accepts lines like "3.14 dummy" also by ignoring everything behind the valid number.

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Andreas Goser 2012-2-16

The published MathWorks solution can be found here: http://www.mathworks.de/support/solutions/en/data/1-2ED43R/index.html

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Answer 2

Pepijn 2012-2-16

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Thanks Jan!

Str2double doens't really do the job either since it isn't converting multiple entries, str2double('1 2') gives NaN. So I'll stick with the workaround below.

Workaround: using an "if sum(isletter(string))==0, do something", first.