Error using exist "The optional second input to exist must be 'var', 'builtin','class', 'dir' or 'file' "

Question

Angelo Figueiredo 2019-2-19

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/445808-error-using-exist-the-optional-second-input-to-exist-must-be-var-builtin-class-dir-or-fi

编辑： Adam Danz 2019-2-19

I don't understand why the following code doesn't work

for k = 1:4
% Create a text file name, and read the file.
	textFileName = ['A' num2str(k) '.txt'];
	if exist(textFileName, 'A')
		fid = fopen(textFileName, 'rt');
		mydata = fread(fid);
		fclose(fid);
	else
		fprintf('File %s does not exist.\n', textFileName);
	end
end

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Steven Lord 2019-2-19

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What do you think this does or want it to do?

if exist(textFileName, 'A')

You expect this if condition to be satisfied if ... ?

Once we know that we may be able to help you write the correct command.

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Answer 1

Adam Danz 2019-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/445808-error-using-exist-the-optional-second-input-to-exist-must-be-var-builtin-class-dir-or-fi#answer_361743

编辑：Adam Danz 2019-2-19

在 MATLAB Online 中打开

It looks like you're checking the existance of a certain file named something like "A1.txt" on the matlab path prior to reading that file. The exist() function has 1 mandatory input and 1 optional input. The optional input allows you to specify the type of result to search for. If you're searching for a file, one of those option for the 2nd input is 'file'.

The correct way to search for the existance of that file on the matlab path is:

if exist(textFileName, 'file')

More info: https://www.mathworks.com/help/matlab/ref/exist.html#f5-453632-searchType