ode45 function error using exist

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Kyriacos Patsalides
评论： Walter Roberson 2020-4-13
I am running my code and when i call ode45 I get an error as followed:
Error using exist The first input to exist must be a string.
Error in odearguments (line 60) if (exist(ode)==2)
Error in ode45 (line 114) [neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...
What does it mean that the first input to exist must be a string?
  1 个评论
Jan
Jan 2017-1-25
Please post the complete error message and the command line, which calls ODE45. Thanks.

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回答（3 个）

Torsten
Torsten 2017-1-25
My guess is that the variable "ode" is not a string.
Best wishes
Torsten.
Jan
Jan 2017-1-25
How do you call the integrator? What is the first input? It should be a function handle, but for backward compatibility functions can be provided as strings also.
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Walter Roberson
Walter Roberson 2017-1-25
I speculate that the poster might be using a very old version of MATLAB, such as MATLAB 5, where ode45 required a string instead of a handle.

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Wan Nabilah
Wan Nabilah 2020-4-13
编辑：Walter Roberson 2020-4-13
I am running my code and i get an error at if ~exist(fullFileName, 'file'). The error is
Error using exist
The first input to exist must be a string
scalar or character vector.
image_folder = 'C:\Users\User\Desktop\Image Pollen';
fullFileName = dir(fullfile(image_folder, '*.jpg'));
%filenames = dir(fullfile(image_folder, '*.jpg'));
% Get the full filename, with path prepended.
total_images = numel(fullFileName);
if ~exist(fullFileName, 'file')
% Didn't find it there. Check the search path for it.
fullFileName = image_folder; % No path this time.
if ~exist(fullFileName, 'file')
% Still didn't find it. Alert user.
errorMessage = sprintf('Error: %s does not exist.', fullFileName);
uiwait(warndlg(errorMessage));
return;
end
Thank you!
  2 个评论
Steven Lord
Steven Lord 2020-4-13
When you call dir with an output argument, the contents of that output argument are not the names of the file or files that dir found. It is a struct array with one element per file found and a number of fields. exist doesn't expect its input to be a struct array.
If you want to check how many files dir found, ask for numel (NUMber of ELements) of that struct. If you want to check if dir found no files, you could instead ask if that output isempty.
Walter Roberson
Walter Roberson 2020-4-13
dinfo = dir(fullfile(image_folder, '*.jpg'));
fullFileName = fullfile(image_folder, {dinfo.name});

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