Matlab报错求助：Undefined function 'myfun' for input arguments of type 'double'

Hey there, I feel your pain—those "undefined function" errors can be so frustrating when you swear you’ve got everything in place. Let’s walk through the most common fixes for this issue, step by step:

• Double-check your function file’s name and syntax
  Matlab is case-sensitive, so make sure your file is exactly named myfun.m (no capital letters, no extra characters). Open the file and verify the first line matches the required format for fsolve:

  function F = myfun(x)
      % Replace this with your actual system of equations
      F = [x(1)^2 + x(2)^2 - 4; x(1) - x(2) - 1];
  end
  

  Ensure you have a closing end statement, the output F is a column vector, and there are no typos in variable names or operators.

• Confirm Matlab’s working directory matches your file location
  Even if you saved myfun.m alongside your other scripts, Matlab might not be looking in that folder. Run pwd in the command window to see your current working directory, then dir to check if myfun.m shows up. If it doesn’t:

  • Use cd /path/to/your/folder to switch to the correct directory, or
  • Right-click the folder in Matlab’s file browser and select Add to Path → Selected Folders and Subfolders

• Make sure your function isn’t being "hidden"
  Sometimes a variable or another script with the same name can override your function. Run which myfun in the command window:

  • If the output points to a variable (not a .m file), run clear myfun to delete the variable and try again.
  • If it points to a different .m file (unlikely for myfun, but possible), rename your function to something unique like my_nonlinear_system.m and update your fsolve call accordingly.

• Test your function directly before using fsolve
  Skip fsolve for a minute and call your function manually with a test input, e.g., myfun([1; 1]). If this throws an error, the problem is in your function’s code—not in fsolve. Fix those syntax or logic issues first, then go back to fsolve.

• Verify your fsolve call syntax
  Ensure you’re passing the function handle correctly with the @ symbol. A valid call should look like this:

  x0 = [0; 0]; % Initial guess for your variables
  x_solution = fsolve(@myfun, x0);
  

  Avoid using string inputs like fsolve('myfun', x0)—while old Matlab versions supported this, function handles are more reliable and less prone to path-related issues.

These steps should cover 99% of cases where this error pops up. Chances are it’s a tiny oversight (like a capital letter in the filename or a missing end) that’s easy to miss after 3 hours of troubleshooting!

内容的提问来源于stack exchange，提问作者Parseval