A flag to indicate whether or not the int function has successfully computed a symbolic integral

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Leo Simon 2016-10-7

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https://ww2.mathworks.cn/matlabcentral/answers/306296-a-flag-to-indicate-whether-or-not-the-int-function-has-successfully-computed-a-symbolic-integral

评论： John D'Errico 2016-10-7

The example below indicates my problem. In the first case, matlab cannot integrate the function provided to it and simply returns the computation I asked it to evaluate. In the second case, matlab is successful and returns a symbolic expression. The output argument of both `int` computations is of class `sym`, but, obviously, one is useful while the other is not. I could, of course, distinguish between them by searching for the string `int` in `char(Ans)` but that's a pretty inelegant thing to do.

Thanks!

 f = @(x)((x+10)*exp(-x)/(sqrt(x*x+20*x)));
  int(f,x,0,1);
  Ans= int((exp(-x)*(x + 10))/(x^2 + 20*x)^(1/2), x, 0, 1) 
  class(Ans) 
  syms b
  Ans = int(x^2,x,0,b)
  class(Ans)

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Answer 1

John D'Errico 2016-10-7

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It appears your only alternative is to inspect the result, as int returns only one argument, with no flag indicating "success" at all. If that result contains the sub-string 'int', then you have not been successful.

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Leo Simon 2016-10-7

That's what I've been doing, but it's clearly not bulletproof, since the variable itself could contain the substring int.

John D'Errico 2016-10-7

You could search for 'int('. That will resolve some problems. Of course, anything that starts with the string 'int(' as the first 4 characters will be clear, but that will miss many cases.

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