Using quad or quad2d to evalute a 1-D integral of 2-D function

Question

David C 2011-11-16

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Hi,

I'm trying to evaluate a 1-D integral (single integral, not double integral). The integrand happens to be a function that takes 2 arguments, call it f(x,y). I am fixing one of the arguments as a constant. So I would like to:

Integrate f(x,2) from x=1 to x=10. Is there a way to do this using quad or quad2d? If not, what would you suggest I use to evaluate this integral?

Thanks, David

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Answer 1

Mike Hosea 2011-11-16

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You need to "bind" one of the arguments to whatever value you choose. This is done with an "anonymous" function.

fx = @(x)f(x,2)

is a function that takes one input argument, x, and returns f(x,2). I have called it fx here, but of course you could call it any valid variable name. So, fx(pi) = f(pi,2). We don't have to name this function. We can just pass it to the integrator, so the answer to your question is

quadgk(@(x)f(x,2),1,10)

Of course this works with quad as well, but why do people use quad anymore when they have quadgk?