Quad error, simple function,

Question

adfnm 2014-8-21

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评论： Roger Stafford 2014-8-21

I am a beginner, so please bear with me

cpn is referenced as such as this is part of a much larger code

clc

clear all

close all

cpn=[29105 8614.9 1701.6 103.47 909.79];

fun=@ (ll) cpn(1)+cpn(2)*((cpn(3)./ll)/sinh(cpn(3)./ll))^2+cpn(4)*((cpn(5)./ll)/cosh(cpn(5)./ll))^2

fun(100)

fun(200)

Q=quad(fun,18,200)

I am getting this error:

Error using quad (line 71) The integrand function must return an output vector of the same length as the input vector.

Error in Inttry (line 11) Q=quad(fun,18,200)

Thanks

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Answer 1

Roger Stafford 2014-8-21

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Your function definition is lacking operators in at least four places, one after "cpn(2)", one after "cpn(4)", and one before each of the two 2's. As such it does not constitute a valid matlab function definition.

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Roger Stafford 2014-8-21

That looks like something that would work properly. Does it work, and is it the function you intended as the integrand?

adfnm 2014-8-21

Yes, that is the function. fun(100) returns a scalar, thats why I am baffled to the cause of this error

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Answer 2

Iain 2014-8-21

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I guess that you've missed off declaring a few operations as elementwise operations, and it's getting out a square matrix instead of a vector.

Each divide, multiply and power should have a "." just before them.

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adfnm 2014-8-21

fun(100) returns a scalar, thats why I am baffled to the cause of this error, It is not the function that is the problem!

Roger Stafford 2014-8-21

在 MATLAB Online 中打开

The documentation of 'quad' at

http://www.mathworks.com/help/matlab/ref/quad.html

gives you the answer: "The function y = fun(x) should accept a vector argument x and return a vector result y, the integrand evaluated at each element of x."

This means that 'quad' doesn't send out just scalar quantities to be evaluated. It asks for entire vectors in x to be evaluated to the corresponding vectors y. That means that your expression which works for scalars but is inappropriate for vectors, will not succeed in 'quad'. You must put in those dots as Iain has recommended if you are to avoid the error messages and obtain a correct result.

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