Asked by Chong
on 22 Aug 2014

I've been puzzling over this last problem in my assignments for the past week or so, and still with no end in sight. Some of the folks here on MathWorks have given me a few pointers, but I still can't make this code work.

function r=myderivative(f,a,tol) y = 0; maxIters = 500; n = 1; while n<=maxIters h = 1/n; while (1/n+1)-h<tol; y = (f(a+h)-f(a))/h; end n = n + 1; y = (f(a+h)-f(a))/h; end r=y; end

The problem in question instructed me to make an m-file mimicking a derivative function, which would apply (f(a+h)−f(a))/h to f(a) again and again (with h turning into 1, then 1/2, then 1/3...) until the successive values dipped under the "tol" value.

However, this current code skips over pretty much all the stuff after "y=0" and spits out 0 as the value for every set of (f,a,tol).

Could anyone spare a bit of help? I would greatly appreciate it.

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Answer by Star Strider
on 22 Aug 2014

You need to track the successive values for the derivative `y` and compare them with the current estimate. The derivative calculation has to go *before* the second `while` block, and the second `while` block has to compare the previous value of the *derivative* with the current value. As long as that difference is >=tol, replace the previous value of the derivative with the current value and continue.

Show 13 older comments

Star Strider
on 23 Aug 2014

`sin(pi/4)` was supposed to be `0.57732...`, it was a trick question, and no one could have gotten it.

Chong
on 23 Aug 2014

Alright, after many hours, much brainstorming, and the sacrifice of a hobo, I finally got the code to work it the way I want. Preserved here for posterity:

function r = myderivative(f,a,tol) h = 1; oldapprox = (f(a+h)-f(a))/h; h = h*(1/h)*(1/(1/h+1)); approx = (f(a+h)-f(a))/h; while (abs(approx-oldapprox)>tol) oldapprox = approx; h=h*(1/h)*(1/(1/h+1)); approx = (f(a+h)-f(a))/h; end r = approx; end

(The real trick is figuring out how to get from 1/n to 1/(n+1) without introducing another bloody variable.)

Star Strider
on 23 Aug 2014

My original code (from a week or so ago):

tol = 1E-12; dfdx = @(f,a,h) (f(a+h)-f(a))./h; dd = Inf; dprv = 0; k1 = 1; while dd > tol h = 1/k1; df = dfdx(f,a,h); dd = df-dprv; dprv = df; k1 = k1+1; end df % Result

produces:

df =

707.1039e-003

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## 1 Comment

## dpb (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/151959#comment_233152

Well, work thru the results of what you've written...for example, look at the following that displays the condition you've calculated for the comparison to

tolin the second loop.I just took out the computations and looked at the looping construct. You don't give a value for

tolbut note that your comparison value never changes as you've got it defined.