[Haskell-cafe] Re: Debugging Newton's method for square roots

Vraj Mohan writes:

I am new to Haskell and need help in debugging my code.

I wrote the following function for calculating square roots using Newton's method:

my_sqrt :: Float -> Float my_sqrt x = improve 1 x where improve y x = if abs (y * y - x) < epsilon then y else improve ((y + (x/y))/ 2) x epsilon = 0.00001

This works for several examples that I tried out but goes into an infinite loop for my_sqrt 96.

Generally it's better to separate out the different parts of the algorithm. So sqrt_step x candidate = (candidate + x/candidate)/2 Now you can try take 20 $ iterate (sqrt_step 2) 1 and watch the convergence. When you're happy with that, you can use something like “head . dropWhile unconverged”.

(The equivalent code is well-behaved on MIT Scheme)

Is it? Is there equivalent code to “my_sqrt :: Float -> Float”? (that might be pertinent). -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2006-09-13)