CODEWITHSUNDEEP
listhaskelllist-comprehension
kaishggy
kaishggy

Reputation: 13

Haskell list comprehension returns empty?

So I'm new to learning Haskell (started on Saturday) and I read a few chapters from online books and I wrote a list comprehension to find the diameter of a circle given its circumference.

ghci> let circle = [(a) | a <- [1..10], 9 / pi == a] but it returns [] I was told this is because [1..10] only counts whole integers and not all the real numbers between 1 and 10.

I tried ghci> let circle = [(a) | a <- [1..10], round (pi * a) == 9] which returns [3.0] but I was wondering if there was a way to get a more precise answer.

Thanks in advance.

Upvotes: 1

Views: 283

Answers (5)

Cirdec
Cirdec

Reputation: 24166

Why don't you just find the diameter of a circle with a given circumference?

diameterOfCircleWithCircumference :: Floating a => a -> a
diameterOfCircleWithCircumference = (/ pi)

Then diameterOfCircleWithCircumference 9 is 2.8947...

Upvotes: 1

Christian Ternus
Christian Ternus

Reputation: 8492

Welcome to Haskell and StackOverflow!

let circle = [(a) | a <- [1..10], 9 / pi == a]

Considering all the real numbers between 1 and 10 is an uncountably infinite set (hat tip to Georg Cantor for that one) I'd be surprised, to say the least, if you managed to make a list comprehension over them. :)

Your reasoning about why this list is empty is correct -- 9/pi will never equal a whole integer.

Other than that, there's no reason to use a list comprehension for this. You can do the same thing by simply dividing by pi:

let diam c = c / pi

Upvotes: 6

Ingo
Ingo

Reputation: 36339

How about:

let circle = [ 9 / pi ]

Upvotes: 1

jwodder
jwodder

Reputation: 57630

This is not what list comprehensions are used for. To get a precise answer to 9 / pi, just calculate 9 / pi directly.

ghci> let diam = 9/pi

Upvotes: 3

DJG
DJG

Reputation: 6563

diameter c = c / pi

or as you get more advanced in Haskell:

diameter = (/ pi)

And then you can call:

Prelude> diameter 9
2.864788975654116

A list comprehension may be useful if you wanted the diameters of the circles with circumferences from 1 to 10, like so:

Prelude> [diameter x | x <- [1..10]]
[0.3183098861837907,0.6366197723675814,0.954929658551372,1.2732395447351628,1.5915494309189535,1.909859317102744,2.228169203286535,2.5464790894703255,2.864788975654116,3.183098861837907]

Upvotes: 1

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