Body Mass Index program in haskell

I'm trying to write a simple program in Haskell that can determine someone's body mass index.

Here's what I have written:

type Height = Float
type Weight = Float
type PeopleStats = [(String, Height, Weight)]

and...

bmi :: Height -> Weight -> Float
bmi heightCm weightKg = weightKg/(heightCm)^2

healthy :: Height -> Weight -> Bool
healthy heightCm weightKg | 25 > index && 18 < 开发者_高级运维index = True
                          | otherwise                = False
  where index = bmi heightCm weightKg

So far, the function "healthy" can calculate someone's BMI, and the function "healthyPeople" returns a boolean statement determining if the person's BMI falls within the limits which is considered normal for a healthy person.

I want to write a function called "healthyPeople".

healthyPeople :: PeopleStats -> [String]

This function needs to take a list of PeopleStats and returns a list of names (Strings) of people who are deemed to be "healthy" from the "healthy" function.

For example:

If I input [("Lee", 65, 185), ("Wang", 170, 100), ("Tsu", 160, 120)] I will get a list of the names of the people whose BMI returns true form the boolean function in "healthy".

Please help !!!!

---

First, I think you probably meant to define bmi as:

bmi :: Height -> Weight -> Float
bmi heightCm weightKg = weightKg/(heightCm/100)^2

since the formula uses height in meters.

Now, here's a step by step way to do it using helper functions. I defined a type:

type PersonStats = (String, Height, Weight)

and some functions on that type:

healthyPerson :: PersonStats -> Bool
healthyPerson (name, h, w) = healthy h w

getName :: PersonStats -> String
getName (name, h, w) = name

With those in place, the final function becomes trivial:

healthyPeople :: PeopleStats -> [String]
healthyPeople people = map getName $ filter healthyPerson people

or in point-free notation:

healthyPeople :: PeopleStats -> [String]
healthyPeople = map getName . filter healthyPerson

First you filter out the healthy people from the list, then you map the list of stats into a list of names.
You can express the entire function in one go without the helpers if you use lambdas.

---

There's a standard Haskell function named filter that does exactly (well, almost) what you want here. It has type (a -> Bool) -> [a] -> [a], i.e., it takes a predicate and a list and returns the members that satisfy the predicate.

You can't apply it directly to PeopleStats because the types don't match up, but it's not hard to write a function to connect the two:

healthyPerson :: (String, Height, Weight) -> Bool
healthyPerson (_, h, w) = healthy h w

healthyPeople :: [(String, Height, Weight)] -> [String]
healthyPeople people = map name $ filter healthyPerson people
  where name (s, _, _) = s

This does what you want.

---

Let's think about what you want to do. You have a list, and you want to (a) select only certain items from the list, and (b) do something to each element of the list. This being Haskell, let's express this in types. The first thing you need—well, it'll have to take a list [a], and a way to check if each element is good. How can it check? Well, it should be a function a -> Bool. And it should give us back a smaller list. In other words, something like [a] -> (a -> Bool) -> [a]. Then we want to take our list and do something to each element. In other words, we'll need a list [a], and a function a -> b. Thus, we'll want something of the type [a] -> (a -> b) -> [b]. Now that we have the types, we're golden: we can use Hoogle to search for them. I highly, highly recommend using Hoogle regularly; it's a Haskell search engine which searches both types—the uniquely awesome part—and function/datatype/typeclass/module/package names. The first function, as it turns out, is the second result for the query: filter :: (a -> Bool) -> [a] -> [a]. This takes a function and a list and returns only the elements of the list for which the function is true. The second function is the first result, map :: (a -> b) -> [a] -> [b], which calls the given function on every element of the given list and returns a list of the results. Note that the arguments have the function, not the list, first; this is more natural, as you'll see shortly.

We want to put these two together for healthyPeople:

healthyPeople :: PeopleStats -> [String]
healthyPeople sts = map (\(n,_,_) -> n) $ filter (\(_,h,w) -> healthy h w) sts

This does what you want. $ is function application, but effectively groups the right-hand side because of its precedence; this allows us to elide parentheses. Here we see why it's nice to have map take its function first; we pass it the name-extracting function ((n,_,_) is a pattern which will match a triple and assign n its first element, ignoring the other two), and then (via $) the filtered list.

This is nice, but not how I'd actually write it. Since sts is the last parameter to the function and to its body, it's unnecessary. In truth, all functions in Haskell take only one argument; this means that if you don't pass enough arguments, you get a function which expects the missing arguments and returns the result. With the help of the function-composition operator ., this gives us

healthyPeople :: PeopleStats -> [String]
healthyPeople = map (\(n,_,_) -> n) . filter (\(_,h,w) -> healthy h w)

And that's probably how I'd write it! You'll find yourself using map and filter a lot; they're real workhorses in functional programming.

There is another idiomatic way you can write healthyPeople; you can use a list comprehension, as follows:

healthyPeople :: PeopleStats -> [String]
healthyPeople stats = [n | (n,h,w) <- stats, healthy h w]

This reads as "construct the list of every n such that (n,h,w) is an element of stats and healthy h w is true. If any of the pattern matches or the predicates fail (you can have more than one of each, though you don't need that here), that element is skipped; otherwise, the left side of the | is executed. It's effectively another way of writing the map/filter version.

---

Edit 1: As many others are saying, your units are off in bmi; you should have heightCm/100. Also, your healthy function has code equivalent to

f x | cond      = True
    | otherwise = False

This is equivalent to writing, in a C-like,

bool f(some_type x) {
  if (cond)
    return true;
  else
    return false;
}

Instead, you should just write

bool f(some_type x) {
  return cond;
}

Or, in this case

f x = cond

This gives you the shorter code

healthy :: Height -> Weight -> Bool
healthy heightCm weightKg = let index = bmi heightCm weightKg
                            in 25 > index && 18 < index

(You can use a where clause too, but here's a let just because I like it better :))

---

First off, note that your definition of BMI is incorrect - you need to convert centimetres into metres:

bmi heightCm weightKg = weightKg/(heightCm/100)^2

With that fixed, I came up with the following:

healthyPeople :: PeopleStats -> [String]
healthyPeople [] = []
healthyPeople ((name, height, weight):xs) | healthy height weight = name : healthyPeople xs
                                          | otherwise = healthyPeople xs

This is fairly straight forward. It uses list-based recursion to recurse over all the elements of the list, and it uses guards similarly to how you used them in the healthy function to switch behaviour based on whether the person at the head of the list is healty or not. If they are healthy, their name is concatted with the result of processing the rest of the list.

Next time, you should try solving the problem yourself and then ask for help (and show what you've tried). You'll learn far more!

---

type Height = Float
type Weight = Float
data PersonStats = PersonStats
  { personName :: String, personHeight :: Height, personWeight :: Weight }

bmi :: Height -> Weight -> Float
bmi heightCm weightKg = weightKg/(heightCm / 100)^2

healthy :: Height -> Weight -> Bool
healthy heightCm weightKg = 25 > index && 18 < index
  where index = bmi heightCm weightKg

healthyPerson :: PersonStats -> Bool
healthyPerson p = healthy (personHeight p) (personWeight p)

healthyPeople :: [PersonStats] -> [String]
healthyPeople = map personName . filter healthyPerson