Transform a Binary Number into a Decimal without recursivity [HASKELL]

Question

I haven't found a way to solve this. I have a list of integer, where which element of the list is a binary digit (0 or 1) so I need to design a function which transforms this list of integers into the proper decimal number.

Example:

Input: [0,1,0] Output: 2

But there is a specific condition, it is neccesary to use list of comprehension and you can't use recursivity.

The problem it is, when I need to know the position of the digit for apply the transform because I can't save the position in the list of comprehension.

Thank you

List comprehensions produce lists, not single Int values. A list comprehension by itself won't do what you want. (It's possible to create a list with a single element whose value is the one you want, but then the list comprehension itself isn't actually doing any of the real work.) — chepner
– chepner, Commented Sep 30, 2018 at 19:27

willeM_ Van Onsem · Accepted Answer · 2018-09-30 17:50:54Z

2

The problem it is, when I need to know the position of the digit for apply the transform because I can't save the position in the list of comprehension.

You can, by using zip and a range, you generate 2-tuples that carry the index, like:

[(idx, val) | (idx, val) <- zip [0..] bin]

will produce a list of 2-tuples: the first element containing the element, and the second the element of data at that position.

So if bin = [0,1,0], then the above list comprehension will result in:

Prelude> [(idx, val) | (idx, val) <- zip [0..] bin]
[(0,0),(1,1),(2,0)]

Since this seems to be the "core problem", I propose that you aim to solve the rest of the problem with the above strategy, or ask a question (edit this one, or ask a new one) if you encouter other problems.

answered Sep 30, 2018 at 17:50

willeM_ Van Onsem

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1 Comment

OnlyDavies Over a year ago

Very useful, I could finally get solved it thanks to you, but I changed one thing, in the zip [0..] bin I used zip [(length (bin)-1), (length (bin) -2) ..] bin] because I wanted to change the positions

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