How can I get the index of the element I am at in haskell when I am using map ?
For example I have this list l = "a+bc?|(de)*fg|h" and I want to know the exact index of the element I am at when I use the map or scanl function.
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2 Answers
Amending Nikita Volkov's answer, you can use a function such as:
-- variant of map that passes each element's index as a second argument to f
mapInd :: (a -> Int -> b) -> [a] -> [b]
mapInd f l = zipWith f l [0..]
Comments
First of all, if you need an index when processing a list it is a certain sign that you're implementing a suboptimal algorithm, because list is not an index-based structure like array. If you need to deal with indexes you better consider using a vector instead.
Concerning your actual question, you can pair the items of your list with incrementing ints with the following code and then map over the result:
Prelude> zip [0..] "a+bc?|(de)*fg|h" :: [(Int, Char)]
[(0,'a'),(1,'+'),(2,'b'),(3,'c'),(4,'?'),(5,'|'),(6,'('),(7,'d'),(8,'e'),(9,')'),(10,'*'),(11,'f'),(12,'g'),(13,'|'),(14,'h')]
2 Comments
Fred Foo
Having indices at your disposal is a pretty common requirement when processing lists (e.g. for error reporting).
Emil Sahlén
There are many uses for an index when processing a list that have nothing to do with using a suboptimal algorithm. So it is a very uncertain sign.