How do I iterate a list?
I am trying to do basi开发者_StackOverflowc list operations with SML.
I want to extract each element of the list and append string to that element and add it back to the list. Example:
List : [A,B,C,D]
String : A
Final List: [AA,AB,AC,AD]
How can I iterate through each element in the list in SML? I can append strings using ^
and concatenate lists using @
but how do I extract each element from the list?
Also can we use something like map or arrays to store these list values and pass it to different functions in SML?
I could just find some vague information about map and no definite information as to how we can use it.
two easy ways to do this:
- fun addstring (x::xs) a = (a^x) :: addstring xs a
= | addstring [] a = []
= ;
val addstring = fn : string list -> string -> string list
- addstring ["A", "B", "C", "D"] "A";
val it = ["AA","AB","AC","AD"] : string list
The above uses pattern matching to destruct the list, performs the operation, then constructs the list again, recursing as it goes.
- fun addstring2 xs a = map (fn x => a^x) xs;
val addstring2 = fn : string list -> string -> string list
- addstring2 ["A", "B", "C", "D"] "A";
val it = ["AA","AB","AC","AD"] : string list
This one is a fair bit simpler (if perhaps a tiny bit harder to read than the explicit cases in addstring.) but it shows how map is used - you specify a function that maps each element from the source to the target domain, give it a list of elements in the source domain, and it returns a list in the target domain.
of course, neither of these do in-place updating of the list, they return the new list.
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