%% Function even_print(List),takes a list and returns a list of only even numbers. Function even_odd(X), takes an integer and tells if it is even or odd.
even_print(List) ->
[X||X<-List, even<-even_odd(X)].
I don't understand why I get this error:
3> seq_erlang:even_print([2,3,4]).
** exception error: no function clause matching
seq_erlang:'-even_print2/1-lc$^1/1-1-'(even) (seq_erlang.erl, line 154)
Just to comment, I have already implemented another function that prints even numbers just fine (so please don't comment with other implementations). I need help with this one only.
That should be even == even_odd(X) instead of using <-. A list comprehension has two types of "clauses": those that map over a list with <-, and those that filter out undesired combinations using a guard or boolean expression that doesn't contain <-.
(And a third one: extract bytes from a binary using <=; but that one is more rarely used.)
Related
I'm trying to create a list of unique by appending to a list, but I'm getting this error.
Error: This expression has type 'a list
but an expression was expected of type unit
in_list is a boolean function that checks whether the value is in the list.
if(in_list x seen_list) then print_string("Already found") else seen_list#x in
List.iter uniq check_list;;
It seems like there must be some small syntactic error I need to fix for the append function. Suggestions?
TL;DR: Lists are immutable in OCaml
According to your code, you seem to believe that lists are mutable in OCaml, and they are not. Hence seen_list#x compute a new list but does not change seen_list.
You could change your code to
let uniq seen_list x =
if in_list x seen_list then
(Printf.printf: "uniq: %d: Already seen.\n" x; seen_list)
else x :: seen_list
in
List.fold_left uniq [] check_list
The uniq function maps a list of integers to a list of integers without repetitions, logging the entries it skips.
This code is obviously intended to be learning material, I guess, nevertheless you should be aware that it most likely implements a Shlemiel the painter's algorithm.
This is a type error, not a syntactic error.
An OCaml function must always return a result of the same type. Right now, when the item is in the list your function tries to return a different type than if the item is not in the list.
Specifically, when the item is already there your function calls print_string, which returns (). This is called unit, and is a placeholder representing no interesting value. When the item isn't already there, your function returns a value of type 'a list. Almost certainly what you need to do is to return a list in all cases.
It's hard to say more without seeing more of your code, but the most usual way to handle this situation is to return the old list when the item is already there and a new, longer list when the item isn't already there.
Update
There are many things to fix in this code, but that's the point of the exercise I assume.
Your next problem seems to be that List.iter is an imperative function, i.e., it wants to do something rather than produce a result. Hence the function that it iterates over the list should return unit (described above). You're using the function uniq instead, which returns a list.
If you want to use a higher-order function like List.iter, which is excellent OCaml style, you will need to use a fold (List.fold_left or List.fold_right), whose purpose is to accumulate a result.
I just started learning Erlang so please bear with me if this question seems a little simple.
Hi guys. I've been thinking about it for a while but nothing I come up with seems to be working.
I am writing an Erlang function that is supposed to take a list as an argument then print the list with my name in front of it. For the purposes of this question, let's say my name is "James".
If I type in testmodule:NameInFront("Legible", "Hey", "Think").
Erlang should return ["James", "Legible", "Hey", "Think"]
This is the code I have so far:
-module(testmodule).
-export([NameInFront/1]).
NameInFront(List)-> ["James"]++[List].
It works just fine when I type in just one word, which I guess it the fault of the NameInFront/1 part but I want it to be able to handle any amount of words I type in. Anyone know how I can get my function to handle multiple inputs? Thank you very much.
I'm not quite sure what you mean: whether you want your function to be variadic (take a flexible number of arguments), or you are having trouble getting your lists to join together properly.
Variadic functions are not the way Erlang works. FunctionName/Arity defines the concrete identity of a function in Erlang (discussed here). So our way of having a function take multiple arguments is to make one (or more) of the arguments a list:
print_terms(Terms) -> io:format("~tp~n", [Terms]).
The io:format/2 function itself actually takes a list as its second function, which is how it deals with a variable number of arguments:
print_two_things(ThingOne, ThingTwo) ->
io:format("~tp~n~tp~n", [ThingOne, ThingTwo]).
In your case you want to accept a list of things, add your name to it, and print it out. This is one way to do it.
name_in_front(ListOfStrings) ->
NewList = ["James" | ListOfStrings],
io:format("~p~n", [NewList]).
Using the ++ operator is another (which is actually a different syntax for a recursive operation which expands to the exact same thing, ):
name_in_front(ListOfStrings) ->
NewList = ["James"] ++ ListOfStrings,
io:format("~tp~n", [NewList]).
But that's a little silly, because it is intended to join two strings together in a simple way, and in this case it makes the syntax look weird.
Yet another way would be to more simply write a function that take two arguments and accomplishes the same thing:
any_name_in_front(Name, ListOfThings) ->
io:format("~tp~n", [[Name | ListOfThings]]).
The double [[]] is because io:format/2 takes a list as its second argument, and you want to pass a list of one thing (itself a list) into a single format substitution slot (the "~tp" part).
One thing to note is that capitalization matters in Erlang. It has a meaning. Module and function names are atoms, which are not the same thing as variables. For this reason they must be lowercase, and because they must be lowercase to start with the convention is to use underscores between words instead of usingCamelCase. Because, well, erlangIsNotCpp.
Play around in the shell a bit with the simple elements of the function you want, and once you have them ironed out write it into a source file and give it a try.
I want to parse the argument list of a Lua function call in C++ using Qt (4.8) in order to avoid a dependency to the Lua interpreter. The comma-separated argument list can be assumed to consist only of string literals and numbers. Eventually the result should be available as a QStringList. The tricky part there is to cope with commas that are part of string arguments as well with the fact that string arguments may use single or double quotes. Until I get to a solution (using regular expressions) myself, somebody might already have dealt with that or a similar problem.
Example:
The argument list string
"Foo", "not 'bar'", 'a, b ,c', 42, 1e-8
should be transformed to a string list containing the items
Foo, not 'bar', a, b, c, 42 and 1e-8
(omitting the quotes per item to avoid confusion)
Not familiar with all the possibilities of your arguments, but the examples you mentioned get correctly matched with this: (?<=")[\w',-]*?(?=")|(?<=^'|\s').*(?='(?:,|$))|[\w-]+, as seen here: https://regex101.com/r/rX7fX7/3
The idea is that you write the "difficult" situations in alternations, preferably to the left, while the less difficult solutions to the right. This way, the engine will first check if a problem situation is present before trying to match whole words.
The current regex doesn't work correctly if quotes/doublequotes appear in middle of the arguments, but your examples didn't have such situations.
I'm a newbie prolog programmer, and for an assignment, I have to have a basic program that succeeds if and only if list X is a list of two elements, with the first as the same as the second.
From my view of prolog, programs seem to be pretty small, so I typed this in:
firstPair(x,x).
When I run it under swipl, I get this as output:
Syntax error: Operator expected
Is there something more that needs to be done? I thought that if I executed this with say, firstPair(1,2). this would be all it would need to know that it is false.
First, lowercase x is not a variable, it's an atom. Make x uppercase to fix the problem:
firstPair(X,X).
Second, you do not type this into the interpreter. Rather, you write it into a file firstPair.pl, and then read that file into Prolog.
At the command prompt, type this:
['firstPair.pl'].
Press enter. Now you can use your firstPair/2 rule.
Finally, since the assignment talks about lists, I think the instructor wanted you to write firstPair/1, not firstPair/2:
firstPair([X,X]).
Your program/fact
firstPair(X,X).
will succeed if the two arguments given it can be unified, whether they are lists, atoms, variables, etc. To meet your specification, a
program that succeeds if and only if list X is a list of two elements,
with the first as the same as the second.
You need something like this:
list_of_two_elements( [X,X] ).
This will succeed if passed a single term that is (or can be unified with) a list of two elements that are, or can be made through unification, identical. For instance, all of the following will succeed:
list_of_two_elements( X ).
on success, the variable X will be unified with a list of two elements containing the same unbound variable, something like [V1,V1].
list_of_two_elements( [1,1] ).
list_of-two_elements( [1,X] ). (on success, X here will have been unified with the integer 1.)
I simply want to pass a list of integers to a function written in C++. I've set up the template (.tm) file and all, and I can successfully call a test function whith scalar arguments. Calling the function with the list argument behaves as though the function was not defined at all. I suspect that the argument types don't match.
In the documentation for templates (http://reference.wolfram.com/mathematica/ref/file/file.tm.html) the datatype for lists is something like "Int32List". When I use that, my C++ function must contain an extra long parameter for the list length. The only example code which uses a list is "sumalist.tm". This example uses IntegerList (a type which doesn't appear in the doku).
When I use Int32List, the mprep result requires a function with an extra integer argument (not long as written in the doku). When I use the undocumented IntegerList type, the extra argument is of type long.
During my experiments with scalar types, I had a similar problem - a c++ function was called properly when using "Integer" in the tm-file, and not recognized with "Integer32".
The "sumalist.tm" example also uses a strange Pattern (list:{___Integer}) about which I didn't find any documentation. I'd also like to understand what the Evaluate line means (I suspect that it's used make the function callable without the curly braces around the list).
So who know which datatypes are really appropriate to call a c++ function with a list - maybe also with reals... ?
The mapping of MathLink data types (e.g., Integer32, Integer32List, ...) to C/C++ types is described on the MathLink template file documentation page.
The page no longer documents the old interface types Integer, Real, IntegerList and RealList. These should no longer be used, because the mapping of these types depends on C types whose bit length is platform and compiler dependent (e.g., long). Use the corresponding new type with explicit bit length instead (i.e., Integer32 or Integer64 instead of Integer). The old interface types are still documented in the somewhat dated MathLink reference guide.
The following talk slides contain a simple MathLink example that shows how to implement a MathLink function that adds a scalar value to a vector of reals. This may serve as a starting point.
I don't know much about MathLink, but I can explain the pattern, list:{___Integer}.
The colon is just the general form for a named pattern, that is symbol:pattern just says that the object referred to by symbol has to match pattern. Indeed, pattern like a_Integer or b__List are really just short forms for a:_Integer and b:__List.
So what we are left with interpreting is {___Integer}. This is a pattern matching a list of arbitrary many (including zero) integers. It works as follows:
{Pattern} is the Pattern for a list whose contents matches Pattern
___Integer is the Pattern for a sequence of zero or more Integers.