# [Haskell] How to substract a list?

Mon Mar 28 07:57:07 EST 2005

```Hi

Bright Sun wrote:
> Hi,
>
> Tks, this is not what I wanted.
>
> I want remove pairs list,
>
> [(5,1),(4,1),(3,1),(2,1),(1,1)] remove [(2,1),(1,1)]
> [(5,1),(4,1),(3,1)]

[(5,1),(4,1),(3,1),(2,1),(1,1)] \\ [(2,1),(1,1)]

should give the right result. Befere using the '\\' operator you may
need to load the List module (in Hugs :load List).

Hope it helps,

>
> so pairs [(2,1),(1,1)] removed.
>
> Tks.
>
> --- Pierre Barbier de Reuille
> <pierre.barbier at cirad.fr> wrote:
> > One answer is the infix operator (\\) defined in the
> > List module (or in
> > Data.List ...)
> >
> > But it's in ont of the "set" operations. For example
> > try :
> >
> > [1..10] \\ [3..6]
> > [1,2,7,8,9,10]
> >
> > But also :
> >
> > ([1..10]++[1..10]) \\ [3..6]
> > [1,2,7,8,9,10,1,2,7,8,9,10]
> >
> > So _all_ the elements with values in [3..6] where
> > removed !!
> > If that's what you want ! Perfect ... if not : what
> > do you want exactly
> > ? List substraction is not uniq.
> >
> > Pierre
> >
> > Bright Sun a crit :
> > > In Haskell, ++ can spends second list argument
> > onto
> > > the end of first list argument.  How to substract
> > the
> > > second list from the first list?
> > >
> > > For example,
> > > [(5,1),(4,1),(3,1),(2,1),(1,1)]
> > > substract
> > > [(2,1),(1,1)]
> > > I want to get result list:
> > > [(5,1),(4,1),(3,1)]
> > >
> > > Tks.
> > >
> > >
> > >
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> >
> > --
> > Pierre Barbier de Reuille
> >
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