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From: amzoti on 29 Jul 2010 23:53
On Jul 29, 7:04 pm, "Michael Robinson" <nos...(a)billburg.com> wrote:
> "Michael Robinson" <nos...(a)billburg.com> wrote in message
>
> news:0Ip4o.47779$YX3.18857(a)newsfe18.iad...>I have an expression in x and y (an output line).  I have values of x and y
> >for which I need to evaluate the expression.  How do I input the values and
> >command mathematica to evaluate the expression?
>
> Another problem.
> Now mathematica is doing something it shouldn't be doing.
> When I put in an expression such as
> i = 3 x^2 + 2 y^2 - 4 y + 1
>
> it should just reprint the expression on the next line.
>
> But now it's putting out a numerical expression.
>
> I've tried using the "clear" command for i, also for x and y, but it just
> keeps doing it.

Recommend you look at what = means versus := versus others
From: gearhead on 30 Jul 2010 09:27
On Jul 29, 11:52 pm, amzoti <amz...(a)gmail.com> wrote:
> On Jul 29, 7:04 pm, "Michael Robinson" <nos...(a)billburg.com> wrote:
>
>
>
> > "Michael Robinson" <nos...(a)billburg.com> wrote in message
>
> >news:0Ip4o.47779$YX3.18857(a)newsfe18.iad...>Ihave an expression in x and y (an output line).  I have values of x and y
> > >for which I need to evaluate the expression.  How do I input the values and
> > >command mathematica to evaluate the expression?
>
> > Another problem.
> > Now mathematica is doing something it shouldn't be doing.
> > When I put in an expression such as
> > i = 3 x^2 + 2 y^2 - 4 y + 1
>
> > it should just reprint the expression on the next line.
>
> > But now it's putting out a numerical expression.
>
> > I've tried using the "clear" command for i, also for x and y, but it just
> > keeps doing it.
>
> Perhaps what you want if f[x_, y_] := 3 x^2 + 2 y^2 - 4 y + 1
>
> Now, you want to plug values in for x and y, for example:
>
> x=0, y=0 is f[0,0] (shift-enter)
>
> x = 3, y = 5 is f[3,5] (shift-enter)
>
> x = 3, y = -11 is f[3,-11] (shift-enter)
>
> Note that x and y can be Z or R or Q
>
> Is that what you are looking for?
>
> Recommendation: read the MMA introduction for basic operations - it
> will help a lot.

I tried the syntax you recommend and it works.
Somebody on craigslist sci forum suggested
the form
x + y /. {x->a, y->2}
and that works too.
I couldn't get the clear command to work but for now I can get around
that by just shutting mathematica down and reopening it.
From: amzoti on 30 Jul 2010 11:29
On Jul 30, 6:27 am, gearhead <nos...(a)billburg.com> wrote:
> On Jul 29, 11:52 pm, amzoti <amz...(a)gmail.com> wrote:
>
>
>
> > On Jul 29, 7:04 pm, "Michael Robinson" <nos...(a)billburg.com> wrote:
>
> > > "Michael Robinson" <nos...(a)billburg.com> wrote in message
>
> > >news:0Ip4o.47779$YX3.18857(a)newsfe18.iad...>Ihavean expression in x and y (an output line).  I have values of x and y
> > > >for which I need to evaluate the expression.  How do I input the values and
> > > >command mathematica to evaluate the expression?
>
> > > Another problem.
> > > Now mathematica is doing something it shouldn't be doing.
> > > When I put in an expression such as
> > > i = 3 x^2 + 2 y^2 - 4 y + 1
>
> > > it should just reprint the expression on the next line.
>
> > > But now it's putting out a numerical expression.
>
> > > I've tried using the "clear" command for i, also for x and y, but it just
> > > keeps doing it.
>
> > Perhaps what you want if f[x_, y_] := 3 x^2 + 2 y^2 - 4 y + 1
>
> > Now, you want to plug values in for x and y, for example:
>
> > x=0, y=0 is f[0,0] (shift-enter)
>
> > x = 3, y = 5 is f[3,5] (shift-enter)
>
> > x = 3, y = -11 is f[3,-11] (shift-enter)
>
> > Note that x and y can be Z or R or Q
>
> > Is that what you are looking for?
>
> > Recommendation: read the MMA introduction for basic operations - it
> > will help a lot.
>
> I tried the syntax you recommend and it works.
> Somebody on craigslist sci forum suggested
> the form
> x + y /. {x->a, y->2}
> and that works too.
> I couldn't get the clear command to work but for now I can get around
> that by just shutting mathematica down and reopening it.

Remove[i] (shift-enter)

The method above is called postfix.

See: http://reference.wolfram.com/mathematica/tutorial/SpecialWaysToInputExpressions.html

Mathematica is an incredible program and allows many methodologies for
getting the job done. They have subtle differences and learning those
allows you to understand what a wonderful tool this is.
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