MaplePrimes - answers and comments on Question, sum() ind subs()

One way

jakubi — Mon, 21 Jan 2008 04:34:38 Z

This is a way that works, though I do not like much: eval(sum(r[i]*y[i],i=1..m),[m=3,sum=add,y[1]=2]);

sum and subs

Robert Israel — Mon, 21 Jan 2008 04:54:04 Z

The short answer to "why" is that sum(r[i]*y[i], i=1..m) does not actually contain y[1]. See for yourself:

> sum(r[i]*y[i], i=1..m);

sum(r[i]*y[i], i=1..m) No y[1] there! One thing you could do is this:

> eval(eval(sum(r[i]*y[i],i=1..m),m=3),y[1]=2);

2*r[1]+r[2]*y[2]+r[3]*y[3] Note: I use eval rather than subs, to have the result evaluated after substitution.

Disregard this post

John Fredsted — Mon, 21 Jan 2008 13:44:59 Z

Please, disregard this post. The previous content was misconceived.

Yes, but you have insert

Dirk — Mon, 21 Jan 2008 13:46:56 Z

Yes, but you have insert i=1..3 instead of i=1..m in your equation above. In this case you would not need the m=3 in subs().

I did not see your response,

Dirk — Mon, 21 Jan 2008 14:08:54 Z

I did not see your response, as I sent my answer. Sorry for that.

It does appear to be the evaluation order.

jaytreiman — Mon, 21 Jan 2008 19:52:54 Z

Here are two ways of trying to do this. The first does not work, the sum is not evaluated. subs(y[1]=2,subs(m=3,sum(r[i]*y[i],i=1..m))); The next works, with a forced evaluation. subs(y[1]=2,eval(subs(m=3,sum(r[i]*y[i],i=1..m)))); If you look at the help on sum in Maple 11 you will find the following statement. "Maple returns the call unevaluated if it cannot find a closed form." Since you do not have a closed form, Maple does not evaluate the sum and there is no "y[1]."