Re: Do loop is faulty

• To: mathgroup at smc.vnet.net
• Subject: [mg55384] Re: [mg55368] Do loop is faulty
• From: "David Park" <djmp at earthlink.net>
• Date: Mon, 21 Mar 2005 03:01:51 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```Whoa! You're going too fast.

Let's check first that Mathematica will integrate an expression of the form
that you have.

NIntegrate[0.3 x Sin[2x], {x, 0, 2}]
0.139333

That looks Ok, so now let's set values of a[i] and b[i]. I'll do three
cases.

Clear[a, b, c];
MapThread[(a[#1] = #2) &, {{1, 2, 3}, {0.2, 0.5, 1.3}}];
Do[b[i] = i, {i, 1, 3}]

There are various ways that could have been done, including individual Set
statements for each a[i] and b[i]. But the above works. Now let's check that
the values actually have been set. (The first time I did this I made an
error and the a values were not actually set so it is worth checking!)

Table[{i, a[i], b[i]}, {i, 3}]
{{1, 0.2, 1}, {2, 0.5, 2}, {3, 1.3, 3}}

That looks good so now let's generate a table of integral values.

Table[NIntegrate[a[i] x Sin[b[i]x], {x, 0, b[i]}], {i, 3}]
{0.0602337, 0.232221, 1.244}

That seems to work.

To make good use of Mathematica it really really pays to set aside some time
and work through most of Part I of The Mathematica Book to become familiar
with the syntax, common commands, and methods of doing things. Plunging
straight into calculations without doing this will be very frustrating.

David Park

From: dumb_founded [mailto:andreajagger_8 at hotmail.com]
To: mathgroup at smc.vnet.net

I define array a[i] and array b[i] of dimension 10.  Then I go through
the following Do loop, or try to, that is.

Array[c,10];
Do[c[i]=NIntegrate[a[i]*x*Sin[b[i]*x],{x,0,b[i]}]

I get numerous error messages saying that b[i] is not a valid limit of
integration.

This is sheer headache.  I will never try to use Mathematica for
numerical analysis again.

All your efforts on this matter are sincerely appreciated.

Thanks

```

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