Date: Jan 21, 2013 12:06 AM
Author: Bill Rowe
Subject: Re: problem with append

On 1/20/13 at 1:21 AM, paolocach@gmail.com (Paolo cachecho) wrote:

>hello i am new to mathmatica.i have the following problem if anyone
>could help.

>For[a = 2, a < 6, a++,
>For[b = 2, b < 6, b++, Appendto[rr = {}, a^b]]]
>rr

>but rr remains empty. how can i solve this thank you

There are two issues with the code above. First you set rr to a
null list for every value of a,b. Second, Appendto should be AppendTo.

This will do what you want

rr={};
For[a = 2, a < 6, a++,
     For[b = 2, b < 6, b++,
   AppendTo[rr, a^b]]]

A faster more efficient way to get the same result would be

rr = Flatten@Outer[#1^#2&, Range[2,5],Range[2,5]]

or

rr = Flatten@Table[a^b,{a,2,5},{b,2,5}]

Using For generally results in slower running Mathematica code.
AppendTo copies the entire array to add another element which
will consume more memory and cpu resources.