Date: Mar 26, 2013 4:05 AM
Author: Bill Rowe
Subject: Re: Table with condition
On 3/24/13 at 11:23 PM, Serych@panska.cz wrote:

>Dear mathgroup, I need to start creating list and continue until

>some condition is not met. For example to generate list of random

>numbers until the value is not 5. Yes, it is theoreticaly possible

>to generate list with sufficient length, and cut it on the right

>place afterwords, but it is very inefficient way if the computation

>is much harder than just generation of random numbers.

>TakeWhile[Table[RandomInteger[{0, 10}], {15}], # != 5 &]

>Using the while cycle is the other way, but I can only print values

>using that method, but I don't know, how to generate classical list:

>r = 0; While[r != 5, r = RandomInteger[{0, 10}]; Print[r]]

>What is the right solution of such a simple problem?

What you might want to use is NestWhileList. Using your example

of RandomInteger you could do it as:

In[9]:= Most@

NestWhileList[(#; RandomInteger[{0, 10}]) &,

RandomInteger[{0, 10}], (# != 5) &]

Out[9]= {4,8,10,3,9,6,4,0,6,3,2,1,7,7,4,8,3,2}

What might be more efficient when each result really doesn't

depend on the previous result could be something like:

In[16]:= list = {}; r = 0;

While[r != 5, list = {list, r = RandomInteger[{0, 10}]}];

Most@Flatten[list]

Out[18]= {2,9,9,0,6,4,0,0,7,7,4,7}

Note, the issue of whether to generate a list of random integers

then use TakeWhile isn't just a question of efficiency. No

matter what length list you generate, there is always some

probability of it not having a 5. By pre-selecting the list

length you eliminate the possibility of having longer lists

which could occur.