
Re: Table with condition
Posted:
Mar 26, 2013 4:05 AM


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 preselecting the list length you eliminate the possibility of having longer lists which could occur.

