Re: extract from list and keep track

• To: mathgroup at smc.vnet.net
• Subject: [mg62167] Re: extract from list and keep track
• From: Bill Rowe <readnewsciv at earthlink.net>
• Date: Sun, 13 Nov 2005 02:08:48 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```On 11/12/05 at 3:31 AM, andrea at radargift.com (Andrea) wrote:

>I have a list of increasing odd numbers. I want to create a table
>that indicates which numbers in this list are 1(mod 8) and 7(mod 8)
>and what position in the list these numbers have. Then I would like
>to have them removed form the list to get a new (reduced) list.

>Example: 3, 7, 9, 21, 43, 57, 63, 71, 75, 99.   I want to form this
>a Table:

>2     7
>3     9
>6    57
>7    63
>8    71

>7 is 7(mod 8) and is the 2nd member in the list, 9 is 1(mod 8) and
>is the 3rd member in the list, etc.

In[5]:=
data = {3, 7, 9, 21, 43, 57, 63, 71, 75, 99};

In[6]:=
Flatten@{Position[data, #][[1]], #1}]&/@
Pick[data, Mod[data, 8], 1 | 7]
Out[6]=
{{2, 7}, {3, 9}, {6, 57}, {7, 63}, {8, 71}}

>The next list would become, having removed the ones in the table:

>3, 21, 43, 75, 99

In[7]:=Pick[data, Mod[data, 8], 3 | 5]
Out[7]={3, 21, 43, 75, 99}
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