Re: Select list elements and exchange them

• To: mathgroup at smc.vnet.net
• Subject: [mg109820] Re: Select list elements and exchange them
• From: Bill Rowe <readnews at sbcglobal.net>
• Date: Wed, 19 May 2010 07:02:31 -0400 (EDT)

```On 5/18/10 at 6:12 AM, ishii.mitsuo.titanium at nsc.co.jp wrote:

>I have a list.

>testList={ {11,{2,3}},{21,{3,4}},{34,{5,6}},{51,{7,8}} }

>1) From this list, select any element list such as {x,{3,4}},

>where X is an arbitrary number.

either

In[5]:= Cases[testList, {_, {3, 4}}]

Out[5]= {{21, {3, 4}}}

or

In[6]:= Select[testList, Last@# === {3, 4} &]

Out[6]= {{21, {3, 4}}}

will do this

>In general, position of {x,{3,4}} in "testList" is not fixed.

>2) Select any element list such as {x,{3,4}} and then  exchange
>{x,{3,4}} to {100,{3,4}}.

>3)Make a new list finally.

>testListNew={ {11,{2,3}},{100,{3,4}},{34,{5,6}},{51,{7,8}} }

This last can be accomplished in one step as:

In[7]:= testList /. {_, {3, 4}} -> {100, {3, 4}}

Out[7]= {{11, {2, 3}}, {100, {3, 4}}, {34, {5, 6}},
{51, {7, 8}}}

```

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