Re: second simple problem

• To: mathgroup at smc.vnet.net
• Subject: [mg99704] Re: second simple problem
• From: gdelfino <gdelfino at gmail.com>
• Date: Mon, 11 May 2009 06:27:37 -0400 (EDT)
• References: <gu666q\$9tf\$1@smc.vnet.net>

```On May 11, 4:19 am, Francisco Gutierrez <fgutiers2... at yahoo.com>
wrote:
> Dear sirs:
> I have the following list:
> ex={1,5,7,4,"M",6,7,8,9,1,"M",3}
> I want to replace the M's in the following way: the first M by 5, and the second by2.
> Thus I have a replacement list
> rL={5,2}
> The problem is to get ={1,5,7,4,5,6,7,8,9,1,2,3}
> How can I do this in the most general form (for any length of ex and any number of values of "M")?
> Thanks
> Francisco

Francisco, this is one solution:

In[1]:= ex = {1, 5, 7, 4, "M", 6, 7, 8, 9, 1, "M", 3}

Out[1]= {1, 5, 7, 4, "M", 6, 7, 8, 9, 1, "M", 3}

In[2]:= rL = {5, 2}

Out[2]= {5, 2}

In[3]:= rules =
MapIndexed[{"M", First[#1]} -> Flatten[{rL[[#2]], #1}] &,
Position[ex, "M"]]

Out[3]= {{"M", 5} -> {5, 5}, {"M", 11} -> {2, 11}}

In[4]:= Map[First, MapIndexed[Join[{#1}, #2] &, ex] /. rules]

Out[4]= {1, 5, 7, 4, 5, 6, 7, 8, 9, 1, 2, 3}

Regards,

Gustavo Delfino

```

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