Re: Adding value to some elements of a list

• To: mathgroup at smc.vnet.net
• Subject: [mg131047] Re: Adding value to some elements of a list
• From: David Bailey <dave at removedbailey.co.uk>
• Date: Thu, 6 Jun 2013 07:28:48 -0400 (EDT)
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• References: <komoi3\$1un\$1@smc.vnet.net>

```On 05/06/2013 08:15, Joerg Lenhardt wrote:
> Hello,
>
> I have two lists. One containing values, another containing indexes to
> the first list. I want to add a value to all elements of the first list
> when its index is in the second list:
>
> a={10,20,30,40,50,60}
>
> b={2,4,6}
>
> c=17
>
> ==>
>
> {10,20+17,30,40+17,50,60+17}
>
> I would be glad if anyone have a simple solution for that problem - I am
> convinced there is one ...
>
> Best regards
> J. Lenhardt
>
In[1]:= a = {10, 20, 30, 40, 50, 60};

b = {2, 4, 6};

c = 17;

In[4]:= ans = a;

In[5]:= ans[[b]] = ans[[b]] + c;

In[6]:= ans

Out[6]= {10, 37, 30, 57, 50, 77}

To understand this better, remember that [[.....]] is a shorthand for
Part - so look that function up.

David Bailey
http://www.dbaileyconsultancy.co.uk

```

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