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Re: Operating on every k-th element of list?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg37096] Re: [mg37080] Operating on every k-th element of list?
*From*: Omega Consulting <omega_consulting at yahoo.com>
*Date*: Wed, 9 Oct 2002 05:25:50 -0400 (EDT)
*Sender*: owner-wri-mathgroup at wolfram.com
At 06:17 AM 10/8/2002, AES wrote:
>I want to apply a function to every k-th element of a long list and
>add the result to the k+1 element.
>
>[Actually k = 3 and I just want to multiply myList[[k]] by a
>constant (independent of k) and add the result to myList[[k+1]] for
>every value of k that's divisible by 3.]
lst = Table[Random[], {20}]
fact = 2
Here's a matrix method. f represents an indexed element of a matrix.
f[x_,x_] := If[Mod[x,3]==0, fact, 1]
f[x_, y_] := If[Mod[x,3]==0&&y\[Equal]x+1, 2, 0]
Create a matrix from the elements.
arr = Array[f,{Length[lst], Length[lst]}];
Then matrix multiply.
newlst1 = lst.arr
Here's another way with the highly underrated MapIndexed.
Create pairs of the nth and n-1th values.
pairs = Transpose[{lst, Prepend[Drop[lst,-1],0]}]
Create a function that takes the nth value (val), the n-1th value
(prevval), and the index (num).
multlst[{val_, prevval_}, {num_}] :=
Switch[Mod[num, 3],
0, fact*val,
1,val+ fact*prevval,
2, val
]
Then, MapIndexed across the pairs.
newlst2=MapIndexed[multlst, pairs]
--------------------------------------------------------------
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