DownValue "associative array" to list?

Question

I have a "sparse array" like this

arr[1.5]=0.4, arr[3.5]=0.7, arr[7]=0.3

which is not really a sparse array since the keys are not all integers. This is like a hash table or associative array.

How can I convert arr into a list? I tried Normal[arr], but it won't work. I would like to be able to compute Total@arr, but first it needs to be converted to a normal list.

In fact this is exactly a hash table. – Oleksandr R. May 02 '13 at 20:21 — Oleksandr R., May 02 '13 at 20:21

score 9 · Answer 1 · answered May 02 '13 at 20:54

9

This might be the general sort of operation you seek.

keyvalpairs = 
 DownValues[arr] /. Verbatim[HoldPattern][arr[k_]] :> k

(* Out[121]= {1.5 :> 0.4, 3.5 :> 0.7, 7 :> 0.3} *)

answered May 02 '13 at 20:54

Daniel Lichtblau

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I was trying to do exactly that but I couldn't figure out the pattern/replacement I needed +1. – s0rce May 03 '13 at 00:53
@sOrce it took me a few tries. That combination of HoldPattern and Verbatim is not something I am accustomed to needing. – Daniel Lichtblau May 03 '13 at 14:56

score 8 · Answer 2 · answered May 02 '13 at 18:39

8

Here is one way to do it:

{arr[1.5] = 0.4, arr[3.5] = 0.7, arr[7] = 0.3}
Total[DownValues[arr][[All, 2]]]

answered May 02 '13 at 18:39

s0rce

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DownValue "associative array" to list?

2 Answers2