Re: Find Position of many elements in a large list.

• To: mathgroup at smc.vnet.net
• Subject: [mg127697] Re: Find Position of many elements in a large list.
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Wed, 15 Aug 2012 03:36:26 -0400 (EDT)
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• Delivered-to: l-mathgroup@wolfram.com
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• References: <20120814082221.E17D66791@smc.vnet.net>

```You did not specify what form of output was desired.

X = Sort[RandomInteger[{0, 50}, 100]];

Y = Union[RandomInteger[{0, 50}, 10]];

Just positions in X of integers common to X and Y.

Flatten[Position[X, #] & /@
Intersection[X, Y]]

{1, 27, 28, 29, 34, 47, 48, 51, 52, 71, 72, 73, 74, 93, 94, 95, 96}

Integers common to X and Y and their positions in X.

{#, Flatten[Position[X, #]]} & /@
Intersection[X, Y]

{{0, {1}}, {17, {27, 28}}, {18, {29}}, {22, {34}}, {28, {47,
48}}, {30, {51}}, {31, {52}}, {40, {71, 72, 73, 74}}, {47, {93, 94, 95,
96}}}

Integers common to X and Y and their position ranges in X.

{#, Flatten[Position[X, #]][[{1, -1}]]} & /@
Intersection[X, Y]

{{0, {1, 1}}, {17, {27, 28}}, {18, {29, 29}}, {22, {34, 34}}, {28, {47,
48}}, {30, {51, 51}}, {31, {52, 52}}, {40, {71, 74}}, {47, {93, 96}}}

Bob Hanlon

On Tue, Aug 14, 2012 at 4:22 AM,  <benp84 at gmail.com> wrote:
> I have a sorted, 1-dimensional list X of 1,000,000 integers, and a sorted, 1-dimensional list Y of 10,000 integers.  Most, but not all, of the elements of Y are also elements of X.  I'd like to know the positions of the elements in X that are also in Y.  What's the fastest way to compute this?
>
> I have an algorithm in mind but it requires lots of custom code and I'm wondering if there's a clever way to do it with built-in functions.  Thanks.
>

```

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