Re: Matrix manipulation

• To: mathgroup at smc.vnet.net
• Subject: [mg17024] Re: [mg16956] Matrix manipulation
• From: BobHanlon at aol.com
• Date: Wed, 14 Apr 1999 02:11:46 -0400
• Sender: owner-wri-mathgroup at wolfram.com

```In a message dated 4/8/99 11:01:46 AM, maa.150 at rsk.dk writes:

>question :   I have 2 matrices with 2 coulomns each.
>The first have 57 rows and the other 200 rows.
>The first coulomn in each matrix contains ID numbers and I
>would like to compare the two matrices and make a new matrix
>containing the ID number and the numbers in the second row ,  so
>the new matrix will be with 3 coulomns and 57 rows.
>how to do that ?
>

Here is one approach:

Generating random data of the form:  {ID_Integer, data_Real}

dataList1 = Union[Table[{Random[Integer, {100, 999}],
Random[]}, {100}], SameTest-> (#1[[1]] ==
#2[[1]]&)];

dataList2 = Union[Table[{Random[Integer, {100, 999}],
Random[]}, {50}], SameTest-> (#1[[1]] ==
#2[[1]]&)];

These lists are already sorted by ID due to the use  of Union to
eliminate duplicate IDs from the random data.  If your list is not
sorted by ID, use

dataList1 = Sort[dataList1, #1[[1]] < #2[[1]]&];

dataList2 = Sort[dataList2, #1[[1]] < #2[[1]]&];

The ID numbers which are common to the two lists are

idList = Intersection[Transpose[dataList1][[1]],
Transpose[dataList2][[1]]];

The elements of the data lists which have the common IDs are

subList1 = Select[dataList1, MemberQ[idList, #[[1]]]&];

subList2 = Select[dataList2, MemberQ[idList, #[[1]]]&];

Merging the lists

result = {#[[1, 1]], #[[1, 2]], #[[2, 2]]}& /@
Transpose[{subList1, subList2}];

Or, if you want to doublecheck that only elements with
common IDs are merged

result2 = Cases[Transpose[{subList1, subList2}],
{{x_, y1_}, {x_, y2_}} -> {x, y1, y2}];

result == result2

True

Note that none of the operations made explicit use of the
length of the lists; consequently, they will work with
lists of any length.

All of these steps can be combined into a single procedure
which would accept two input lists and output the merged list.

Bob Hanlon

```

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