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# How can I pick the first and last item from various lists?

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I’m looking for help to extracting the first and last value of a given list.

I have a binary parameter L(p,i) which will be given varios list of data.

For example:

List p=1:

i=1 → 0

i=2 → 1

i=3 → 1

i=4 → 0

List p=2:

i=1 → 1

i=2 → 0

i=3 → 1

i=4 → 1

I’m looking for a command which will extract the first i which is set to 1 on a given list. In this Example for the list p=1 it would be i=2 and for the list p=2 it would be i=1.

I’m also looking for a command which will extract the last i which is set to 1 on a given list. In this example for the list p=1 it would be i=3 and for the list p=2 it would be i=4.

I tried the “First” and “Last” command which always lead to using i=1 (for the command “First”) even though in the example p=1 the value of i=1 is 0 and not 1.

Any help would be appreciated.

Thanks a lot for the answer, that worked perfect for my problem.

What you could do is create an element parameter that has the range attribute set to the set that corresponds to the index i and that is indexed over index p.

You can then give this element parameter the following definition:

first( i | L(p,i) )

(or last in case of the second question).

You can read this definition as “The first i SUCH THAT there is a non-zero value for L(p,i)”

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