Return Index of Element Matching Value?

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PT_
3
Site Admin (Posts: 2088)

Return Index of Element Matching Value?
26 Sep 2015 09:29:53 am

What is the smallest or fastest way to return the index of a list, where the number is ~2, starting from the index of A?
So
Code:

A=2, L1 = {~1,7,~2,1,~1,6,~2}


would return 3

and
Code:

A=5, L1 = {~1,7,~2,1,~1,6,~2}


would return 7

jonbush
1
Power User (Posts: 364)

26 Sep 2015 04:16:37 pm

You can use this to find the index, where N is the number to search for, and A is the starting index:

Code:




A+sum(not(cumSum(N=seq(L1(X),X,A,dim(L1

If you are going to run this many times, replace X with [recursiven].

lirtosiast
1
Power User (Posts: 338)

26 Sep 2015 05:48:25 pm
Last edited by lirtosiast on 26 Sep 2015 06:20:13 pm; edited 2 times in total

Here is a routine without seq( to return the first instance of N starting from A. Since it computes the entire list, it will be slower when A is close to dim(L1 and both are large.

Code:







cumSum(1 or L1 


min(Ans+ᴇ9(Ans<A or N≠L1

Thanks to jonbush for fixing

jonbush
1
Power User (Posts: 364)

26 Sep 2015 06:10:22 pm

lirtosiast wrote:

Here is a routine without seq( to return the first instance of N starting from A. Since it computes the entire list, it will be slower when A is close to dim(L1 and both are large.

Code:




cumSum(1 or L1


min(E9(Ans<A)+Ans*(N=L1

This is almost correct. The code below should work properly:

Code:




cumSum(1 or L1


min(E9(Ans<A or N!=L1)+Ans

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