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Viz's avatar
Viz
Brass Contributor
Jan 05, 2021

Lambda Example: Generate Fibonacci series

In this post, I would like to explain how I have used Lambda to create a function to generate a Fibonacci series array.   This example can also be used to understand how to create an array where th...
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lori_m's avatar
lori_m
Iron Contributor
Jan 06, 2021

Viz  I should have replied here instead of the other post for further discussion.

Have also tweaked recursive formula for the n=1 case.

FIB:
=LAMBDA(n,
       IF(n<=2,
       SEQUENCE(n,,0),
       LET(b,FIB(n-1),
            IF(SEQUENCE(n)<n,b,INDEX(b,n-1)+INDEX(b,n-2))
       )))

 

  • Viz's avatar
    Viz
    Brass Contributor
    Jan 06, 2021

    lori_m 

     

    Wow! your solution is super elegant!

     

    I was initially trying something closer to your first solution but I could not crack the logic to extend the size of array. And the algorithm I used to extend the array made the function very slow.

     

    I am still getting my head around your function to understand how the array is getting extended. But thank you very much for sharing these solutions. It was very useful.

  • SergeiBaklan's avatar
    SergeiBaklan
    Diamond Contributor
    Jan 06, 2021

    lori_m 

    If add error handling as

    =LAMBDA(n,
       IF(n<>INT(n),"use integer as argument",
       IF(n<=2,
       SEQUENCE(n,,0),
       LET(b,FIB(n-1),
            IF(SEQUENCE(n)<n,b,INDEX(b,n-1)+INDEX(b,n-2))
       ))))

    it returns errors message as expected for FIB(10.1), but the spill as

    for FIB(10+1e-14). Correction could be

    =LAMBDA(n,
   IF(n<>INT(n),"use integer as argument",
      LET(m, INT(n),
         IF(m<=2,
         SEQUENCE(m,,0),
         LET(b,FIB(m-1),
         IF(SEQUENCE(m)<m,b,INDEX(b,m-1)+INDEX(b,m-2))
   )))))

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