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Hello, I'm trying to use a function that uses a power that is less than 1, and for some reason pico will always evaluate it to 0. Is this just a limitation of pico? and any ideas on alternatives to achieve a similar result?

sin( x ^ ( 1 / 3 ))

Cart #33485 | 2016-12-14 | Code ▽ | Embed ▽ | License: CC4-BY-NC-SA

P#33486 2016-12-14 10:16 ( Edited 2016-12-14 22:53)

I concur that Pico-8 doesn't do the right thing for fractional powers. I get 1 for every power between 0 and 1, and 0 for every power less than 0.

?9^2  -- 81
?9^1  -- 9
?9^(0.5)  -- 1, should be 3
?9^0  -- 1
?9^(-0.5)  -- 0, should be 0.3333
?9^(-1)  -- 0, should be 0.1111

These operations work in Lua 5.3, so this seems specific to Pico-8's fixed point implementation.

If you're desperate you could use a lookup table, but I would agree that this should get fixed.

P#33491 2016-12-14 12:33 ( Edited 2016-12-14 17:34)

I'm sure there's a more efficient way to write this, but here's a function based on the Newton method:

function nroot(k,n)
 if(k<0 and n%2==0) return 0 --nan
 if n==1 then return k
 elseif n==2 then return sqrt(k) end

 local num=abs(k)
 local lowbnd,highbnd
 if num<1 then
  lowbnd=sqrt(num)
  highbnd=1
 else
  lowbnd=1
  highbnd=sqrt(num)
 end
 local guess=lowbnd+(highbnd-lowbnd)/2
 local precis=0.001
 local oneoverroot=1/n

 local deltaguess=32767
 while abs(deltaguess)>precis do
  deltaguess=oneoverroot*(num/guess^(n-1)-guess)
  guess=guess+deltaguess
 end

 return guess*sgn(k)
end

print("cuberoot of 4: "..nroot(4,3))
print("7-root of -17: "..nroot(-17,7))
print("5-root of 0.4: "..nroot(0.4,5))

(since x^(1/3) == nroot(x,3))

P#33498 2016-12-14 17:27 ( Edited 2016-12-14 22:30)

I don’t believe the ^ operator is supposed to be supported (at least it’s not in the documentation). In this thread I provided a pow() function that uses some kind of exponentiation by squaring, it has a pretty good speed vs. precision trade-off.

P#33502 2016-12-14 17:53 ( Edited 2016-12-14 22:53)

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