Anyone have some spin code to find largest value in an array?

Don M · 2021-03-12 22:19

A byte array of 16 bytes.

Thanks.

Don M · 2021-03-12 22:27

I think I found the answer... https://forums.parallax.com/discussion/136846/how-to-get-the-maximum-or-minimum-value-of-an-array

JonnyMac · 2021-03-12 22:52

You could make a general-purpose method that will work with any array type of any size:

pub find_max(p_array, bsize, count) : maxval | check

  maxval := negx 

  repeat count
    case bsize
      1 :
        check := byte[p_array]
        p_array += 1

      2 :
        check := word[p_array]
        p_array += 2

      4 :
        check := long[p_array]
        p_array += 4

    if (check > maxval)
      maxval := check

As Phil points out, all the math done in this method uses longs, so there is not need to check size when setting the initial value of maxval.

Phil Pilgrim (PhiPi) · 2021-03-12 22:56

Jon,

I think you can do this regardless of bsize:

maxval := negx

since the math is all done in longs anyway. Saves an if.

-Phil

Don M · 2021-03-12 23:02

Well I thought I did. Evidently I don't know how to use it.

VAR

byte Buff1[16], MaxResult

In this array I have these numbers stored: $01, $02, $05, $14, $FE, $00, $00, $00, $00, $00, $38, $00, $00, $00, $00, $00

I tried this method from @Kuroneco

pub

MaxResult := maxval2(@Buff1, 16)
term.hex(MaxResult, 2)

pub maxval2(addr, n)

addr := (addr - @addr) >> 2
result := addr[addr++]
repeat n - 1
result #>= addr[addr++]

I'm not sure what it's supposed to give me. The actual largest number ($FE) or the index of the largest number (4). It gives me 0.

Help!

DigitalBob · 2021-03-12 23:07

This is my demo

JonnyMac · 2021-03-12 23:51

Good point, Phil.

Phil Pilgrim (PhiPi) · 2021-03-12 23:59

Saving another if:

maxval #>= check

Of course, that might be totally opaque to a lot of people. Reason enough not to use it, I s'pose.

-Phil

JonnyMac · 2021-03-13 00:01

Since you only care about bytes, here's a simple version. I tested on a byte array with 25 values and it found the correct maximum.

pub get_max(p_array, count) : maxval | check

  repeat count
    check := byte[p_array++]
    if (check > maxval)
      maxval := check

What you're doing with the array pointer seems convoluted. When using pointer, you need to tell Spin what size value you are taking from that pointer -- in this case, a byte.

You could shorten the code a bit...

pub get_max(p_array, count) : maxval

  repeat count
    maxval #>= byte[p_array++]

... but the infinitesimally small performance gain is not worth the non-obvious code (my opinion, of course). I always strive to write obvious code.

Anyone have some spin code to find largest value in an array?

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