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

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

A byte array of 16 bytes.

Thanks.

Comments

  • JonnyMacJonnyMac Posts: 7,619
    edited 2021-03-12 23:53

    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)Phil Pilgrim (PhiPi) Posts: 23,001
    edited 2021-03-12 22:59

    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 MDon M Posts: 1,635

    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!

  • JonnyMacJonnyMac Posts: 7,619
    edited 2021-03-12 23:51

    Good point, Phil.

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,001
    edited 2021-03-13 00:00

    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

  • JonnyMacJonnyMac Posts: 7,619
    edited 2021-03-13 00:15

    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.

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