Finding Max Value Below Zero - go

Question

I'm trying to get the max value from a diff, n[i] - n[i-1], timeseries. The first value is always zero from the slice, here is the code:
func MaxBelowZero(n ...float64) float64 {
var maxValue float64
if len(n) == 1 {
return n[0]
} else if len(n) == 0 {
return 0.
}
for i := range n {
if i == 0 {
maxValue = math.SmallestNonzeroFloat64
continue
}
if maxValue < n[i] && n[i] < 0 {
maxValue = n[i]
}
}
return maxValue
}
var sliceTest = []float64{0, 1, 2, -1, -2, -10, 10, 20}
MaxBelowZero(sliceTest...)
Output: 5e-324
It supossed to be -1. What am I doing wrong? I would appreciate some help.
The code in playground: link

Answer 1

math.SmallestNonzeroFloat64 is the number closest to 0 that is not 0, not the number furthest from zero. Try this:
go playground
func MaxBelowZero(values ...float64) float64 {
if len(values) == 0 {
return 0
} else if len(values) == 1 {
return values[0]
}
max := -math.MaxFloat64
for _, n := range values {
if n >= 0 {
continue
}
if n > max {
max = n
}
}
return max
}

Answer 2

func MaxBelowZero(n ...float64) float64 {
maxValue := -math.MaxFloat64 // Initial value must be negative
if len(n) == 1 {
return n[0]
} else if len(n) == 0 {
return 0.
}
for i := 1; i < len(n); i++ {
diff := n[i] - n[i-1] // Correct logic here
if diff > maxValue && diff < 0 {
maxValue = diff
}
}
return maxValue
}

Answer 3

You could reverse sort the slice, then find first negative number:
package main
import "sort"
func main() {
a := []float64{0, 1, 2, -1, -2, -10, 10, 20}
sort.Slice(a, func(d, e int) bool {
return a[e] < a[d]
})
n := sort.Search(len(a), func(n int) bool {
return a[n] < 0
})
println(a[n] == -1)
}
Or sort by sign, then by absolute value:
package main
import "math"
type sFunc func(a, b float64) bool
var sFuncs = []sFunc{
func(a, b float64) bool {
return math.Copysign(1, a) < math.Copysign(1, b)
},
func(a, b float64) bool {
return math.Abs(a) < math.Abs(b)
},
}
Result:
package main
import (
"fmt"
"sort"
)
func main() {
floats := []float64{0, 1, 2, -1, -2, -10, 10, 20}
sort.Slice(floats, func(a, b int) bool {
fa, fb := floats[a], floats[b]
for _, sf := range sFuncs {
if sf(fa, fb) {
return true
}
if sf(fb, fa) {
break
}
}
return false
})
fmt.Println(floats) // [-1 -2 -10 0 1 2 10 20]
}
https://golang.org/pkg/sort#Search
https://golang.org/pkg/sort#Slice