[go-nuts] More goroutines breaking program?

[m***@gmail.com]

I implemented one of the Project Euler problems, problem 14, in Go. If I
run it with 4 goroutines it works fine, but if I run it with 8 goroutines,
it gets the wrong answer. So far I haven't been able to see why. Here's my
code:

package main

import (
"fmt"
"runtime"
)

func steps(n uint64) uint64 {
if n == 0 { return 0 }
if n == 1 { return 0 }

var steps uint64
var i = n

for i != 1 {
for (i & 1) == 1 {
i = ((3 * i) + 1) / 2
steps += 2
}

for (i & 1) == 0 {
i /= 2
steps++
}
}

return steps
}

func maxStart(start, end uint64, result chan uint64) {
var max uint64
var maxStart uint64
for i := start; i < end; i++ {
steps := steps(i)
if steps > max {
max = steps
maxStart = i
}
}
result <- maxStart
}

func main() {
const MAX = 1000000
const GOROUTINES = 4

const PER_GOROUTINE = MAX / GOROUTINES

runtime.GOMAXPROCS(runtime.NumCPU())

results := make(chan uint64)

var current uint64
for i := 0; i < GOROUTINES; i++ {
go maxStart(current, current + PER_GOROUTINE, results)
current += PER_GOROUTINE
}

var maxStart uint64
for i := 0; i < GOROUTINES; i++ {
start := <-results
if start > maxStart {
maxStart = start
}
}

fmt.Println(maxStart)
}

Surely I am being stupid? Ideas? Thanks!

[Sam Mortimer]

From a super quick look - is it because 10000/8 is not an exact integer value ? (And current is an integer type.)

[Fredrik Ehnbom]

Your goroutines send the index of the "maxStart" variable within its own
block, but that tells you nothing of how many steps each goroutine actually
took. You need to make your goroutines send both the index and the number
of steps it took.

Post by m***@gmail.com
I implemented one of the Project Euler problems, problem 14, in Go. If I
run it with 4 goroutines it works fine, but if I run it with 8 goroutines,
it gets the wrong answer. So far I haven't been able to see why. Here's my
package main
import (
"fmt"
"runtime"
)
func steps(n uint64) uint64 {
if n == 0 { return 0 }
if n == 1 { return 0 }
var steps uint64
var i = n
for i != 1 {
for (i & 1) == 1 {
i = ((3 * i) + 1) / 2
steps += 2
}
for (i & 1) == 0 {
i /= 2
steps++
}
}
return steps
}
func maxStart(start, end uint64, result chan uint64) {
var max uint64
var maxStart uint64
for i := start; i < end; i++ {
steps := steps(i)
if steps > max {
max = steps
maxStart = i
}
}
result <- maxStart
}
func main() {
const MAX = 1000000
const GOROUTINES = 4
const PER_GOROUTINE = MAX / GOROUTINES
runtime.GOMAXPROCS(runtime.NumCPU())
results := make(chan uint64)
var current uint64
for i := 0; i < GOROUTINES; i++ {
go maxStart(current, current + PER_GOROUTINE, results)
current += PER_GOROUTINE
}
var maxStart uint64
for i := 0; i < GOROUTINES; i++ {
start := <-results
if start > maxStart {
maxStart = start
}
}
fmt.Println(maxStart)
}
Surely I am being stupid? Ideas? Thanks!

[m***@gmail.com]

Oh duh, thanks!