CODEWITHSUNDEEP
go
user1616534
user1616534

Reputation: 21

Why does this go code fail?

I have written some go code in FP style to generate primes:

package main
import (
    "fmt"
)

func gen_number_stream() func() (int, bool) {
    i := 1
    return func() (int, bool) {
        i += 1
        return i, true
    }
}

func filter_stream(stream func() (int, bool), f func(int) bool) func() (int, bool) {
    return func() (int, bool) {
        for i, ok := stream(); ok; i, ok = stream() {
            if f(i) {
                return i, true
            }
        }
    return 0, false
    }
}

func sieve(stream func() (int, bool)) func() (int, bool) {
    return func() (int, bool) {
        if p, ok := stream(); ok {
            remaining := filter_stream(stream, func(q int) bool { return q % p != 0 })
            stream = sieve(remaining)
            return p, true
        }
        return 0, false
    }
}


func take(stream func() (int, bool), n int) func() (int, bool) {
    return func() (int, bool) {
        if n > 0 {
            n -= 1
            return stream()
        }
        return 0, false
    }
}

func main() {

    primes := take(sieve(gen_number_stream()), 50)

    for i, ok := primes(); ok; i, ok = primes() {
        fmt.Println(i)
    }

}

when I run this code, it becomes slower and slower and finally gets a runtime error like this:

runtime: out of memory:  [...]

here is a version of python code, and it just runs fine:

def gen_numbers():
    i = 2
    while True:
        yield i
        i += 1

def sieve(stream):
    p =  stream.next()
    yield p
    for i in sieve( i for i in stream if i % p != 0 ):
        yield i

def take(stream,n):
    for i,s in enumerate(stream):
        if i == 50: break
        yield s

def main():
    for i in take(sieve(gen_numbers()),50):
        print i


if __name__ == '__main__':
    main()

I wonder why and how to fix it. Is it a problem of my code or golang compiler? Thanks!

PS: Sorry for my poor english.

Upvotes: 2

Views: 218

Answers (2)

Jeremy Wall
Jeremy Wall

Reputation: 25275

The problem is your sieve function which is recursive. I suspect that you are blowing your stack by continually calling sieve recursively in a loop.

func sieve(stream func() (int, bool)) func() (int, bool) {
    return func() (int, bool) {
        if p, ok := stream(); ok {
            remaining := filter_stream(stream, func(q int) bool { return q % p != 0 })
            stream = sieve(remaining) // just keeps calling sieve recursively which eventually blows your stack.
            return p, true
        }
        return 0, false
    }
}

Upvotes: 2

Max
Max

Reputation: 6394

You reuse stream

    if p, ok := stream(); ok {
        remaining := filter_stream(stream, func(q int) bool { return q % p != 0 })

BUT for every new "p" you must create a new "stream2"

    if p, ok := stream(); ok {
        stream2 := ....
        remaining := filter_stream(stream2, func(q int) bool { return q % p != 0 })

Upvotes: 1

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