CODEWITHSUNDEEP
pythongogenerator
Pierre Dittgen
Pierre Dittgen

Reputation: 23

golang: read file generator

I'm learning go language and try to rewrite some of my Python code using golang. I wrote a generator function that reads a text file line by line and send (using yield keyword) only "valid" lines (blank lines are ignored, uncompleted lines are recomposed).

Sample file (myfile.txt):

#123= FOOBAR(1.,'text');

#126= BARBAZ('poeazpfodsp',
234,56);

parse.py:

#!/usr/bin/python
def validlines(filename):
    with open(filename) as fdin:
        buff = ''
        for line in fdin.readlines():
            line = line.strip()
            if line == '':
                continue
            buff += line
            if line[-1] != ';':
                continue
            yield buff
            buff = ''
        fdin.close()

for line in validlines('myfile.txt'):
    print(line)

displays:

#123= FOOBAR(1.,'text');
#126= BARBAZ('poeazpfodsp',234,56);

Now, I try to do it the same way using a closure in golang:

parse.go:

package main

import (
    "bufio"
    "fmt"
    "os"
    "strings"
)

func validLines(filename string) (func() (string, bool)) {

    file, _ := os.Open(filename)
    scanner := bufio.NewScanner(file)

    return func() (string, bool) {
        buff := ""
        for scanner.Scan() {
            line := scanner.Text()
            line = strings.TrimSpace(line)

            if line == "" {
                continue
            }
            buff += line
            if line[len(line)-1] != ';' {
                continue
            }
            return buff, true
        }

        file.Close()
        return "", false
    }
}

func main() {
    vline := validLines("myfile.txt")
    for line, ok := vline(); ok; {
        fmt.Println(line)
    }
}

displays:

#123= FOOBAR(1.,'text');
#123= FOOBAR(1.,'text');
#123= FOOBAR(1.,'text');
#123= FOOBAR(1.,'text');
#123= FOOBAR(1.,'text');
#123= FOOBAR(1.,'text');
#123= FOOBAR(1.,'text');
...

What's the right way to do it in golang?

Upvotes: 2

Views: 1726

Answers (4)

Vasilii Petrov
Vasilii Petrov

Reputation: 21

Just little addition to pav5000's answer, you should make the channel buffered:
c := make(chan string, 1)
Otherwise, it will be reading the whole file into a channel and there is no point in using it.

Upvotes: 2

pav5000
pav5000

Reputation: 143

In Go you can use channels instead of yield, it's very convenient.

package main

import (
    "bufio"
    "fmt"
    "os"
    "strings"
)

func ValidLines(filename string) (c chan string) {
    c = make(chan string)
    buff := ""
    go func() {
        file, err := os.Open(filename)
        if err != nil {
            close(c)
            return
        }

        reader := bufio.NewReader(file)
        for {
            line, err := reader.ReadString('\n')
            if err != nil {
                close(c)
                return
            }
            line = strings.TrimSpace(line)
            if line == "" {
                continue
            }
            buff += line
            if line[len(line)-1] != ';' {
                continue
            }
            c <- buff
            buff = ""
        }
    }()
    return c
}

func main() {
    for line := range ValidLines("myfile.txt") {
        fmt.Println(line)
    }
}

Upvotes: 6

timestee
timestee

Reputation: 1096

small change:

func main() {
    vline := validLines("myfile.txt")
    line, ok := vline()
    for ok  {
        fmt.Println(line)
        line, ok = vline()
    }
}

Upvotes: 1

T. Claverie
T. Claverie

Reputation: 12256

This is the for loop of your main which is the problem. With this syntax, here is what it does:

  1. Initialize line and ok with a first call to vline()
  2. If ok is true, run the loop once
  3. Update something at the end of the block, and go to 2.

So, your problem is that you never update lineand ok. Here is the correct version:

for line, ok := vline(); ok; line, ok = vline() { ... }

Upvotes: 1

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