Python & Go | Check if lines in one file are in another file

matching-lines.go

package main

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

func main() {

	var userLogins int

	usersFile, err := os.Open("users_list.txt")
	if err != nil {
		panic(err)
	}
	defer usersFile.Close()

	loginsFile, err := os.Open("logins.txt")
	if err != nil {
		panic(err)
	}
	defer loginsFile.Close()

	usersScanner := bufio.NewScanner(usersFile)
	loginsScanner := bufio.NewScanner(loginsFile)

	for usersScanner.Scan() {

		user := strings.ToLower(usersScanner.Text())

		for loginsScanner.Scan() {

			login := strings.ToLower(loginsScanner.Text())
			if user == login {
				userLogins++
			}
		}

		if err := loginsScanner.Err(); err != nil {
			panic(err)
		}
	}

	if err := usersScanner.Err(); err != nil {
		panic(err)
	}

	fmt.Printf("%d matching user logins\n", userLogins)
}

matching_lines.py

users_fh = open("users_list.txt")
logins_fh = open("logins.txt")

matching_user_logins = 0

for user in users_fh:
    user = user.strip().lower()

    # https://stackoverflow.com/questions/34553559/python-nested-loops-only-working-on-the-first-pass
    logins_fh.seek(0)

    for login in logins_fh:
        login = login.strip().lower()
        if user == login:
            matching_user_logins += 1

users_fh.close()
logins_fh.close()

print("{} matching user logins".format(matching_user_logins))

My goal is to reproduce in Go.

The following did not work.

package main

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

func main() {

	var userLogins int

	usersFile, err := os.Open("users_list.txt")
	if err != nil {
		panic(err)
	}
	defer usersFile.Close()

	loginsFile, err := os.Open("logins.txt")
	if err != nil {
		panic(err)
	}
	defer loginsFile.Close()

	usersScanner := bufio.NewScanner(usersFile)
	loginsScanner := bufio.NewScanner(loginsFile)

	for usersScanner.Scan() {

		user := strings.ToLower(usersScanner.Text())

		for loginsScanner.Scan() {

			login := strings.ToLower(loginsScanner.Text())
			if user == login {
				userLogins++
			}
		}

		if err := loginsScanner.Err(); err != nil {
			panic(err)
		}
	}

	if err := usersScanner.Err(); err != nil {
		panic(err)
	}

	fmt.Printf("%d matching user logins\n", userLogins)
}

Follow-up:

Looks like the Python script posted here suffers from the same problem: the file object iterated over in the inner loop is not reset and doesn't allow a full read more than once.

Evidently what I first tested does not match the code provided here 1:1, otherwise I would have noticed the Python error sooner.

---

EDIT: I've updated the original Python script to reflect the change.

From Tim Heckman via #newbies Gophers slack

@Adam Chalkley if you intend to use it multiple times, just have the outer
loop be responsible for creating the inner loop's scanner.

That way on each iteration on the outer loop, you have a new scanner ready
to go.

It gets complex though.

A bufio.Scanner takes an io.Reader, those can only be read forward. So you'd
also need to Seek the file back to position 0. So that it can start reading
from the beginning again.

So open file, hit outer loop, seek the inner's file back to position 0,
create the scanner, invoke inner loop.

Added Go code as a separate "file" to this Gist. I'm thinking I'd be better served by setting up a small repo for this work.

Added Go code as a separate "file" to this Gist. I'm thinking I'd be better served by setting up a small repo for this work.

I spun off local test code to prototype this.

Note to self: Look in the check-if-lines-in-file-are-in-another-file branch.