Putting text file into slice then compare

Question

I am writing a program where I take usernames line by line in a .txt file and verify that the username is in it. I have then created a slice and converted the file into a string and appended it to the string slice. I'm now trying to loop over the slice using a for loop to find a username that's in the file and compare it to another string variable that contains the username. I want to check for and see if it is in the slice. What is the best way to do this? I've tried comparing the elements in range using a for loop of the slice with the variable with the username I want to check but it's not working. So in other words I want to find out the best way to take a txt file that contains a list of just usernames added to it from top to bottom, have my program read (loop over) from that text file, and compare it with a predetermined item (username) in a string variable with what's in the text file and determine if it matches or not.

Username (string variable) == Username (in text file)

*Also the username variable will be based on what a user of the program enters into it. So I'm trying to ultimately check if when a user enters their username the program will verify if it's in the file or not. Thanks.

Code example I've tried:

var readSystemCtl []string
readSystemCtl = append(readSystemCtl, string(file))
for _, username := range of readSystemCtl {
    if username == input {
        //Continue program if true 
        break
    }else {
        //Do something else
    }
}

Note: string(file) is the text file i'm trying to read from; and input, is the string variable that the user will had input for their username that is predetermined earlier in the program.

Answer 1

As a Go programmer, I quickly translated your overall application requirements into a Go program. I then compared it to your code. I don't understand why you use a linear search of a Go slice. Why not use a random access to a Go map?

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Before comparing user names for equality, trim white space (strings.TrimSpace) and, for case insensitivity, convert to lowercase (strings.ToLower).

---

users.go

package main

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

func loadUsers(r io.Reader) (map[string]bool, error) {
    users := make(map[string]bool)
    scnr := bufio.NewScanner(r)
    for scnr.Scan() {
        user := strings.TrimSpace(scnr.Text())
        if len(user) > 0 {
            user = strings.ToLower(user)
            users[user] = true
        }
    }
    if err := scnr.Err(); err != nil {
        return nil, err
    }
    return users, nil
}

func loadUsersFile(name string) (map[string]bool, error) {
    f, err := os.Open(name)
    if err != nil {
        return nil, err
    }
    defer f.Close()
    users, err := loadUsers(f)
    if err != nil {
        return nil, err
    }
    return users, nil
}

func isUser(users map[string]bool, user string) bool {
    user = strings.TrimSpace(user)
    user = strings.ToLower(user)
    return users[user]
}

func main() {
    users, err := loadUsersFile("users.txt")
    if err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }

    scnr := bufio.NewScanner(os.Stdin)
    for scnr.Scan() {
        user := scnr.Text()
        fmt.Print("User: ", user)
        if isUser(users, user) {
            fmt.Println(" found")
        } else {
            fmt.Println(" not found")
        }
    }
    if err := scnr.Err(); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

---

$ cat users.txt
username1
Username2
UserName3
$ go build users.go
$ ./users
User Name X
User: User Name X not found
username3
User: username3 found
$ 

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