Why does fmt.Scan() behave in a weird way?-CodePudding

I'm trying to validate the user input. If the user inputs an integer number it works like expected. However, if the user inputs a non-integer string, the variable userTickets gets assigned value 0, but prints Try again! It must be more than zero: many times. To be exact, it prints len(input) times and I don't understand why.

Also tried achieving desired result using fmt.Scanf("%d", &usertickets) but got an identical result.

Why does it behave this way and how can I write a workaround for it?

package main

import "fmt"

func main() {

    var remainingTickets uint = 50
    var userTickets uint

    fmt.Print("Enter the number of tickets you want to purchase: ")
    fmt.Scan(&userTickets)

    for userTickets > remainingTickets {
        fmt.Printf("We only have %v tickets available!\n", remainingTickets)
        fmt.Print("Try again! Enter the number of tickets: ")
        fmt.Scan(&userTickets)
    }

    for userTickets == 0 {
        fmt.Print("Try again! It must be more than zero: ")
        fmt.Scan(&userTickets)
    }

    fmt.Printf("Remaining tickets: %v\n", remainingTickets-userTickets)

}

CodePudding user response：

Scan is able to determine that the input isn't numeric without reading the entire contents of stdin. This is why you validation logic loops for len(input) when non-numeric. While you can use a Scanner as well (and people do recommend that approach), below is an approach similar to yours. Note that all validation checking is done within a single "for" loop as well:

package main

import (
    "fmt"
    "strconv"
)

func main() {

    var remainingTickets uint64 = 50
    
    fmt.Print("Enter the number of tickets you want to purchase: ")

    for {
        var userTickets string
        fmt.Scanln(&userTickets)

        // parse to make sure we get a positive (unsigned) integer
        u64, err := strconv.ParseUint(userTickets,10,64)
        // make sure it's a postive integer and not equal to zero
        if err != nil || u64==0{
            fmt.Print("Try again! You must enter a number greater than zero: ")
            continue
        }

        // check to make sure we have enough tickets left
        if u64 > remainingTickets {
            fmt.Printf("We only have %v tickets available!\n", remainingTickets)
            fmt.Print("Try again! Enter the number of tickets: ")
            continue
        }

        // update remaining tickets
        remainingTickets -= u64
        break
    }

    fmt.Printf("Remaining tickets: %d\n", remainingTickets)
}