Why does the number 01000 convert to 512?

I started to learn Go a few days ago and while learning about structs I came across and instance where I wanted to create a contacts structure and the zip code of 01000 was printed to the terminal as 512 instead on 01000

Here is the block of code:

package main

import "fmt"

type contactInfo struct {
	email   string
	zipCode int
}

type person struct {
	firstName string
	lastName  string
	contact   contactInfo
}

func main() {
	jim := person{
		firstName: "Jim",
		lastName:  "Party",
		contact:   contactInfo{email: "jim.party@gmail.com", zipCode: 01000},
	}

	fmt.Printf("%+v", jim)
}

Here is the output to the terminal :

{firstName:Jim lastName:Party contact:{email:jim.party@gmail.com zipCode:512}}%  

I used the command go run main.go and was expecting 01000 as the zip code but I got 512.

Because the 0 prefix indicates octal. We can read about this in Go spec:

> ebnf
> octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
>

So the literal string 01000 is interpreted as 1 * 8^3 + 0 * 8^2 + 0 * 8^1 + 0 * 8^0 = 512.

As of Go 1.13, you can (and probably "should") use the convention of 0o instead of 0, when you intend to use octal, for improved readability. That is to say, it would be better to write 0o1000, as that is more visually distinct and clear that octal was intended.

<hr>

In your specific case, you should treat zip codes as strings, not as numbers.