Challenge 3 — Solution

Task: Write a program that declares city, population := "London",
8900000 on one line, then deliberately tries city := "Paris" again
further down in main(). Run it, note the compiler error, then fix it
using = instead of := for the second assignment.

// Step 1 — the broken version (does NOT compile):
package main

import "fmt"

func main() {
	city, population := "London", 8900000
	fmt.Println(city, population)

	city := "Paris"
	fmt.Println(city)
}

// Running "go run broken.go" produces:
// ./broken.go:9:7: no new variables on left side of :=


// Step 2 — the fixed version:
package main

import "fmt"

func main() {
	city, population := "London", 8900000
	fmt.Println(city, population)

	city = "Paris"
	fmt.Println(city)
}


Expected output (fixed version):

London 8900000
Paris

Notes:
- "no new variables on left side of :=" appears because city already
  exists — := requires at least one genuinely new variable on its
  left side, and here there are none.
- Switching to city = "Paris" (no colon) correctly reassigns the
  existing variable instead of trying to redeclare it.
- population is untouched in either version, since the challenge
  only reassigns city.