Go Tricky: A Bug with For-Range Loops and Pointer

Spent way too much time on a slice and 'for range' problem while working on my API. I thought I knew it all, but turns out, I didn't

Photo by Daniel Jensen on Unsplash

As it happens, this problem is a variant of the LoopVar issues. Fortunately, it will be addressed in Go 1.22.

---

Ever thought you understood something, but then had a problem with it?

That’s what happened to me with Go’s for range and I spent way too much time fixing an issue with a slice in my API flow.

After I finally solved it, I thought others could learn from my experience and guess what? A teammate ran into the same problem, making me realize I should share my story.

---

I wanted to filter even numbers from a list, turn them into pointers, and then modify or do something with them.

Here’s the code:

func main() {
  pool := []int{1, 2, 3, 4, 5, 6}
  even := []*int{}
  
  for _, v := range pool {
    if v%2 == 0 {
      even = append(even, &v)
    }
  }
  
  for i := range even {
    fmt.Print(*even[i], " ")
  }

  // .... Doing something complex with even numbers to 
  // modify them without touching the original pool
}

Want to try guessing what shows up on the screen when you run this?

Spoiler alert, from the title of this article, you can guess it’s not what I thought would happen so it prints 6 6 6 instead of 2 4 6.

“Why did that happen?”

Well, the for range loop creates a copy of the element in the array when you use a variable like v so it doesn’t actually use the real element from the array.

The variable v is initialized once and reused in the loop. Let me recast that for range loop into a standard for loop to clear things up:

for i, v := 0, 0; i < len(pool); i++ {
  v = pool[i]

  if v%2 == 0 {
    even = append(even, &v)
  }
}

When you append &v to the ‘even’ slice, you’re actually storing the address of the v variable, which changes to 2, 4, and finally 6 in the loop.

So, in the end, all the elements in the even slice share the same memory address, which is the address of the v variable (unfortunately).

For instance, when I printed the memory addresses in the ‘even’ slice, they were all the same:

for i := range even {
  fmt.Print(even[i], " ")
}

// 0x1400001a228 0x1400001a228 0x1400001a228

---

And that wraps up my little lesson.

Hopefully, this has been insightful for other developers, including my teammate who stumbled into the same issue and good luck with your coding.