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Predict the Output of Go Code Storing Pointers to a for-Loop Variable

ZooxSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSoftware Engineering Fundamentals

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A Go output-prediction question about a for loop that appends the address of its loop variable on every iteration and prints the stored values afterwards. It tests loop-variable semantics across Go language versions, escape analysis, and whether such pointers are safe to dereference.

Without running it, predict what this Go program prints, and explain why.

package main

import "fmt"

func main() {
	var a []*int
	for i := 0; i < 3; i++ {
		a = append(a, &i)
	}
	for _, b := range a {
		fmt.Println(*b)
	}
}

This was an output-prediction exercise: the code alone, with a request for its output.

Clarifying Questions

  • Which Go release builds the program, and which language version does its module declare in go.mod?
  • Should the answer also cover whether the program can crash, or only what it prints?

Hints

Hint 1

Count the variables: Work out how many distinct variables the three `&i` expressions can point to, and what each one holds when the second loop reads it.

Hint 2

Consider the toolchain: Ask whether your answer would be the same for every Go release, and what in a build selects the rules for a loop's variable.

Follow-up questions

  • A candidate argues that `i` goes out of scope when the loop ends, so dereferencing the stored pointers panics. Is that right, and what does the compiler actually do with `i`?
  • Replace the loop body with `go func() { fmt.Println(i) }()` and wait for the goroutines to finish. What can it print under each rule?
  • If the loop body sometimes runs `i++` itself, which value does the next iteration start from under the per-iteration rule?
  • Does giving every iteration its own variable cost anything at run time, and in which cases?