Why do all values ​​become the last element when using for range in Go language to traverse slices and store maps?

Go Map iteration trap: Why all values ​​point to the last element?

When used in combination with map data structures for...range Go, a common trap is prone to occur: when traversing the slice and adding elements to map , all map values ​​eventually point to the last element. This article will analyze its reasons through code examples and provide solutions.

Let's look at a piece of code:

 type Student struct {
    Name string
    Age int
}

func main() {
    m := make(map[string]*Student)
    students := []Student{
        {Name: "pprof.cn", Age: 18},
        {Name: "Test", Age: 23},
        {Name: "Blog", Age: 28},
    }

    for _, stu := range students {
        m[stu.Name] = &stu // The problem is here}
    for k, v := range m {
        fmt.Println(k, "=>", v.Name)
    }
}

The expected result of this code is to store each Student structure in students slice into map . However, the run results show that all map values ​​point to the last element "blog":

 <code>pprof.cn => 博客测试=> 博客博客=> 博客</code>

The root of the problem: variable scope of for...range loop

The problem is that in the for...range loop, stu variable does not create a new variable every iteration. It points to the same memory address in the students slice. Each iteration, the value of stu is updated, but its memory address remains the same. So when the loop is over, all pointers in map point to the final value of stu (the last element).

Solution: Create a new copy of the variable

To solve this problem, we need to create a copy of the stu variable in each iteration, making sure that map stores different memory addresses. The modified code is as follows:

 for _, stu := range students {
    newStu := stu // Create a copy m[newStu.Name] = &newStu
}

Or, a more concise way is to create a new Student structure directly:

 for _, stu := range students {
    m[stu.Name] = &Student{Name: stu.Name, Age: stu.Age} // Create a new Student instance}

With the above modification, each value in map will point to a different memory address, avoiding the case where all values ​​point to the last element. Running the modified code will get the expected results.

Hopefully this explanation will help you understand the details of for...range loops and map data structures in Go and avoid such traps.

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