Change values while iterating

Question

Let's suppose I have these types:

type Attribute struct {
    Key, Val string
}
type Node struct {
    Attr []Attribute
}

and that I want to iterate on my node's attributes to change them.

I would have loved to be able to do:

for _, attr := range n.Attr {
    if attr.Key == "href" {
        attr.Val = "something"
    }
}

but as attr isn't a pointer, this wouldn't work and I have to do:

for i, attr := range n.Attr {
    if attr.Key == "href" {
        n.Attr[i].Val = "something"
    }
}

Is there a simpler or faster way? Is it possible to directly get pointers from range?

Obviously I don't want to change the structures just for the iteration and more verbose solutions are no solutions.

Answer 1

No, the abbreviation you want is not possible.

The reason for this is that range copies the values from the slice you're iterating over. The specification about range says:

Range expression                          1st value             2nd value (if 2nd variable is present)
array or slice  a   [n]E, *[n]E, or []E   index    i  int       a[i]       E

So, range uses a[i] as its second value for arrays/slices, which effectively means that the value is copied, making the original value untouchable.

This behavior is demonstrated by the following code:

x := make([]int, 3)

x[0], x[1], x[2] = 1, 2, 3

for i, val := range x {
    println(&x[i], "vs.", &val)
}

The code prints you completely different memory locations for the value from range and the actual value in the slice:

0xf84000f010 vs. 0x7f095ed0bf68
0xf84000f014 vs. 0x7f095ed0bf68
0xf84000f018 vs. 0x7f095ed0bf68

So the only thing you can do is to either use pointers or the index, as already proposed by jnml and peterSO.