Can't modify the xml node value by golang?

I can't modify the c node's value in golang. i want get the some nodes value (is ok),and reset some nodes value (such as between "<v></v>") as the following, but there are some thing wrong with it. how to do it? youe are welcome to give some help:

    package main
import (
"fmt"
"regexp"
)
type C struct {
XMLName xml.Name `xml:"c"`
V       string   `xml:"v,omitempty"`
R       string   `xml:"r,attr"`
T       string   `xml:"t,attr,omitempty"`
S       string   `xml:"s,attr"`
}
type Row struct {
XMLName xml.Name `xml:"row"`
R       string   `xml:"r,attr"`
C       []C      `xml:"c"`
Spans   string   `xml:"spans,attr"`
}
type Result struct {
XMLName xml.Name `xml:"sheetData"`
Row     []Row    `xml:"row"`
}
func main() {
input := `
<sheetData>
<row r="2" spans="1:15">
<c r="A2" s="5" ><v>{{range .txt}}</v></c>
<c r="B2" s="5" t="s"><v>1</v></c>
<c r="C2" s="5" t="s"><v>2</v></c>
<c r="D2" s="5" t="s"><v>3</v></c>
<c r="E2" s="5" />
<c r="K2" s="6" t="s"><v>21</v></c>
</row> 
<row r="3" spans="1:15">
<c r="A3" s="5" t="s"><v>0</v></c>
<c r="B3" s="5" t="s"><v>1</v></c>
<c r="C3" s="5" t="s"><v>2</v></c>
<c r="D3" s="5" t="s"><v>3</v></c>
<c r="E3" s="5" />
<c r="K3" s="6" t="s"><v>21</v></c>
</row> 
</sheetData>`
v := Result{}
err := xml.Unmarshal([]byte(input), &v)
if err != nil {
fmt.Printf("error: %v", err)
return
}
for _, r := range v.Row {
for _, c := range r.C {       
c.V="25"          //i want the set some c node value.
fmt.Printf("%v %v %v\n", c.V, c.R,c.T)
}
}
output, err := xml.MarshalIndent(&v, "", "")
if  err != nil {
fmt.Printf("error: %v\n", err)
} 
fmt.Println(string(output))   //but the c node value is still original
}          
}  

What wrong with the above? how to set some node valule in golang?

Here's a working example (using identation for XML): Playground

Explanation:

You're copying your structs in your for-loop.
When writing

for _, r := range v.Row {

r is a copy of the value(s) in v.Row.
If you then try to change the value, you will just change the copy, but the original value won't change.

You should write your loop like

for i := range v.Row {

and access the struct with v.Row[i] instead.

The same applies to your inner loop, which should be written like this:

for j := range v.Row[i].C {

Then you can change the column like

v.Row[i].C[j].V = "25"

Optionally you could get a reference to the column by writing c := &v.Row[i].C[j] and then change values as c.V = "25"