Learn go language 0x03: understand the dependency between variables and initialization order

In this paper, Go Dependencies between variables in language , It's also Go Examples in language specifications , It just expands a little .

* Code

package main  
import  "fmt"  
  
var (  
 a = b + c 
 b = f() 
 c = f() 
 d = 3  
)  

func f() int {
      
  d++ 
  return d  
}  
  
func main() {
      
 fmt.Printf("a = %d, b = %d, c = %d, d = %d\n", a, b, c, d)  
}

You can rely on intuition , Think about what the output is .
First
thinking
Examination
One
Next
.
.
.

The output is ,a = 9, b = 4, c = 5, d = 5, Did you do it right ？

From this code ：

var (  
 a = b + c 
 b = f() 
 c = f() 
 d = 3  
)  

func f() int {
      
  d++ 
  return d  
}  

• a rely on b and c
• b rely on d
• c rely on d

therefore , The initialization order is d,b,c,a.
This dependency can be drawn as a tree , Start from the leaf node of the tree , Until the root of the tree .

When printing , It will be initialized in a way similar to the subsequent traversal of the tree .
for example , Print a when , Need to compute a, Calculation a when , It needs to be calculated first b and c.
Calculation b when , Need to compute d, And then execute d The initialization d=3, here d by 3.
Then you can calculate b 了 , perform f(), here d by 4, therefore ,b by 4 .
And then calculate c , perform f(), here d by 5, therefore ,c by 5 .
And then calculate a,a=4+5, therefore a by 9 .

So the result is ：a = 9, b = 4, c = 5, d = 5

* Reference resources

https://golang.google.cn/ref/spec#Package_initialization