big.NewInt匿名变量的怪异行为

Question

In the code snippet below, I want to make a very simple calculation d = a + b + c; a, b, c = b, c, d. It iterates several times.

In the first try, I make an anonymous variable big.NewInt(0).Add(a, b) to get the result of a + b, then add it and c to get the final result of a + b + c. But from the second iteration, d.Add(big.NewInt(0).Add(a, b), c) changed the value of c, then b, then a, and of course, the final result is wrong.

However, the second try's method gave me the right answer. Can anyone tell me why, please?

package main

import (
    "fmt"
    "math/big"
)

func main() {
    // first try
    a := big.NewInt(1)
    b := big.NewInt(2)
    c := big.NewInt(3)
    d := big.NewInt(0)
    for i := 0; i < 5; i++ {
        // d = a + b + c
        d.Add(big.NewInt(0).Add(a, b), c)
        fmt.Println(a, b, c, d)
        // a <- b, b <- c, c <- d
        a, b, c = b, c, d
        fmt.Println(a, b, c, d)
    }
    fmt.Println(d)

    // second try
    a = big.NewInt(1)
    b = big.NewInt(2)
    c = big.NewInt(3)
    d = big.NewInt(0)
    for i := 0; i < 5; i++ {
        // d = a + b + c
        d = big.NewInt(0).Add(big.NewInt(0).Add(a, b), c)
        fmt.Println(a, b, c, d)
        // a <- b, b <- c, c <- d
        a, b, c = b, c, d
        fmt.Println(a, b, c, d)
    }

    fmt.Println(d)
}

Answer 1

math/big.NewInt returns a pointer to an Int, so after your first iteration, both c and d point to the same object. This means, in your second iteration, when you add something to d, the same change will be reflected in c. Then you copy c to b so b will also point to the same object, and on the next iteration b to a, so after your third iteration, all 4 variables point to the same Int.

Your second loop works because you create a new Int for d each time, so every time you shift the pointers, a new Int (the one which was created for d) gets moved to c, then to b, then to a.