So I am new to Go and fairly inexperienced with programming in general so I hope I don't get downvoted again for asking stupid questions. I am working my way through the project euler problems and at problem 25 "1000-digit Fibonacci number" I encountered what seems to be strange behavior. The following is the code I wrote that resulted in this behavior.
package main
import (
"fmt"
"math/big"
)
func main() {
index := 2
l := new(big.Int)
pl := big.NewInt(1)
i := big.NewInt(1)
for {
l = i
i.Add(i, pl)
pl = l
index++
if len(i.String()) == 1000 {
break
}
}
fmt.Println(i, "
index: ", index)
}
Naturally this did not generate the correct answer so in the process of determining why I discovered that I had inadvertently discovered a neat way to generate powers of 2. I made the following changes and this did generate the correct result.
package main
import (
"fmt"
"math/big"
)
func main() {
index := 2
l := new(big.Int)
pl := big.NewInt(1)
i := big.NewInt(1)
for {
l.Set(i)
i.Add(i, pl)
pl.Set(l)
index++
if len(i.String()) == 1000 {
break
}
}
fmt.Println(i, "
index: ", index)
}
My question is what is happening in the first example that causes each big Int variable to be set to the value of i and why this did not generate an error if this was not the correct way to assign a big Int var value? Is i = l, etc a legitimate big Int operation that is simply incorrect for this situation?
The lines
l = i
and
pl = l
aren't doing what you think they are.
l, pl, and i are pointers, and assigning them to each other copies the pointer value, not the big.Int value.
After executing l = i, l is now the same pointer value as i, pointing to the same big.Int. When you use l.Set(i), it sets l's big.Int value to i's big.Int value, but l and i still point to two separate values.