How to pass variables/parameters to big.NewInt()

Question content

The following code snippet fails to compile on N := big.NewInt(n) with the following error ：

<code>
cannot use n (variable of type int) as int64 value in argument to
big.NewInt
</code>

func Factorial(n int) *big.Int {
    var result = new(big.Int)
    i := new(big.Int)
    N := big.NewInt(n)
    for i.Cmp(N) < 0 {
        result = result.Mul(i, result)
        i = i.Add(i, new(big.Int))
    }
    return result
}

If I pass an int64 literal (i.e. N := big.NewInt(1)) it works. But I need a way to convert an int64 variable or argument/argument to big.Int. What did I do wrong? Doesn't Go support this at all?

Correct answer

This error is because https://pkg.go.dev/math/big# NewInt function uses int64 value as argument, not int type. Perform the required type conversion:

N := big.NewInt(int64(n))

In addition, the calculation logic can be written very simply as

func Factorial(n int) *big.Int {
    result, one := big.NewInt(1), big.NewInt(1)
    bigN := big.NewInt(int64(n))
    for bigN.Cmp(&big.Int{}) == 1 {
        result.Mul(result, bigN)
        bigN.Sub(bigN, one)
    }
    return result
}

https://www.php.cn/link/861f8aa2598860c0023f399e992eb747

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