[英]How do I calculate a multiple factorial using num_bigint in Rust?
我正在尝试使用Num-BigInt库计算 Rust 中的阶乘。 我已经到了可以计算阶乘的地步:
use num_bigint::BigUint;
use num_traits::{One, Zero, FromPrimitive};
fn factorial(n: usize) -> BigUint {
let mut f: BigUint = One::one();
for i in 1..(n+1) {
let bu: BigUint = FromPrimitive::from_usize(i).unwrap();
f = f * bu;
}
f
}
pub fn main() {
println!("Starting calculation...");
println!("{}", factorial(5));
}
我想做一个双阶乘,比如:
pub fn main() {
println!("Starting calculation...");
println!("{}", factorial(factorial(5)));
}
但是,这会引发以下错误,因为数据类型不同:
error[E0308]: mismatched types
--> src/main.rs:16:30
|
16 | println!("{}", factorial(factorial(5)));
| ^^^^^^^^^^^^ expected `usize`, found struct `BigUint`
如何使用BigUint
而不是 usize 重复此usize
?
问题是您首先要使用usize
,然后使用BigUint
作为 function 参数,而您的参数设置为usize
。
要解决此问题,您应该使您的factorial
function通用,然后只允许那些对您的特定方法有意义的类型。
我举了你的例子并将其扩展为允许所有未签名的 integer 类型:
use num_bigint::BigUint;
use num_traits::{One, FromPrimitive, Unsigned};
use num::{Integer, NumCast, ToPrimitive};
fn factorial<N: Unsigned + Integer + ToPrimitive>(n: N) -> BigUint {
let mut f: BigUint = One::one();
let end: usize = NumCast::from(n).unwrap();
for i in 1..(end + 1) {
let bu: BigUint = FromPrimitive::from_usize(i).unwrap();
f = f * bu;
}
f
}
pub fn main() {
println!("Starting calculation...");
println!("{}", factorial(factorial(5 as u32)));
}
然而,这将导致不允许factorial(5)
,因为5
默认情况下被视为i32
,可以签名。 如果您希望允许签名类型,而不是在运行时失败,您可以执行以下操作:
use num_bigint::BigUint;
use num_traits::{One, FromPrimitive};
use num::{Integer, NumCast, ToPrimitive};
fn factorial<N: Integer + ToPrimitive>(n: N) -> BigUint {
let mut f: BigUint = One::one();
let end: usize = NumCast::from(n).expect("Number too big or negative number used.");
for i in 1..(end + 1) {
let bu: BigUint = FromPrimitive::from_usize(i).unwrap();
f = f * bu;
}
f
}
pub fn main() {
println!("Starting calculation...");
println!("{}", factorial(factorial(5)));
}
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